Distribution of the wingless gene product in Drosophila embryos: a protein involved in cell-cell communication

A soluble form of Wnt-1 protein with mitogenic activity on mammary epithelial cells

The proto-oncogene Wnt-1 plays an essential role in fetal brain development and causes hyperplasia and tumorigenesis when activated ectopically in the mouse mammary gland. When expressed in certain mammary epithelial cell lines, the gene causes morphological transformation and excess cell proliferation at confluence. Like other members of the mammalian Wnt family, Wnt-1 encodes secretory glycoproteins which have been detected in association with the extracellular matrix or cell surface but which have not previously been found in a soluble or biologically active form. We show here that conditioned medium harvested from a mammary cell line expressing Wnt-1 contains soluble Wnt-1 protein and induces mitogenesis and transformation of mammary target cells. By immunodepletion of medium containing epitope-tagged Wnt-1, we show that at least 60% of this activity is specifically dependent on Wnt-1 protein. These results provide the first demonstration that a mammalian Wnt protein can act as a diffusible extracellular signaling factor.

The Drosophila cubitus interruptus protein and its role in the wingless and hedgehog signal transduction pathways

The segment polarity gene cubitus interruptus (ci) is required to maintain expression of the wingless gene and to specify naked cuticle within each epidermal segment. Antibodies were generated against the ci protein and used to analyze its pattern of expression. By stage 11, post-transcriptional regulation of ci is observed. ci transcript levels are uniform across the anterior compartment, but protein levels are higher next to the compartment boundaries. The distribution of the ci protein is altered in fused, hedgehog and wingless mutants suggesting cell-cell signaling may regulate ci protein levels. The role of ci in cell-cell signaling and pattern formation was examined in double mutants of ci with patched and zeste-white3/shaggy.

Schnurri is required for Drosophila Dpp signaling and encodes a zinc finger protein similar to the mammalian transcription factor PRDII-BF1

Cytokines of the TGF beta superfamily regulate many aspects of cellular function by activating receptor complexes consisting of two distantly related serine/threonine kinases. Previous studies have indicated that Drosophila dpp uses similar signaling complexes and strictly requires the punt and thick veins receptors to transduce the signal across the membrane. Here, we show that the schnurri (shn) gene is required for many aspects of dpp signaling. Genetic epistasis experiments indicate that shn functions downstream of the dpp signal and its receptors. The shn gene encodes a large protein similar to a family of mammalian zinc finger transcription factors. The shn protein might therefore act as a nuclear target in the dpp signaling pathway directly regulating the expression of dpp-responsive genes.

Murine Wnt-11 and Wnt-12 have temporally and spatially restricted expression patterns during embryonic development

The Wnt gene family encodes a set of signalling molecules implicated in the development of a wide range of organisms. We have recently cloned partial cDNA sequences of murine Wnt-11 and Wnt-12. Here, we describe the spatio-temporal expression patterns of both genes during mouse embryogenesis. Wnt-11 expression is first detected within the truncus arteriosus from 8.25 dpc. By 9.5 dpc, Wnt-11 expression is detected in the somites at the medial junction of the dermatome and the myotome. Wnt-11 transcripts are also detected in limb bud mesenchyme from the time the bud is first visible. Wnt-12 is detected in the apical ectodermal ridge from 10.5 dpc. The implications of these expression patterns are discussed.

Differential requirements for segment polarity genes in wingless signaling

The segment polarity genes wingless and engrailed are required throughout development of Drosophila. During early embryogenesis, these two genes are expressed in adjacent domains, in an inter-dependent way. Later, their expression is regulated by different mechanisms and becomes maintained by auto-regulation. To dissect the genetic requirements for the initial signaling between wingless and engrailed expressing cells, we have previously used a transgenic Drosophila strain that expresses wingless under the control of the heat shock promoter (HS-wg). Focusing on the later phases of wingless and engrailed regulation, we have now extended these studies, using embryos carrying various combinations of segment polarity mutations and the HS-wg transgene. We confirm some of the existing models of regulation of the expression of wingless and engrailed. In addition, we find that HS-wg embryos require engrailed for induction of ectopic endogenous wingless expression. Signaling from engrailed cells to this novel wingless expression domain is dependent on hedgehog but also on porcupine. We further demonstrate a novel requirement for hedgehog in maintenance of expression of engrailed itself.

Phosphorylation of the Drosophila engrailed protein at a site outside its homeodomain enhances DNA binding

The engrailed gene encodes a homeodomain-containing phosphoprotein that binds DNA. Here, we show that engrailed protein is posttranslationally modified in embryos and in embryo-derived cultured cells but is essentially unmodified when expressed in Escherichia coli. Engrailed protein produced by bacteria can be phosphorylated in nuclear extracts prepared from Drosophila embryos, and phosphotryptic peptides from this modified protein partly reproduce two-dimensional maps of phosphotryptic fragments obtained from metabolically labeled engrailed protein. The primary embryonic protein kinase modifying engrailed protein is casein kinase II (CK-II). Analysis of mutant proteins revealed that the in vitro phosphoacceptors are mainly clustered in a region outside the engrailed homeodomain and identified serines 394, 397, 401, and 402 as the targets for CK-II phosphorylation. CK-II-dependent phosphorylation of an N-truncated derivative of engrailed protein purified from bacteria increased its DNA binding 2-4-fold.

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.

Spatial regulation of segment polarity gene expression in the anterior terminal region of the Drosophila blastoderm embryo

The effects of mutations in five anterior gap genes (hkb, tll, otd, ems and btd) on the spatial expression of the segment polarity genes, wg and hh, were analyzed at the late blastoderm stage and during subsequent development. Both wg and hh are normally expressed at blastoderm stage in two broad domains anterior to the segmental stripes of the trunk region. At the blastoderm stage, each gap gene acts specifically to regulate the expression of either wg or hh in the anterior cephalic region: hkb, otd and btd regulate the anterior blastoderm expression of wg, while tll and ems regulate hh blastoderm expression. Additionally, btd is required for the first segmental stripe (mandibular segment) of both hh and wg at blastoderm stages. The subsequent segmentation of the cephalic segments (preantennal, antennal and intercalary) appears to be dependent on the overlap of the wg and hh cephalic domains as defined by these gap genes at the blastoderm stage. None of these five known gap genes are required for the activation of the labral segment domains of hh and wg, which are presumably either activated directly by maternal pathways or by an unidentified gap gene.

Quantitative effects of hedgehog and decapentaplegic activity on the patterning of the Drosophila wing

BACKGROUND

Members of the hedgehog (hh) gene family encode a novel class of proteins implicated in positional signalling in both invertebrates and vertebrates. In Drosophila, the hh gene has been shown to regulate patterning of the imaginal discs, the precursors of the insect limbs. In a remarkably similar fashion, the function and expression of the sonic hedgehog (shh) gene is closely associated with the 'zone of polarizing activity' (ZPA) that controls antero-posterior patterning of the vertebrate limb. Both of these functions suggest a role for hedgehog family proteins as morphogens. An alternative possibility, however, is that hh and its homologues act to control the expression of other instructive signalling molecules.

RESULTS

We have explored this issue by examining the effects on Drosophila wing patterning of ectopically expressing varying levels of hh and shh, as well as of the putative hh target gene, decapentaplegic (dpp), a member of the transforming growth factor-beta family of signalling molecules. We find that different levels of hh activity can induce graded changes in the patterning of the wing, and that zebrafish shh acts in a similar though attenuated fashion. Varying levels of ectopic hh and shh activity can differentially activate transcription of the patched and dpp genes. Furthermore, ectopic expression of dpp alone is sufficient to induce the pattern alterations caused by ectopic hh or shh activity.

CONCLUSION

Thus, hh family proteins can elicit different responses in a dose-dependent manner in the imaginal disc. The principal function of hh, however, is to activate transcription of dpp at the compartment boundary, thereby establishing a source of dpp activity that is the primary determinant of antero-posterior patterning.

Relationships between complex Delta expression and the specification of retinal cell fates during Drosophila eye development

Analysis of retinal development in Delta (Dl) temperature-sensitive mutants reveals requirements for Delta function in the specification of all retinal cells, including photoreceptors, cone cells, pigment cells and cells that make up interommatidial bristles. In situ hybridization and immunohistochemistry indicate that Delta is expressed dynamically during the specification of different cell types. Comparisons of Delta expression patterns with developmental defects in Dl mutants implies that Delta functions in a cell-nonautonomous manner in the specification of photoreceptors. Delta protein resides predominantly in subcellular vesicles located primarily at the apical ends of developing retinal cells. Localization of Delta protein in Dl and shibire tsl mutants implies that Delta is targeted to the cell surface, but is efficiently removed via endocytosis, resulting in vesicular accumulation.

Mutations that alter the timing and pattern of cubitus interruptus gene expression in Drosophila melanogaster

The cubitus interruptus (ci) gene is a member of the Drosophila segment polarity gene family and encodes a protein with a zinc finger domain homologous to the vertebrate Gli genes and the nematode tra-1 gene. Three classes of existing mutations in the ci locus alter the regulation of ci expression and can be used to examine ci function during development. The first class of ci mutations causes interruptions in wing veins four and five due to inappropriate expression of the ci product in the posterior compartment of imaginal discs. The second class of mutations eliminates ci protein early in embryogenesis and causes the deletion of structures that are derived from the region including and adjacent to the engrailed expressing cells. The third class of mutations eliminates ci protein later in embryogenesis and blocks the formation of the ventral naked cuticle. The loss of ci expression at these two different stages in embryonic development correlates with the subsequent elimination of wingless expression. Adults heterozygous for the unique ciCe mutation have deletions between wing veins three and four. A similar wing defect is present in animals mutant for the segment polarity gene fused that encodes a putative serine/threonine kinase. In ciCe/+ and fused mutants, the deletions between wing veins three and four correlate with increased ci protein levels in the anterior compartment. Thus, proper regulation of both the ci mRNA and protein appears to be critical for normal Drosophila development.

Positional signaling by hedgehog in Drosophila imaginal disc development

We describe a dominant gain-of-function allele of the segment polarity gene hedgehog. This mutation causes ectopic expression of hedgehog mRNA in the anterior compartment of wing discs, leading to overgrowth of tissue in the anterior of the wing and partial duplication of distal wing structures. The posterior compartment of the wing is unaffected. Other imaginal derivatives are affected, resulting in duplications of legs and antennae and malformations of eyes. In mutant imaginal wing discs, expression of the decapentaplegic gene, which is implicated in the hedgehog signaling pathway, is also perturbed. The results suggest that hedgehog protein acts in the wing as a signal to instruct neighboring cells to adopt fates appropriate to the region of the wing just anterior to the compartmental boundary.

Involvement of wnt1 and pax2 in the formation of the midbrain-hindbrain boundary in the zebrafish gastrula

The secreted signalling molecule encoded by the wnt1 gene and the paired box-containing pax2 gene are thought to play an integral role in patterning the zebrafish rostral nervous system. Using a double-label analysis, we compare the expression patterns of wnt1 RNA and pax2 protein during zebrafish embryogenesis to determine whether they were expressed in identical or overlapping patterns in individual embryos. During gastrulation, wnt1 RNA was detected in a pattern similar but not identical to the pax2 protein. Later, wnt1 and pax2 co-localize to the midbrain-hindbrain boundary. Exogenous retinoic acid, a teratogen that is known to affect the formation of the midbrain-hindbrain boundary, has a profound affect on both wnt1 and pax2 expression at gastrulation. Furthermore, when pax2 is overexpressed in zebrafish embryos, the wnt1 pattern of expression expands ventrally in the prospective rostral neuroepithelium. Despite the widespread and random distribution of exogenous pax2 RNA, it alone is unable to induce wnt1 expression in other ectopic sites. These results are consistent with the coordinate expression of wnt1 and pax2 being in a pathway responsible for establishing the midbrain-hindbrain boundary and support the earlier interpretation that pax2 may regulate wnt1 expression [Krauss et al., 1992], although only in a subset of embryonic cells. These data suggest that a predisposition for the regionalization of the central nervous system exists at gastrulation.

Control of Drosophila head segment identity by the bZIP homeotic gene cnc

Mutational analysis of cap'n'collar (cnc), a bZIP transcription factor closely related to the mammalian erythroid factor NF-E2 (p45), indicates that it acts as a segment-specific selector gene controlling the identity of two cephalic segments. In the mandibular segment, cnc has a classical homeotic effect: mandibular structures are missing in cnc mutant larvae and replaced with duplicate maxillary structures. We propose that cnc functions in combination with the homeotic gene Deformed to specify mandibular development. Labral structures are also missing in cnc mutant larvae, where a distinct labral primordia is not properly maintained in the developing foregut, as observed by the failure to maintain and elaborate patterns of labral-specific segment polarity gene expression. Instead, the labral primordium fuses with the esophageal primordium to contribute to formation of the esophagus. The role of cnc in labral development is reciprocal to the role of homeotic gene forkhead, which has an identical function in the maintenance of the esophageal primordium. This role of homeotic selector genes for the segment-specific maintenance of segment polarity gene expression is a unique feature of segmentation in the preoral head region of Drosophila.

Distinct pathways for autocrine and paracrine Wingless signalling in Drosophila embryos

Two secreted proteins, Wingless and Hedgehog, instruct cell fates within the segmented epidermis of Drosophila embryos (reviewed in ref. 5). Wingless (Wg) is expressed by the most posterior cells in each parasegment; Hedgehog (Hh) is expressed in the most anterior cells of the next parasegment. Immediately after gastrulation, the two cell types are mutually dependent. Local Wg signalling stabilizes Hh expression and local Hh signalling stabilizes Wg expression. Direct Wg autoregulation (autocrine signalling) is masked by its paracrine role in maintaining hh, which in turn maintains wg. I have used zeste-white3 (zw3) and patched (ptc) mutant backgrounds to uncouple genetically this positive-feedback loop and to study autocrine Wg signalling. I report here that direct Wg autoregulation differs from Wg signalling to adjacent cells in the importance of fused (fu), smoothened (smo) and cubitus interruptus (ci) relative to zw3 and armadillo (arm). I also find that Wg autoregulation during this early hh-dependent phase differs from later Wg autoregulation by lack of gooseberry (gsb) participation.

Wingless can bring about a mesoderm-to-ectoderm induction in Drosophila embryos

By means of nuclear transplantations, we make mosaics in which largely wingless- embryos contain patches of wingless+ cells. In these genetic mosaics, using a standard assay for wingless function (the maintenance of engrailed expression), we uncover an induction across germ layers: Wingless made in the mesoderm can sustain engrailed expression in the ectoderm. This result makes clear that Wingless is expressed in the mesoderm until at least one hour after gastrulation and may function in this germ layer in the wild type.

Role of the teashirt gene in Drosophila midgut morphogenesis: secreted proteins mediate the action of homeotic genes

Homeotic genes control the development of embryonic structure by coordinating the activities of downstream ‘target’ genes. The identities and functions of target genes must be understood in order to learn how homeotic genes control morphogenesis. Drosophila midgut development is regulated by homeotic genes expressed in the visceral mesoderm, where two of their target genes have been identified. Both encode secreted proteins. The Ultrabithorax (Ubx) homeotic gene activates transcription of the decapentaplegic (dpp) gene, which encodes a TGF beta class protein, while in adjacent mesoderm cells the abdominal-A (abd-A) homeotic gene activates transcription of the wingless (wg) gene, which encodes a Wnt class protein. The homeotic genes Antennapedia (Antp) and Sex combs reduced (Scr) act in more anterior midgut regions. Here we report the identification of another homeotic gene target in the midgut mesoderm, the teashirt (tsh) gene, which encodes a protein with zinc finger motifs. tsh is necessary for proper formation of anterior and central midgut structures. Antp activates tsh in anterior midgut mesoderm. In the central midgut mesoderm Ubx, abd-A, dpp, and wg are required for proper tsh expression. The control of tsh by Ubx and abd-A, and probably also by Antp, is mediated by secreted signaling molecules. By responding to signals as well as localized transcription regulators, the tsh transcription factor is produced in a spatial pattern distinct from any of the homeotic genes.

Differential transformation of mammary epithelial cells by Wnt genes

The mouse Wnt family includes at least 10 genes that encode structurally related secreted glycoproteins. Wnt-1 and Wnt-3 were originally identified as oncogenes activated by the insertion of mouse mammary tumor virus in virus-induced mammary adenocarcinomas, although they are not expressed in the normal mammary gland. However, five other Wnt genes are differentially expressed during development of adult mammary tissue, suggesting that they may play distinct roles in various phases of mammary gland growth and development. Induction of transformation by Wnt-1 and Wnt-3 may be due to interference with these normal regulatory events; however, there is no direct evidence for this hypothesis. We have tested Wnt family members for the ability to induce transformation of cultured mammary cells. The results demonstrate that the Wnt gene family can be divided into three groups depending on their ability to induce morphological transformation and altered growth characteristics of the C57MG mammary epithelial cell line. Wnt-1, Wnt-3A, and Wnt-7A were highly transforming and induced colonies which formed and shed balls of cells. Wnt-2, Wnt-5B, and Wnt-7B also induced transformation but with a lower frequency and an apparent decrease in saturation density. In contrast, Wnt-6 and two other family members which are normally expressed in C57MG cells, Wnt-4 and Wnt-5A, failed to induce transformation. These data demonstrate that the Wnt genes have distinct effects on cell growth and should not be regarded as functionally equivalent.

Homologs of the mouse Brachyury gene are involved in the specification of posterior terminal structures in Drosophila, Tribolium, and Locusta

The Brachyury (T) gene is required for notochord differentiation in vertebrates. We have identified a Drosophila gene, the T-related gene (Trg), with high similarity to T within a stretch of approximately 200 amino acids, the DNA-binding domain of T. Trg is expressed throughout embryogenesis, first at the blastoderm stage in the hindgut primordium under the control of the terminal gap genes tll and hkb, and then until the end of embryogenesis in the differentiating hindgut. Drosophila embryos deficient for Trg do not form the hindgut, a phenotype that can be rescued by a Trg transgene. Thus, a common feature of T and Trg is their requirement in specifying the development of a single embryonic structure. Homologs of Trg are also expressed in the developing hindgut of Tribolium and Locusta embryos suggesting a highly conserved function of Trg in insects. This conservation and the high similarity of T and Trg raise the question of a common evolutionary origin of the hindgut of insects and the notochord of chordates.

Expression of a novel Toll-like gene spans the parasegment boundary and contributes to hedgehog function in the adult eye of Drosophila

Many proteins involved in signal transduction and cell adhesion are characterized by the presence of an extracellular domain with repeated copies of a leucine-rich motif (LRR). Here we report the isolation and characterization of a novel gene, tlr (for Toll-like receptor), which encodes a protein containing multiple LRRs in its presumed extracellular domain, a single transmembrane segment and homology to the cytoplasmic domain of the interleukin 1 receptor in its presumed intracellular domain. The pattern of tlr expression at the extended germ band stage is characterized by 15 transverse stripes in the gnathal and trunk segments, with four patches of expression corresponding to head segments and an additional patch of expression in the presumptive hindgut. The segmentally repeated tlr stripes in the trunk overlap both the wingless and engrailed stripes and thus span the parasegment boundary. The tlr stripes require pair rule gene function for their establishment and later become dependent upon segment-polarity gene function for their maintenance. Segmental modulation of tlr expression later in the tracheal system is dependent upon the function of the homeotic genes of the bithorax complex. The tlr gene also is prominently expressed in the imaginal discs. In the eye disc, this expression occurs in two stripes at the anterior and posterior margins of the morphogenetic furrow; this expression is consistent with a genetic interaction between a tlr mutation and an eye-specific allele of hedgehog. All of these data combine to suggest a role for tlr in interactions between cells at critical boundaries during development.

Modulation of wingless signaling by Notch in Drosophila

Extensive genetic and molecular analyses indicate that Notch acts as a transmembrane receptor in an evolutionarily conserved cell interaction mechanism that appears to control a common step in the progression of an uncommitted cell towards the differentiated state. In Drosophila, Notch mutations were shown to affect the development of a broad spectrum of tissues, including the wing. We found that mutations in the segment polarity gene wingless are capable of acting as dominant enhancers of notchoid, a recessive Notch allele affecting the wing. The Wingless protein is homologous to the mammalian proto-oncoprotein Wnt-1 and is thought to act as the signal in a cell interaction mechanism that specifies differentiation of the embryonic epidermis as well as imaginal structures such as the wing. Although some components of the Wingless signal transduction pathway have been identified, the receptor for Wingless remains elusive. This genetic link between the Wingless and Notch pathways has been further examined by determining the relative expression patterns and subcellular localization of Notch and Wingless in mutant and wild-type backgrounds. We find that Notch is necessary for the implementation of the Wingless signal in specifying normal wing development. We discuss the possibility that Notch is directly involved in the reception of Wingless in the light of current models for the developmental action of Notch.

Differential transformation of mammary epithelial cells by Wnt genes

The mouse Wnt family includes at least 10 genes that encode structurally related secreted glycoproteins. Wnt-1 and Wnt-3 were originally identified as oncogenes activated by the insertion of mouse mammary tumor virus in virus-induced mammary adenocarcinomas, although they are not expressed in the normal mammary gland. However, five other Wnt genes are differentially expressed during development of adult mammary tissue, suggesting that they may play distinct roles in various phases of mammary gland growth and development. Induction of transformation by Wnt-1 and Wnt-3 may be due to interference with these normal regulatory events; however, there is no direct evidence for this hypothesis. We have tested Wnt family members for the ability to induce transformation of cultured mammary cells. The results demonstrate that the Wnt gene family can be divided into three groups depending on their ability to induce morphological transformation and altered growth characteristics of the C57MG mammary epithelial cell line. Wnt-1, Wnt-3A, and Wnt-7A were highly transforming and induced colonies which formed and shed balls of cells. Wnt-2, Wnt-5B, and Wnt-7B also induced transformation but with a lower frequency and an apparent decrease in saturation density. In contrast, Wnt-6 and two other family members which are normally expressed in C57MG cells, Wnt-4 and Wnt-5A, failed to induce transformation. These data demonstrate that the Wnt genes have distinct effects on cell growth and should not be regarded as functionally equivalent.

Pattern formation in the visual centers of the Drosophila brain: wingless acts via decapentaplegic to specify the dorsoventral axis

A stepwise morphogenetic program of cell division and cell fate determination generates the precise neuronal architecture of the visual centers of the Drosophila brain. Here, we show that the assembly of the target structure for ingrowing retinal axons involves cell-cell interactions mediated by the secreted product of the wingless (wg) gene. wg, expressed in two symmetrical domains of the developing brain, is required to induce and maintain the expression of the secreted decapentaplegic (dpp) gene product in adjacent domains. wg and dpp function are required for target field neurons to adopt their proper fates and to send axons into the developing target structure. These observations implicate a cascade of diffusible signaling molecules in patterning the visual centers of the Drosophila brain.

Wnt-1-dependent regulation of local E-cadherin and alpha N-catenin expression in the embryonic mouse brain

E-cadherin is transiently expressed in local regions of the embryonic mouse brain, which include several patchy areas on the mesencephalon and diencephalon and their roof plate and part of cerebellar rudiments. In the present study, we compared this E-cadherin expression with that of Wnt-1, which occurs in specific zones in the embryonic brain, and found certain spatiotemporal relations between them: Wnt-1 expression tended to run parallel or overlap with peripheries of the E-cadherin-positive areas. For example, in the dorsal midline, Wnt-1 was expressed at the middle of the roof plate, while E-cadherin was absent in the middle zone but detected in two arrays of marginal roof plate cells. Furthermore, alpha N-catenin, a cadherin-associated protein, was found to occur at the roof plate of the mesencephalon and diencephalon, coinciding with Wnt-1 expression. The expression of these molecules was then studied in two alleles of the Wnt-1 mutation, Wnt-1sw and Wnt-1neo. In mice homozygous for these mutant genes, E-cadherin expression in the roof plate was up-regulated; the middle E-cadherin-negative zone disappeared. Moreover, E-cadherin expression in the roof plate began earlier in the mutant mice than in wild-type mice. On the contrary, alpha N-catenin expression in the dorsal midline was suppressed in these mutants. These changes in cadherin and catenin expression occurred at the level of mRNA expression. These results suggest that the Wnt-1 signal is, either directly or indirectly, involved in the regulation of expression of E-cadherin and alpha N-catenin in restricted regions of the embryonic brain. This mechanism may contribute to the patterning of the expression of these adhesion-related proteins in the embryonic brain.

Cis-acting regulatory sequences governing Wnt-1 expression in the developing mouse CNS

The protooncogene Wnt-1 encodes a short-range signal which is first expressed in, and appears to demarcate, the presumptive midbrain. Absence of Wnt-1 expression leads to the loss of this region of the brain. By the end of neural tube closure, expression of Wnt-1 extends down much of the dorsal midline of the central nervous system (CNS). Expression is exclusively limited to the CNS at this and later stages. We have investigated the regulation of Wnt-1 during mouse development. Analysis of the embryonic expression of Wnt-1-lacZ reporter constructs spanning nearly 30 kb of the Wnt-1 locus identified a 5.5 kb cis-acting 3′ enhancer element which confers correct temporal and spatial expression on the lacZ gene. Interestingly embryos express Wnt-1-lacZ transgenes in migrating neural crest cells which are derived from the dorsal CNS. Ectopic expression of the Wnt-1-lacZ transgenes may result from perdurance of beta-galactosidase activity in migrating neural crest cells originating from a Wnt-1-expressing region of the dorsal CNS. Alternatively, ectopic expression may arise from transient de novo activation of the transgenes in this cell population. These results are a first step towards addressing how regional cell signaling is established in the mammalian CNS. In addition, transgene expression provides a new tool for the analysis of neural crest development in normal and mutant mouse embryos.

Band 6 protein, a major constituent of desmosomes from stratified epithelia, is a novel member of the armadillo multigene family

Desmosomes are intercellular adhering junctions characteristic of epithelial cells. Several constitutive proteins--desmoplakin, plakoglobin and the transmembrane glycoproteins desmoglein and desmocollin--have been identified as fundamental constituents of desmosomes in all tissues. A number of additional and cell type-specific constituents also contribute to desmosomal plaque formation. Among these proteins is the band 6 polypeptide (B6P). This positively charged, non-glycosylated protein is a major constituent of the plaque in stratified and complex glandular epithelia. Using an overlay assay we show that purified keratins bind in vitro to B6P. Thus B6P may play a role in ordering intermediate filament networks of adjacent epithelial cells. To characterize the structure of B6P in the desmosome we have isolated cDNA clones representing the entire coding sequence. The predicted amino acid sequence of human B6P shows strong sequence homology with a murine p120 protein, which is a substrate of protein tyrosine kinase receptors and of p60v-src. P120 and B6P show amino-terminal domains differing distinctly in length and sequence. These are followed in both proteins by 460 residues that display a series of imperfect repeats corresponding to the repeats in the cadherin binding proteins armadillo, plakoglobin and beta-catenin. Over this repeat region B6P and p120 share 33% sequence identity (54% similarity). These sequence characteristics define B6P as a novel member of the armadillo multigene family and raise the question of whether the structural proteins B6P, plakoglobin, beta-catenin and armadillo share some function. Since armadillo, plakoglobin, beta-catenin and p120 seem involved in signal transduction this may also hold for B6P. The amino-terminal region of B6P (residues 1 to 263) shows no significant homology to any known protein sequence. It may therefore be involved in unique functions of B6P.

Drosophila mode of metamerization in the embryogenesis of the lepidopteran insect Manduca sexta

Insect embryos have been classified as intermediate- and short-germ embryos, in which posterior segments are thought to be generated sequentially from an uncommitted growth zone, or as long-term embryos, such as Drosophila melanogaster, which develop primordia for all segments simultaneously. In Drosophila the coordinated activities among a three-tiered cascade of zygotic segmentation genes subdivide the embryo into progressively smaller units along the anterior-posterior axis. The mode of pattern specification in lepidopteran embryos has not been determined, although on morphological grounds they have been characterized as intermediate-germ insects. We have cloned orthologues of Drosophila segmentation genes from the tobacco hawkmoth Manduca sexta and have found that the blastoderm expression patterns of these genes show a molecular prepatterning typical of Drosophila. Thus, successive segment formation in Manduca embryos may not be due to sequential addition but rather may be the consequence of a lateral compression of the embryo proceeding in an anterior-to-posterior progression. These data challenge the view that the classification of insect development according to morphological criteria can serve as a reliable indicator of the molecular mechanisms underlying segmentation.

Evidence for a mitogenic effect of Wnt-1 in the developing mammalian central nervous system

The analysis of mutant alleles at the Wnt-1 locus has demonstrated that Wnt-1-mediated cell signalling plays a critical role in development of distinct regions of the embryonic central nervous system (CNS). To determine how these signals participate in the formation of the CNS, we have ectopically expressed this factor in the spinal cord under the control of the Hoxb-4 Region A enhancer. Ectopic Wnt-1 expression causes a dramatic increase in the number of cells undergoing mitosis in the ventricular region and a concomitant ventricular expansion. Although this leads to consistent changes in the relative proportions of dorsal and ventral regions, Wnt-1 does not appear to act as a primary patterning signal. Rather, our experiments indicate that Wnt-1 can act as a mitogen in the developing CNS.

Drosophila wingless: a paradigm for the function and mechanism of Wnt signaling

The link between oncogenesis and normal development is well illustrated by the study of the Wnt family of proteins. The first Wnt gene (int-1) was identified over a decade ago as a proto-oncogene, activated in response to proviral insertion of a mouse mammary tumor virus. Subsequently, the discovery that Drosophila wingless, a developmentally important gene, is homologous to int-1 supported the notion that int-1 may have a role in normal development. In the last few years it has been recognized that int-1 and Wingless belong to a large family of related glyco-proteins found in vertebrates and invertebrates. In recognition of this, members of this family have been renamed Wnts, an amalgam of int and Wingless. Investigation of Wnt genes in Xenopus and mouse indicates that Wnts have a role in cell proliferation, differentiation and body axis formation. Further analysis in Drosophila has revealed that Wingless function is required in several developmental processes in the embryo and imaginal discs. In addition, a genetic approach has identified some of the molecules required for the transmission and reception of the Wingless signal. We will review recent data which have contributed to our growing understanding of the function and mechanism of Drosophila Wingless signaling in cell fate determination, growth and specification of pattern.

Developmental consequences of unrestricted expression of the abd-A gene of Drosophila

The abdominal-A (abd-A) gene of Drosophila specifies abdominal segments two through eight (A2-A8). We have found that the uniform expression of the abd-A protein under heat shock control transforms embryonic segments anterior to the abd-A domain into an abdominal segment of the A2-A6 type. Posterior abdominal segments and telson undergo little or no transformation, that is, the abd-A product is phenotypically suppressed posterior to its realm of expression. The comparison of wildtype embryos with embryos carrying the heat shock-abd-A construct but no abd-A endogenous product indicate that some elements of the pattern-like shape of denticle belts or ventral pits depend on the amount of abd-A protein. The concurrent expression of abd-A and other homeotic genes suggest competition for common downstream genes. This competition has been studied by looking at the expression on the visceral mesoderm of a gene downstream of abd-A, decapentaplegic, when two homeoproteins with opposing effects on its transcription are simultaneously induced.

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

The secreted glycoproteins encoded by Wnt genes are thought to function as intercellular signaling molecules which convey positional information. Localized expression of Wingless protein is required to specify the fate of ventral cells in the developing Drosophila leg. We report here that Wingless acts through inactivation of the shaggy/zeste white 3 protein kinase to specify ventral cell fate in the leg. Ectopic expression of Wingless outside its normal ventral domain has been shown reorganize the dorsal-ventral axis of the leg in a non-autonomous manner. Using genetic mosaics, we show that cells that lack shaggy/zeste white 3 activity can influence the fate of neighboring cells to reorganize dorsal-ventral pattern in the leg, in the same manner as Wingless-expressing cells. Therefore, clones of cells that lack shaggy/zeste white 3 activity exhibit all of the organizer activity previously attributed to Wingless-expressing cells, but do so without expressing wingless. We also show that the organizing activity of ventral cells depends upon the location of the clone along the dorsal-ventral axis. These findings suggest that Wingless protein does not function as a morphogen in the dorsal-ventral axis of the leg.

The gene serpent has homeotic properties and specifies endoderm versus ectoderm within the Drosophila gut

The gut of Drosophila consists of ectodermally derived foregut and hindgut and endodermally derived midgut. Here I show that the gene serpent plays a key role in the development of the endoderm. serpent embryos lack the entire midgut and do not show endodermal differentiation. They gastrulate normally and form proper amnioproctodeal and anterior midgut invaginations. However, the prospective anterior midgut cells acquire properties that are usually found in ectodermal foregut cells. In the posterior region of the embryo, the prospective posterior midgut forms an additional hindgut which is contiguous with the normal hindgut and which appears to be a serial duplication, not a mere enlargement of the hindgut. The fate shifts in both the anterior and the posterior part of the srp embryo can be described in terms of homeotic transformations of anterior midgut to foregut and of posterior midgut to hindgut. serpent appears to act as a homeotic gene downstream of the terminal gap gene huckebein and to promote morphogenesis and differentiation of anterior and posterior midgut.

Interactions of decapentaplegic, wingless, and Distal-less in the Drosophila leg

The genes decapentaplegic, wingless, and Distalless appear to be instrumental in constructing the anatomy of the adult Drosophila leg. In order to investigate how these genes function and whether they act coordinately, we analyzed the leg phenotypes of the single mutants and their inter se double mutant compounds. In decapentaplegic the tarsi frequently exhibit dorsal deficiencies which suggest that the focus of gene action may reside dorsally rather than distally. In wingless the tarsal hinges are typically duplicated along with other dorsal structures, confirming that the hinges arise dorsally. The plane of symmetry in double-ventral duplications caused by decapentaplegic is virtually the same as the plane in double-dorsal duplications caused by wingless. It divides the fate map into two parts, each bisected by the dorsoventral axis. In the double mutant decapentaplegic wingless the most ventral and dorsal tarsal structures are missing, consistent with the notion that both gene products function as morphogens. In wingless Distal-less compounds the legs are severely truncated, indicating an important interaction between these genes. Distal-less and decapentaplegic manifest a relatively mild synergism when combined.

Localized expression of sloppy paired protein maintains the polarity of Drosophila parasegments

During germ-band extension in the Drosophila embryo, intercellular communication is required to maintain gene expression patterns initiated at cellular blastoderm. For example, the wingless (wg) single-cell-wide stripe in each parasegment (PS) is dependent on a signal from the adjacent, posterior cells, which express engrailed (eN). This signal is thought to be the hedgehog (hh) gene product, which antagonizes the activity of patched (ptc), a repressor of wg expression. Genetic evidence indicates that the hh signal is bidirectional, but wg transcription is only derepressed on the anterior side of the en/hh stripes. To explain the asymmetric response of the wg promoter to the hh signal, current models predict that each PS is divided into cells that are competent to express either wg or en, but not both. The sloppy paired (slp) locus contains two transcription units, both encoding proteins containing a forkhead domain, a DNA-binding motif. Removal of slp gene function causes embryos to exhibit a severe pair-rule/segment polarity phenotype. We show that the en stripes expand anteriorly in slp mutant embryos and that slp activity is an absolute requirement for maintenance of wg expression at the same time that wg transcription is dependent on hh. The slp proteins are expressed in broad stripes just anterior of the en-positive cells, overlapping the narrow wg stripes. We propose that by virtue of their ability to activate wg and repress en expression, the distribution of the slp proteins define the wg-competent and en-competent groups. Consistent with this hypothesis, ubiquitous expression of slp protein throughout the PS abolishes en expression and, in ptc mutant embryos, results in a near ubiquitous distribution of wg transcripts. In addition to demonstrating the role of slp in maintaining segment polarity, our results suggest that slp works in, or parallel with, the ptc/hh signal transduction pathway to regulate wg transcription.

Subcellular localization of the segment polarity protein patched suggests an interaction with the wingless reception complex in Drosophila embryos

The product of the segment polarity gene patched is a transmembrane protein involved in the cell communication processes that establish polarity within the embryonic segments of Drosophila. Monoclonal antibodies have been raised against the patched protein, and by immunoelectron microscopy part of the patched staining is found associated with discrete regions of the lateral plasma membrane of the embryonic epidermal cells. Using a mutation affecting endocytosis (shibire) we find that patched is a membrane-bound protein, which is internalized by endocytosis, and that the preferential sites of accumulation resemble the described localization of the cell-cell adhesive junctions of the epidermal cells. patched partially co-localizes with the wingless protein in the wingless-expressing and nearby cells, in structures that seem to be endocytic vesicles. These data suggest the interaction of patched protein with elements of the reception complex of wingless, as a way to control the wingless expression.

Biological activity of soluble wingless protein in cultured Drosophila imaginal disc cells

The phenotypes caused by mutations in Wnt genes suggest that their gene products are involved in cell-to-cell communication. Wnt genes indeed encode secreted molecules, but soluble active Wnt protein has not been found. We have developed a novel cell culture assay for the Drosophila Wnt gene wingless, using a Drosophila imaginal disc cell line (cl-8; ref. 13), and measured effects on the adherens junction protein armadillo, a known genetic target of wingless. Transfection of a temperature-sensitive wingless complementary DNA into cl-8 cells increases the levels of the armadillo protein. The wingless protein does not affect the rate of synthesis of armadillo, but leads to increased stability of an otherwise rapidly decaying armadillo protein. The wingless protein in the extracellular matrix and soluble medium from donor cells also increases the levels of armadillo protein. The protein in the medium acts fast and is inhibited by an antibody to wingless protein, demonstrating that Wnt products can act as soluble extracellular signalling molecules.

Compartment boundaries and the control of Drosophila limb pattern by hedgehog protein

Drosophila limbs are subdivided into anterior and posterior compartments which derive from adjacent cell populations founded early in development. Evidence is now provided that posterior cells organize growth and cell patterning in both compartments by secreting hedgehog protein and that hedgehog protein acts indirectly by inducing neighbouring anterior cells to secrete decapentaplegic or wingless protein.

Wnt-1 modulates cell-cell adhesion in mammalian cells by stabilizing beta-catenin binding to the cell adhesion protein cadherin

Wnt-1 homologs have been identified in invertebrates and vertebrates and play important roles in cellular differentiation and organization. In Drosophila, the products of the segment polarity genes wingless (the Wnt-1 homolog) and armadillo participate in a signal transduction pathway important for cellular boundary formation in embryonic development, but functional interactions between the proteins are unknown. We have examined Wnt-1 function in mammalian cells in which armadillo (beta-catenin and plakoglobin) is known to bind to and regulate cadherin cell adhesion proteins. We show that Wnt-1 expression results in the accumulation of beta-catenin and plakoglobin. In addition, binding of beta-catenin to the cell adhesion protein, cadherin, is stabilized, resulting in a concomitant increase in the strength of calcium-dependent cell-cell adhesion. Thus, a consequence of the functional interaction between Wnt-1 and armadillo family members is the strengthening of cell-cell adhesion, which may lead to the specification of cellular boundaries.

Drosophila hedgehog acts as a morphogen in cellular patterning

The patterning of cell types in embryogenesis is specified by signals emanating from specialized organizer regions. We demonstrate that engrailed-expressing cells in the Drosophila epidermis have organizer properties. These cells influence the pattern of cell type differentiation across the segment. We show that this function is mediated by the hedgehog (hh) gene. The results of modulating the levels of hh in the embryo suggest that hh acts as a morphogen, specifying distinct cell fates by a concentration-dependent mechanism. We present a model that integrates the role of hh with that of the wingless signal in establishing the segmental array of cell type diversity.

Conservation of wingless patterning functions in the short-germ embryos of Tribolium castaneum

During embryogenesis, all insects reach a conserved, or phylotypic, stage at which all future segments are present. Different insects, however, arrive at this stage by overtly different pathways. In the long-germ insect Drosophila melanogaster, segmentation of the entire embryo occurs nearly simultaneously and results from the action of a cascade of transcriptional regulatory factors that operate in the acellular environment of the syncytial blastoderm. In short-germ insects, segmentation occurs in an anterior-to-posterior sequence, within a cellular environment, and might then be dependent on intercellular signalling. To compare the molecular mechanisms of segmentation, we have isolated a homologue of the Drosophila wingless gene, a mediator of cell-cell communications, from the short-germ beetle Tribolium castaneum. The principal features of wingless expression patterns in Drosophila are conserved in Tribolium, including its early deployment in rostral and caudal domains in the blastoderm, its segmental iteration in cells immediately anterior to cells expressing the engrailed gene, and its later restriction to a ventral sector of the developing appendages.

wingless signal and Zeste-white 3 kinase trigger opposing changes in the intracellular distribution of Armadillo

wingless/wnt-1 signaling directs cell fate during development. Genetic analysis in Drosophila identified genes that may encode components of the wingless signal transduction system. Drosophila Armadillo, homolog of vertebrate beta-catenin, is required for wingless signaling. Unlike armadillo RNA, Armadillo protein accumulates non-uniformly in different cells of each embryonic segment. We found that cells alter their intracellular distribution of Armadillo in response to Wingless signal, accumulating increased levels of cytoplasmic Armadillo relative to those of membrane-associated protein. Levels of cytoplasmic Armadillo are also regulated by Zeste-White 3 kinase. Analysis of double mutants demonstrates that Armadillo's role in wingless signaling is direct, and that Armadillo functions downstream of both wingless and zeste-white 3. We present a model for the role of Armadillo stripes in transduction of wingless signal.

dishevelled is required during wingless signaling to establish both cell polarity and cell identity

The dishevelled gene of Drosophila is required to establish coherent arrays of polarized cells and is also required to establish segments in the embryo. Here, we show that loss of dishevelled function in clones, in double heterozygotes with wingless mutants and in flies bearing a weak dishevelled transgene leads to patterning defects which phenocopy defects observed in wingless mutants alone. Further, polarized cells in all body segments require dishevelled function to establish planar cell polarity, and some wingless alleles and dishevelled; wingless double heterozygotes exhibit bristle polarity defects identical to those seen in dishevelled alone. The requirement for dishevelled in establishing polarity in cell autonomous. The dishevelled gene encodes a novel intracellular protein that shares an amino acid motif with several other proteins that are found associated with cell junctions. Clonal analysis of dishevelled in leg discs provides a unique opportunity to test the hypothesis that the wingless dishevelled interaction species at least one of the circumferential positional values predicted by the polar coordinate model. We propose that dishevelled encodes an intracellular protein required to respond to a wingless signal and that this interaction is essential for establishing both cell polarity and cell identity.

Hedgehog is a signaling protein with a key role in patterning Drosophila imaginal discs

The segment polarity genes hedgehog and engrailed are expressed in identical posterior-compartment-specific patterns in both Drosophila embryos and imaginal discs. We show here that the hedgehog protein is secreted, and it can cross embryo parasegment borders and the anterior-posterior compartment border of imaginal discs to neighboring cells that express neither engrailed nor hedgehog. In these cells, it is localized in discrete punctate structures that are sequestered within the polarized epithelium. Analysis of animals that have expressed hedgehog ectopically, or of a mutant that expresses hedgehog abnormally in the anterior compartment of the wing disc, indicates that hedgehog is involved in regulating patched. In the embryo, hedgehog regulation of patched apparently facilitates patched and wingless expression. In the discs, hedgehog regulation of patched and other genes in the anterior compartment helps to establish the proximodistal axis. We propose that the cell-cell communication mediated by hedgehog links the special properties of compartment borders with specification of the proximodistal axis in imaginal development.

dishevelled and armadillo act in the wingless signalling pathway in Drosophila

The Wnt genes encode conserved secreted proteins that play a role in normal development and tumorigenesis. Little is known about the signal transduction pathways of Wnt gene products. One of the best characterized Wnt family members is the Drosophila segment polarity gene wingless. We have investigated whether segment polarity genes with a wingless-like phenotype mediate the wingless signal. We used a wingless transgene controlled by a heat-shock promoter for genetic epistasis experiments. We show that wingless acts through dishevelled and armadillo to affect the expression of the homeobox gene engrailed and cuticle differentiation.

Components of wingless signalling in Drosophila

The determination of specific cell fates and polarity within each segmental unit of the Drosophila embryo involves the products of the segment polarity genes. One of these, wingless (wg), encodes a secreted protein that is homologous to the mammalian proto-oncogene Wnt-1 (refs 4, 5). In the embryonic epidermis, wg is expressed in a single row of cells within each segmental unit, although its activity is required for the correct patterning of most of the epidermis. Initially Wg signals to adjacent posterior cells, maintaining engrailed (en) expression. Later during embryogenesis, wg specifies the differentiation of naked cuticle. Wg signalling functions by inactivating or antagonizing the activity of zestewhite 3 (zw3). We have investigated the requirement in the Wg signal transduction pathway for the three genes armadillo (arm), dishevelled (dsh) and porcupine (porc), all of which have embryonic mutant phenotypes similar to wg. Our results indicate that dsh and porc act upstream of zw3, and arm acts downstream of zw3.

Evolution of distinct developmental functions of three Drosophila genes by acquisition of different cis-regulatory regions

It is generally accepted that the specific function of a gene depends on its coding sequence. The three paired-box and homeobox genes paired (prd), gooseberry (gsb) and gooseberry neuro (gsbn) have distinct developmental functions in Drosophila embryogenesis. During the syncytial blastoderm stage, the pair-rule gene prd activates segment-polarity genes, such as gsb, wingless (wg), and engrailed (en), in segmentally repeated stripes. After germ-band extension, gsb maintains the expression of wg, which in turn specifies the denticle pattern by repressing a default state of ubiquitous denticle formation in the ventral epidermis. In addition, gsb activates gsbn, which is expressed mainly in the central nervous system, suggesting that gsbn is involved in neural development. Here we show that, despite the functional difference and the considerably diverged coding sequence of these genes, their proteins have conserved the same function. The finding that the essential difference between genes may reside in their cis-regulatory regions exemplifies an important evolutionary mechanism of how function diversifies after gene duplication.

Regulation of the early development of the nervous system by growth factors

Development of the nervous system, although patterned by intrinsic genetic expression, appears to be dependent on growth factors for many of the differentiation steps that generate the wide variety of neurons and glia found in the both the central and peripheral nervous system. By using in vitro assays, including clonal analysis, the precise function of the various growth factors and the differentiation potential of the various neural populations has begun to be described. This review discusses some of the recent findings and examines how neuronal differentiation may result from the interaction of several growth factors.