Organizing activity of wingless protein in Drosophila

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.

Cell interaction between compartments establishes the proximal-distal axis of Drosophila legs

The appendage primordia of Drosophila are subdivided into compartments by the localized expression of transcription factors. Interaction between cells in adjacent compartments establishes organizing centres responsible for generating spatial pattern and promoting cell proliferation in the developing appendages. Localized expression of hedgehog (hh) in the posterior compartment of the leg imaginal disc directs expression of wingless (wg) in ventral-anterior cells and decapentaplegic (dpp) in dorsal-anterior cells near the anterior-posterior compartment boundary; wg then acts to specify ventral cell fate and to organize the dorsal-ventral axis of the leg. Interaction between wg-expressing ventral cells and dorsal cells near the anterior-posterior compartment boundary promotes axis formation in the leg. Here we show that the combined action of wg-expressing cells in the ventral-anterior compartment and dpp-expressing cells in the dorsal-anterior compartment activates expression of Distal-less, a gene required for proximal-distal axis formation in the limbs. These results demonstrate that sequential interaction between anterior-posterior and dorsal-ventral compartments establishes the proximal-distal axis of the limbs.

Induction of a mirror-image duplication of anterior wing structures by localized hedgehog expression in the anterior compartment of Drosophila melanogaster wing imaginal discs

The segment polarity gene hedgehog (hh) encodes a secretory protein involved in cell-cell communication in Drosophila melanogaster. The hh gene is expressed in the posterior compartment and is essential for the establishment and maintenance of the anterior/posterior-compartment boundary of each embryonic parasegment [Ingham, P.W., Nature 366 (1993) 560-562]. To clarify possible hh functions in adult appendage formation, we isolated a fly line (h9D) associated with a wing malformation from among fly lines with an hh transgene whose expression is under the control of trapped enhancers. In h9D flies, the ectopic expression of hh occurred in the anterior edge of wing pouch in the wing disc. This abnormal hh expression resulted in not only a mirror-image duplication and ectopic outgrowth in the anterior wing compartment, but also the ectopic expression of patched and decapentaplegic, strongly suggesting that the hh product serves as a morphogen or an inducer essential for wing development, including the proximal/distal axis formation.

Cell interactions in the control of size in Drosophila wings

The vein locus (vn) includes lethal alleles (designated also defective dorsal discs) that prevent growth of dorsal discs and in viable genetic combinations reduce the number of cells of the adult wing. Those effects are prominent in genetic mosaics. Cell proliferation is reduced in all regions of the wing blade in a local autonomous way. These effects are more extreme when mutant clones occupy full intervein regions bordering veins. Clones have, in addition, nonautonomous effects (accommodation) in the proliferation of wild-type cells of the same wing. These effects are more extreme in double mutant vn (ddd) and ve (rhomboid) allelic combinations. Developmental analysis shows that cell proliferation stops earlier in larval development the stronger the vn allele considered. A model is discussed of how cell proliferation is controlled by cellular interactions.

Site-specific recombination: developments and applications

Site-specific DNA recombination systems have considerable utility in manipulating DNA and can facilitate many cloning and gene transfer techniques. The ability of a number of recombinases to catalyze efficient DNA recombination in higher eukaryotes has important and exciting consequences for precise chromosome and transgene engineering. Exploitation of these recombinases will facilitate the generation of transgenic animal disease models and help elucidate the function of developmental genes.

Molecular biology of embryonic development: how far have we come in the last ten years?

The successes of molecular developmental biology over the last ten years have been particularly impressive in those directions favored by its major paradigms. New technologies have both guided and been guided by the progress of the field. I review briefly some of the major insights into embryonic development that have derived from research in four specific areas: early embryogenesis of various forms; 'pattern formation'; evolutionary conservation of regulatory elements; and spatial mechanisms of gene regulation. There remain many major problem areas, some of which may require new orientations to solve.

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.

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.

Pattern formation and eyespot determination in butterfly wings

Butterfly wings display pattern elements of many types and colors. To identify the molecular processes underlying the generation of these patterns, several butterfly cognates of Drosophila appendage patterning genes have been cloned and their expression patterns have been analyzed. Butterfly wing patterns are organized by two spatial coordinate systems. One system specifies positional information with respect to the entire wing field and is conserved between fruit flies and butterflies. A second system, superimposed on the general system and involving several of the same genes, operates within each wing subdivision to elaborate discrete pattern elements. Eyespots, which form from discrete developmental organizers, are marked by Distal-less gene expression. These circular pattern elements appear to be generated by a process similar to, and perhaps evolved from, proximodistal pattern formation in insect appendages.

Pattern formation in the vertebrate neural plate

Recent advances have been made in the understanding of the cellular and molecular mechanisms involved in the formation and patterning of the neural plate of vertebrate embryos. Both planar and vertical signaling pathways appear to be involved in the neural induction and axial patterning of the neural plate. The neural plate, behaving as a developmental field, might be patterned by signals emanating from boundary regions: the organizer region and the midline and edges of the neural plate. Here, A. Ruiz i Altaba describes a possible model for anteroposterior patterning involving ;lanar signals for amphibian, avian and mammalian embryos, compares the axial patterning of the neural plate with the patterning of insect epithelia, and discussed possible roles of noggin, follistatin and hedgehog-related genes in neural induction and patterning.

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.

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.

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.

Organization of wing formation and induction of a wing-patterning gene at the dorsal/ventral compartment boundary

The appendages of arthropods and vertebrates possess a third, proximodistal patterning axis that is established after the primary anteroposterior and dorsoventral body axes by mechanisms that are largely unknown. The vestigial gene is required for formation of the entire Drosophila wing, and the dorsal/ventral boundary is shown to organize wing formation and vestigial gene expression. Interactions between dorsal and ventral cells in the growing imaginal disc induce vestigial gene expression through a discrete, extraordinarily conserved imaginal disc-specific enhancer. The link between dorsal/ventral compartmentalization and wing formation distinguishes the development of this sheet-like appendage from that of legs and antennae.

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.

Floor plate and motor neuron induction by vhh-1, a vertebrate homolog of hedgehog expressed by the notochord

The differentiation of distinct cell types in the ventral neural tube depends on local inductive signals from the notochord. We have isolated a vertebrate homolog of the Drosophila segment polarity gene hedgehog (hh) from zebrafish and rat, termed vhh-1. vhh-1 is expressed in the node, notochord, floor plate, and posterior limb bud mesenchyme. Each of these cell groups has floor plate inducing activity, suggesting that the vhh-1 gene may encode a floor plate-inducing molecule. Widespread expression of rat vhh-1 in frog embryos leads to ectopic floor plate differentiation in the neural tube. In vitro tests for the signaling functions of vhh-1 demonstrate that COS cells expressing the rat vhh-1 gene induce floor plate and motor neuron differentiation in neural plate explants. vhh-1 may, therefore, contribute to the floor plate and motor neuron inducing activities of the notochord.

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.

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.

Pattern formation. The beginning and the end of insect limbs

Recent results shed light on the mechanisms underlying pattern formation in the development of Drosophila imaginal discs, which give rise to the appendages of the adult fly.

The Drosophila segment polarity gene dishevelled encodes a novel protein required for response to the wingless signal

The Drosophila Wnt-1 homolog, wingless (wg), is involved in the signaling of patterning information in several contexts. In the embryonic epidermis, Wg protein is secreted and taken up by neighboring cells, in which it is required for maintenance of engrailed transcription and accumulation of Armadillo protein. The dishevelled (dsh) gene mediates these signaling events as well as wg-dependent induction across tissue layers in the embryonic midgut. dsh is also required for the development processes in which wg functions in adult development. Overall, cells lacking dsh are unable to adopt fates specified by Wg. dsh functions cell autonomously, indicating that it is involved in the response of target cells to the Wg signal. dsh is expressed uniformly in the embryo and encodes a novel protein with no known catalytic motifs, although it shares a domain of homology with several junction-associated proteins. Our results demonstrate that dsh encodes a specific component of Wg signaling and illustrate that Wnt proteins may utilize a novel mechanism of extracellular signal transduction.

The use of photoactivatable reagents for the study of cell lineage in Drosophila embryogenesis

Photoactivatable lineage tracers represent a major advance for clonal analysis in the early embryo and the study of cell movements. Any cell in the blastoderm can be marked, and the nuclear localization of the signal allows excellent resolution in identifying the daughters of individual cells. Although the technique is limited by the availability of the water-soluble caged fluorescein and its derivatives for synthesis of the complete tracer, these may become commercially available in the future. The use of caged rhodamine derivatives or antibody amplification of the signal may greatly extend the developmental period over which marked clones can be identified.

Mutations in the segment polarity genes wingless and porcupine impair secretion of the wingless protein

We have characterized the molecular nature of mutations in wingless (wg), a segment polarity gene acting during various stages of Drosophila development. Embryo-lethal alleles have undergone mutations in the protein-encoding domain of the gene, including deletions and point mutations of conserved residues. In a temperature sensitive mutation, a conserved cysteine residue is replaced by a serine. In embryo-viable alleles, the wg transcriptional unit is not affected. Immunostaining of mutant embryos shows that the embryo-lethal alleles produce either no wg antigen or a form of the protein that is retained within cells. Interestingly, embryos mutant for the segment polarity gene porcupine show a similar retention of the wg antigen. We have also transfected wild type wg alleles into Drosophila tissue culture cells, which then display wg protein on the cell surface and in the extracellular matrix. In similar experiments with mutant alleles, the proteins are retained in intracellular compartments and appear not to be secreted. These data provide further evidence that wg acts as a secreted factor and suggest that porcupine provides an accessory function for wg protein secretion or transport.

Association of the APC gene product with beta-catenin

Mutations in the human APC gene are linked to familial adenomatous polyposis and to the progression of sporadic colorectal and gastric tumors. To gain insight into APC function, APC-associated proteins were identified by immunoprecipitation experiments. Antibodies to APC precipitated a 95-kilodalton protein that was purified and identified by sequencing as beta-catenin, a protein that binds to the cell adhesion molecule E-cadherin. An antibody specific to beta-catenin also recognized the 95-kilodalton protein in the immunoprecipitates. These results suggest that APC is involved in cell adhesion.

Expression of Wnt-1 in PC12 cells results in modulation of plakoglobin and E-cadherin and increased cellular adhesion

The Wnt-1 gene plays an essential role in fetal brain development and encodes a secreted protein whose signaling mechanism is presently unknown. In this report we have investigated intracellular mechanisms by which the Wnt-1 gene induces morphological changes in PC12 pheochromocytoma cells. PC12 cells expressing Wnt-1 show increased steady-state levels of the adhesive junction protein plakoglobin, and an altered distribution of this protein within the cell. This effect appears similar to a modulation of the plakoglobin homolog, Armadillo, that occurs in Drosophila embryos in response to the Wnt-1 homolog, wingless (Riggleman, B., P. Schedl, and E. Wieschaus. 1990. Cell. 63:549-560). In addition, PC12/Wnt-1 cells show elevated expression of E-cadherin and increased calcium-dependent cell-cell adhesion. These results imply evolutionary conservation of cellular responses to Wnt-1/wingless and indicate that in certain cell types Wnt-1 may act to modulate cell adhesion mechanisms.

Reversibility of the differentiated state in somatic cells

Analysis of de novo gene activation in multinucleated heterokaryons has shown that the differentiated state, although stable, is not irreversible, and can be reprogrammed in the presence of appropriate combinations of trans-acting regulatory molecules. These properties have been exploited to design strategies for identifying novel regulators of cellular differentiation.

Xwnt-11: a maternally expressed Xenopus wnt gene

We have isolated and characterized a novel Xenopus wnt gene, Xwnt-11, whose expression pattern and overexpression phenotype suggest that it may be important for dorsal-ventral axis formation. Xwnt-11 mRNA is present during oogenesis and embryonic development through swimming tadpole stages. Xwnt-11 mRNA is ubiquitous in early oocytes and is localized during mid-oogenesis. By late oocyte stages, Xwnt-11 mRNA is localized to the vegetal cortex, with some mRNA in the vegetal cytoplasm. After egg maturation, Xwnt-11 mRNA is released from the vegetal cortex and is found in the vegetal cytoplasm. This early pattern of Xwnt-11 mRNA localization is similar to another vegetally localized maternal mRNA, Vg1 (D. A. Melton (1987) Nature 328, 80–82). In the late blastula, Xwnt-11 mRNA is found at high levels in the dorsal marginal zone. As gastrulation proceeds, Xwnt-11 mRNA appears in the lateral and ventral marginal zone and, during tadpole stages, it is found in the somites and first branchial arch. Injection of Xwnt-11 mRNA into UV-ventralized embryos can substantially rescue the UV defect by inducing the formation of dorsal tissues. The rescued embryos develop somitic muscle and neural tube; however, they lack notochord and anterior head structures.

Activity of Wnt-1 as a transmembrane protein

The product of the Wnt-1 proto-oncogene is a cysteine-rich glycoprotein that plays a crucial role in the development of the vertebrate central nervous system. Wnt-1 protein is secreted but remains associated with the cell surface and extracellular matrix. The function of Wnt-1 in several different biological settings can be carried out by cells that receive the Wnt signal from adjacent cells. Ectopic expression of Wnt-1 in certain mammary gland cell lines, such as C57MG, causes morphological transformation; C57MG cells can also be transformed by a paracrine mechanism when mixed with other cell types secreting Wnt-1 protein. To ask whether Wnt-1 protein can function while bound to the cell of origin, a variety of cell types were programmed to produce chimeric proteins containing the complete sequence of mature Wnt-1 protein fused to part or all of the transmembrane protein CD4 or CD8. The chimeras were found at the cell surface of transfected cells and did not appear to be proteolytically processed. In autocrine and paracrine transformation assays with C57MG cells and in an axis induction assay in Xenopus laevis embryos, the Wnt-1/CD4 or CD8 fusions retained significant activity, as did a secreted chimera containing the CD8 extracellular domain but lacking the transmembrane domain. However, a chimera lacking a spacer between the Wnt-1 and the transmembrane domains was weakly active and only in autocrine transformation. These results show that tethering Wnt-1 to the cell surface still allows Wnt-1-mediated cell-to-cell signaling.

Pattern formation in the limbs of Drosophila: bric a brac is expressed in both a gradient and a wave-like pattern and is required for specification and proper segmentation of the tarsus

We have identified the gene bric a brac and show that it is required for pattern formation along the proximal-distal axis of the leg and antenna of Drosophila. In bric a brac mutant legs, the bristle pattern of the three central tarsal segments is transformed towards the pattern of the most proximal tarsal segment. In addition, bric a brac mutant legs and antennae have segmentation defects. bric a brac encodes a nuclear protein that shares a highly conserved domain with two transcription factors from Drosophila. bric a brac function is dosage dependent and is required in a graded manner for the specification of tarsal segments. The graded requirement for bric a brac correlates with its graded expression pattern, suggesting that the concentration of BRIC A BRAC protein specifies segment identity in the tarsus.

Wnt-5a and Wnt-7a are expressed in the developing chick limb bud in a manner suggesting roles in pattern formation along the proximodistal and dorsoventral axes

The Wnt gene family encodes a group of secreted signalling molecules that have been implicated in the regulation of cell fate and pattern formation during embryogenesis. We have examined the patterns of expression of two members of the chicken Wnt family, Wnt-5a and Wnt-7a, during development of the chick limb bud. Wnt-5a is expressed in the apical ectodermal ridge which directs outgrowth of limb mesoderm. Wnt-5a also exhibits three quantitatively distinct domains of expression along the proximodistal (PD) axis of the limb mesoderm that may correspond to the regions which will give rise to the three distinct PD segments of the limb, the autopod, zeugopod, and stylopod. In contrast, Wnt-7a expression in the limb bud is specifically limited to the dorsal ectoderm. These observations suggest possible roles for Wnt-5a and Wnt-7a in pattern formation along the PD and dorsoventral axes of the developing chick limb bud. In addition, Wnt-5a and Wnt-7a exhibit spatially discrete domains of expression in several other regions of the chick embryo consistent with developmental roles for these genes in a variety of other tissues.

Axis specification in the developing Drosophila appendage: the role of wingless, decapentaplegic, and the homeobox gene aristaless

The wingless (wg) and decapentaplegic (dpp) genes of Drosophila encode homologs of secreted growth factors and are required for the correct patterning of the appendages. We show that the presumptive tips of both the leg and wing, the distal extreme of the proximodistal axis, are characterized by the close association of cells expressing wg, dpp, and the homeobox gene aristaless (al). Ectopic expression of wg can induce both ectopic al expression and a duplication of the proximodistal axis (the development of supernumerary legs), but only in regions expressing high levels of dpp. Ectopic al expression can induce a duplication of the proximodistal axis in the wing, suggesting that it may be directly involved in axis specification. The proximodistal axis may be specified via a mechanism involving a direct interaction between cells expressing wg, dpp, and possibly al.

Mouse Wnt genes exhibit discrete domains of expression in the early embryonic CNS and limb buds

Mutation and expression studies have implicated the Wnt gene family in early developmental decision making in vertebrates and flies. In a detailed comparative analysis, we have used in situ hybridization of 8.0- to 9.5-day mouse embryos to characterize expression of all ten published Wnt genes in the central nervous system (CNS) and limb buds. Seven of the family members show restricted expression patterns in the brain. At least three genes (Wnt-3, Wnt-3a, and Wnt-7b) exhibit sharp boundaries of expression in the forebrain that may predict subdivisions of the region later in development. In the spinal cord, Wnt-1, Wnt-3, and Wnt-3a are expressed dorsally, Wnt-5a, Wnt-7a, and Wnt-7b more ventrally, and Wnt-4 both dorsally and in the floor plate. In the forelimb primordia, Wnt-3, Wnt-4, Wnt-6 and Wnt-7b are expressed fairly uniformly throughout the limb ectoderm. Wnt-5a RNA is distributed in a proximal to distal gradient through the limb mesenchyme and ectoderm. Along the limb's dorsal-ventral axis, Wnt-5a is expressed in the ventral ectoderm and Wnt-7a in the dorsal ectoderm. We discuss the significance of these patterns of restricted and partially overlapping domains of expression with respect to the putative function of Wnt signalling in early CNS and limb development.

Intrinsic activity of the Lin-12 and Notch intracellular domains in vivo

The lin-12 gene of C. elegans and the Notch gene of D. melanogaster encode structurally related transmembrane proteins that mediate intercellular signaling. We show that truncated forms of these proteins consisting of only the intracellular domains cause cell fate transformations associated with constitutive activity in their respective organisms. This activity does not depend on endogenous gene function. Our results indicate that the intracellular domains of Lin-12 and Notch have intrinsic activity and that the principal role of the extracellular domains in the intact proteins is to regulate this activity. Our results also suggest that equivalent truncated forms of lin-12/Notch family members in vertebrates, including known oncogenes, are similarly active.

Simple and efficient generation of marked clones in Drosophila

BACKGROUND

Cell lineage analysis and mosaic analysis of mutations are important techniques that are used to study the development of many organisms. Unfortunately, the methods employed for such analyses are usually inefficient, technically demanding or labor intensive. In Drosophila, the most common methodology used for the generation of mosaic animals is mitotic recombination, which is induced by X-rays. Although this technique is simple, it has the undesirable characteristics of a low efficiency and a high rate of cell death. Furthermore, although a large number of marker systems has been employed to detect mitotic recombinants, none allows easy identification of clones for all cell types.

RESULTS

A system is described here that allows a highly efficient generation of clones with the concomitant expression of an easily detectable cellular marker. This method can be applied to cell lineage and mosaic analysis in Drosophila. The site-specific yeast FLP recombinase, under the control of a heat shock-inducible promoter, efficiently catalyses mitotic recombination specifically at the site of a FLP recombination target (FRT). In this system, recombination fuses the alpha-tubulin promoter to the lacZ gene, allowing transcription of the marker. Recombinant cells and their progeny can, therefore, be detected by standard assays for beta-galactosidase. Of particular importance is the fact that only the cells of interest stain, thus allowing their simple detection in any tissue.

CONCLUSIONS

We demonstrate that, by intermolecular recombination, we can use FLIP recombinase to generate marked clones efficiently in embryonic, larval and adult tissues. This simple and efficient technique is well suited to cell-lineage analysis and can be easily extended to the generation and detection of mutant clones in mosaic animals.