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

The role of Wnt genes in vertebrate development

Over the past decade, many potential candidates for molecules involved in pattern formation in the vertebrate embryo have been identified. Manipulation of the expression of some of these factors has generated fascinating results that have allowed investigators to address their roles in embryogenesis. One such family consists of a group of putative cell signaling molecules related to the proto-oncogene Wnt-1. An accumulating body of evidence suggests that the Wnt-family plays a major role in several aspects of vertebrate development.

Relationship between Wnt-1 and En-2 expression domains during early development of normal and ectopic met-mesencephalon

Grafting a met-mesencephalic portion of neural tube from a 9.5-day mouse embryo into the prosencephalon of a 2-day chick embryo results in the induction of chick En-2 (ChickEn) expression in cells in contact with the graft (Martinez et al., 1991). In this paper we investigate the possibility of Wnt-1 being one of the factors involved in En-2 induction. Since Wnt-1 and En-2 expression patterns have been described as diverging during development of the met-mesencephalic region, we first compared Wnt-1 and En-2 expression in this domain by in situ hybridization in mouse embryos after embryonic day 8.5. A ring of Wnt-1-expressing cells is detected encircling the neural tube in the met-mesencephalic region at least until day 12.5. This ring consistently overlapped with the En-2 expression domain, and corresponds to the position of this latter gene's maximal expression. We subsequently studied ChickEn ectopic induction in chick embryos grafted with various portions of met-mesencephalon. When the graft originated from the level of the Wnt-1-positive ring, ChickEn induction was observed in 71% of embryos, and in these cases correlated with Wnt-1 expression in the grafted tissue. In contrast, this percentage dropped significantly when the graft was taken from more rostral or caudal parts of the mesencephalic vesicle. Taken together, these results are compatible with a prolonged role of Wnt-1 in the specification and/or development of the met-mesencephalic region, and show that Wnt-1 could be directly or indirectly involved in the regulation of En-2 expression around the Wnt-1-positive ring during this time. We also provide data on the position of the Wnt-1-positive ring relative to anatomical boundaries in the neural tube, which suggest a more general role for the Wnt-1 protein as a positional signal involved in organizing the met-mesencephalic domain.

Molecular organization and embryonic expression of the hedgehog gene involved in cell-cell communication in segmental patterning of Drosophila

hedgehog is a segment polarity gene necessary to maintain the proper organization of each segment of the Drosophila embryo. We have identified the physical location of a number of rearrangement breakpoints associated with hedgehog mutations. The corresponding hh RNA is expressed in a series of segmental stripes starting at cellular blastoderm in the posterior portion of each segment. This RNA is localized predominantly within nuclei until stage 10, when the localization becomes primarily cytoplasmic. Expression of hh RNA in the posterior compartment is independent of most other segment polarity genes, including en, until the late extended germ-band stage (stage 11). Sequence analysis of the hedgehog locus suggests the protein product is a transmembrane protein, which may, therefore, be directly involved in cell-cell communication.

orthodenticle activity is required for the development of medial structures in the larval and adult epidermis of Drosophila

Lethal alleles of orthodenticle (= otd) cause abnormalities in the embryonic head that reflect an early role in anterior pattern formation. In addition, otd activity is required for the development of the larval and adult epidermis. Clonal analysis of both viable and lethal alleles shows that the adult requirement for otd is restricted to medial regions of certain discs. When otd activity is reduced or removed, some medial precursor cells produce bristles and cuticle characteristic of more lateral structures. Similar medial defects are observed in the larval epidermis of embryos homozygous for lethal otd alleles. Antibodies to otd recognize a nuclear protein found at high levels in the medial region of the eye antennal discs, the leg discs, the genital discs and along the ventral midline of the ventral epidermis of the embryo. These results suggest that the otd gene product is required to specify medial cell fates in both the larval and adult epidermis.

The argos gene encodes a diffusible factor that regulates cell fate decisions in the Drosophila eye

The argos gene encodes a protein that is required for viability and that regulates the determination of cells in the Drosophila eye. A developmental analysis of argos mutant eyes indicates that the mystery cells, which are usually nonneuronal, are transformed into extra photoreceptors, and that supernumerary cone cells and pigment cells are also recruited. Clonal analysis indicates that argos acts nonautonomously and can diffuse over the range of several cell diameters. Conceptual translation of the argos gene suggests that it encodes a secreted protein.

apterous, a gene required for imaginal disc development in Drosophila encodes a member of the LIM family of developmental regulatory proteins

The apterous (ap) gene is required for the normal development of the wing and haltere imaginal discs in Drosophila melanogaster. ap encodes a new member of the LIM family of developmental regulatory genes. The deduced amino acid sequence of ap predicts a homeo domain and a cysteine/histidine-rich domain known as the LIM domain. In these domains ap is highly similar to the mec-3 and lin-11 proteins of Caenorhabditis elegans and to the vertebrate insulin enhancer-binding protein isl-1. ap is presumably required for transcriptional regulation of genes involved in wing and haltere development. The nature of the defects in homozygous null mutant flies is consistent with the pattern of ap expression in the larval imaginal discs. ap is also expressed in a complex pattern in the embryo, including portions of the peripheral nervous system (PNS) and central nervous system (CNS). A requirement for ap expression in the larval and adult CNS may be the underlying cause of the defects in hormone production and vitellogenesis described for ap mutations.

Diversification of the Wnt gene family on the ancestral lineage of vertebrates

Diversification of the Wnt genes, a family of powerful developmental regulator molecules, is inferred by molecular evolutionary analyses. Fifty-five recently determined partial sequences from a variety of vertebrates and invertebrates, together with 17 published sequences, mostly from the mouse and Drosophila melanogaster, are analyzed. Wnt-1 through -7 originated before the last common ancestor of arthropods and deuterostomes lived. Another round of gene duplication, involving Wnt-3, -5, -7, and -10, occurred after the echinoderm lineage arose, on the ancestral lineage of jawed vertebrates. Increased constraints were imposed on the Wnt genes when jawed vertebrates originated, as indicated by an overall 4-fold lower rate of amino acid replacements in jawed vertebrates compared with invertebrates and jawless vertebrates. The Wnt genes are thus inferred to have undergone a disproportionately high amount of structural and functional evolution in the relatively short time (approximately 100 million years) between the origin of the echinoderm lineage and the first diversification of jawed vertebrates. A model is presented for the relationship of functional diversification of developmental regulators and their rates of amino acid replacement.

Differential regulation of the Wnt gene family during pregnancy and lactation suggests a role in postnatal development of the mammary gland

The mouse Wnt family comprises at least 10 members sharing substantial amino acid identity with the secreted glycoprotein Wnt-1/int-1. Two of these, Wnt-1 and Wnt-3, are implicated in mouse mammary tumor virus-associated adenocarcinomas, although neither member is normally expressed in the mammary gland. These results suggest the presence of active cellular pathways which mediate the action of Wnt-1 and Wnt-3 signals. An understanding of the normal role of these signalling pathways is clearly necessary to comprehend the involvement of Wnt-1 and Wnt-3 in mammary tumorigenesis. We demonstrate here that five Wnt family members are expressed and differentially regulated in the normal mouse mammary gland. In addition, some of these genes are also expressed in both Wnt-1-responsive and nonresponsive mammary epithelial cell lines. We propose that Wnt-mediated signalling is involved in normal regulation of mammary development and that inappropriate expression of Wnt-1, Wnt-3, and possibly other family members can interfere with these signalling pathways.

The midbrain-hindbrain phenotype of Wnt-1-/Wnt-1- mice results from stepwise deletion of engrailed-expressing cells by 9.5 days postcoitum

Mice homozygous for null alleles of the putative signaling molecule Wnt-1 have a reproducible phenotype: loss of the midbrain and adjacent cerebellar component of the metencephalon. By examining embryonic expression of the mouse engrailed (En) genes, from 8.0 to 9.5 days postcoitum, we demonstrate that Wnt-1 primarily regulates midbrain development. The midbrain itself is required for normal development of the metencephalon. Thus, the observed neonatal phenotype is explained by a series of early events, within 48 hr of neural plate induction, that leads to a complete loss of En domains in the anterior central nervous system. Wnt-1 and a related gene, Wnt-3a, are coexpressed from early somite stages in dorsal aspects of the myelencephalon and spinal cord. We suggest that functional redundancy between these two genes accounts for the lack of a caudal central nervous system phenotype.

Epimorphin: a mesenchymal protein essential for epithelial morphogenesis

A novel 150 kd protein expressed on the surface of mesenchymal cells of mouse embryonic tissues was identified. A monoclonal antibody to this molecule inhibited various processes of epithelial morphogenesis, such as hair follicle growth and lung epithelial tubular formation, in organ cultures of these tissues. Sequence analysis of cDNA encoding this protein revealed that it had 289 amino acids with a hydrophobic stretch at the C-terminus. NIH 3T3 cells transfected with the cDNA of this protein expressed the exogenous 150 kd protein on their surface. When lung epithelial cells were cocultured with these transfected cells, they showed normal tubular morphogenesis, but not with untransfected NIH 3T3 cells. These results indicate that this protein, termed epimorphin, plays a central role in epithelial-mesenchymal interactions.

Differential regulation of the Wnt gene family during pregnancy and lactation suggests a role in postnatal development of the mammary gland

The mouse Wnt family comprises at least 10 members sharing substantial amino acid identity with the secreted glycoprotein Wnt-1/int-1. Two of these, Wnt-1 and Wnt-3, are implicated in mouse mammary tumor virus-associated adenocarcinomas, although neither member is normally expressed in the mammary gland. These results suggest the presence of active cellular pathways which mediate the action of Wnt-1 and Wnt-3 signals. An understanding of the normal role of these signalling pathways is clearly necessary to comprehend the involvement of Wnt-1 and Wnt-3 in mammary tumorigenesis. We demonstrate here that five Wnt family members are expressed and differentially regulated in the normal mouse mammary gland. In addition, some of these genes are also expressed in both Wnt-1-responsive and nonresponsive mammary epithelial cell lines. We propose that Wnt-mediated signalling is involved in normal regulation of mammary development and that inappropriate expression of Wnt-1, Wnt-3, and possibly other family members can interfere with these signalling pathways.

Expression of a Wnt gene in embryonic epithelium of the leech

A new member of the Wnt class of cell-cell communication molecules was identified in the leech Helobdella triserialis, on the basis of a conserved 86 amino acid coding sequence and exon structure. This gene, htr-wnt-A, is not an obvious homolog of any one of the previously described wnt class proteins. The embryonic expression of htr-wnt-A has been characterized at the cellular level, using nonradioactive in situ hybridization and polyclonal antibodies generated via a novel method of antigen presentation. Subcellular localization of the htr-wnt-A protein was examined by the use of immunofluorescence and confocal microscopy. htr-wnt-A is among the first zygotically expressed genes in Helobdella, appearing first in a single cell of the eight-cell embryo. In early development it is expressed within a stereotyped subset of micromeres and later, in a seemingly dynamic and stochastic pattern, by cells in a micromere-derived provisional embryonic epithelium. Its spatial and temporal expression pattern make it a candidate for participation in the regulation of cell fate in the O/P equivalence group.

Mutational analysis of mouse Wnt-1 identifies two temperature-sensitive alleles and attributes of Wnt-1 protein essential for transformation of a mammary cell line

The proto-oncogene Wnt-1 encodes a cysteine-rich, secretory glycoprotein implicated in virus-induced mouse mammary cancer and intercellular signaling during vertebrate neural development. To attempt to correlate structural motifs of Wnt-1 protein with its function, 12 mutations were introduced singly and in several combinations into the coding sequence of Wnt-1 cDNA by site-directed mutagenesis. Mutant alleles in a retroviral vector were tested for their ability to transform the mouse mammary epithelial cell line C57MG in two ways: by direct infection of C57MG cells and by infection of NIH3T3 cells that serve as donors of Wnt-1 protein to adjacent C57MG cells in a secretion-dependent (paracrine) assay. In addition, the synthesis and secretion of mutant proteins were monitored in multiple cell types by immunological assays. Deletion of the signal peptide demonstrated that transformation in both direct and paracrine assays depends upon entry of Wnt-1 protein into the endoplasmic reticulum. Changes in potential proteolytic processing sites (two basic dipeptides and a probable signal peptidase cleavage site) did not adversely impair biological activity or protein processing and uncovered a second site for cleavage by signal peptidase. Replacement of each of the four asparagine-linked glycosylation sites did not affect transforming activity at normal temperatures, but one glycosylation site mutant was found to be temperature-sensitive for transformation. An allele encoding a protein that lacks all four glycosylation sites was also transformation competent. In two of four cases, substitution of serine for a cysteine residue impaired transforming activity at the usual temperature, and transformation was temperature sensitive in a third case, implying that at least some of the highly conserved cysteine residues are important for Wnt-1 function.

Secretory and inductive properties of Drosophila wingless protein in Xenopus oocytes and embryos

Like its vertebrate homologues, Xenopus wnt-8 and murine wnt-1, we find that Drosophila wingless (wg) protein causes axis duplication when overexpressed in embryos of Xenopus laevis after mRNA injection. In many cases, the secondary axes contain eyes and cement glands, which reflect the induction of the most dorsoanterior mesodermal type, prechordal mesoderm. We show that the extent of axis duplication is dependent on the embryonic site of expression, with ventral expression leading to a more posterior point of axis bifurcation. The observed duplications are due to de novo generation of new axes as shown by rescue of UV-irradiated embryos. The true dorsal mesoderm-inducing properties of wg protein are indicated by its ability to generate extensive duplications after mRNA injection into D-tier cells of 32-cell embryos. As revealed by lineage mapping, the majority of these D cell progeny populate the endoderm; injections into animal blastomeres at this stage are far less effective in inducing secondary axes. However, when expressed in isolated animal cap explants, wg protein induces only ventral mesoderm, unless basic fibroblast growth factor is added, whereupon induction of muscle and occasionally notochord is seen. We conclude that in intact embryos, wg acts in concert with other factors to cause axis duplication. Immunolocalisation studies in embryos indicate that wg protein remains localised to the blastomeres synthesizing it and has a patchy, often perinuclear distribution within these cells, although some gets to the surface. In oocytes, the pool of wg protein is entirely intracellular and relatively unstable. When the polyanion suramin is added, most of the intracellular material is recovered in the external medium.

The state of engrailed expression is not clonally transmitted during early Drosophila development

In Drosophila embryos, boundaries of lineage restriction separate groups of cells, or compartments. Engrailed is essential for specification of the posterior compartment of each segment, and its expression is thought to mark this compartment. Using a new photo-activatable lineage tracer, we followed the progeny of single embryonic cells marked at the blastoderm stage. No clones straddled the anterior edges of engrailed stripes (the parasegment border). However, posterior cells of each stripe lose engrailed expression, producing mixed clones. We suggest that stable expression of engrailed by cells at the anterior edge of the stripe reflects, not cell-intrinsic mechanisms, but proximity to cells that produce Wingless, an extracellular signal needed for maintenance of engrailed expression. If control of posterior cell fate parallels control of engrailed expression, cell fate is initially responsive to cell environment and cell fate determination is a later event.

Altering the insertional specificity of a Drosophila transposable element

Vectors derived from the Drosophila P element transposon are widely used to make transgenic Drosophila. Insertion of most P-element-derived vectors is nonrandom, but they exhibit a broad specificity of target sites. During experiments to identify cis-acting regulatory elements of the Drosophila segmentation gene engrailed, we identified a fragment of engrailed DNA that, when included within a P-element vector, strikingly alters the specificity of target sites. P-element vectors that contain this fragment of engrailed regulatory DNA insert at a high frequency near genes expressed in stripes.

The mouse Wnt-1 gene can act via a paracrine mechanism in transformation of mammary epithelial cells

The mouse Wnt-1 gene plays an essential role in fetal brain development and can contribute to tumorigenesis when activated aberrantly in the mammary gland. The gene encodes secretory glycoproteins associated with the extracellular or pericellular matrix, and it has been proposed that Wnt-1, as well as its Drosophila homolog wingless, may function in intercellular signalling. We show here that fibroblasts expressing Wnt-1 protein, although not transformed themselves, are able to elicit morphological transformation of neighboring C57MG mammary epithelial cells in coculture experiments. Heparin inhibits this effect, possibly by displacing Wnt-1 protein from its normal site of action. Our results indicate that the Wnt-1 gene can act via a paracrine mechanism in cell culture and strongly support the notion that in vivo the gene may function in cell-to-cell communication.

The mouse Wnt-1 gene can act via a paracrine mechanism in transformation of mammary epithelial cells

The mouse Wnt-1 gene plays an essential role in fetal brain development and can contribute to tumorigenesis when activated aberrantly in the mammary gland. The gene encodes secretory glycoproteins associated with the extracellular or pericellular matrix, and it has been proposed that Wnt-1, as well as its Drosophila homolog wingless, may function in intercellular signalling. We show here that fibroblasts expressing Wnt-1 protein, although not transformed themselves, are able to elicit morphological transformation of neighboring C57MG mammary epithelial cells in coculture experiments. Heparin inhibits this effect, possibly by displacing Wnt-1 protein from its normal site of action. Our results indicate that the Wnt-1 gene can act via a paracrine mechanism in cell culture and strongly support the notion that in vivo the gene may function in cell-to-cell communication.

A phosphoglycerate kinase-related gene conserved between Trypanosoma brucei and Crithidia fasciculata

Trypanosoma brucei and Crithidia fasciculata both contain three different phosphoglycerate kinase (PGK) genes, A, B and C, in a tandem array. The genes B and C encode the major PGKs: the cytosolic and glycosomal PGKs, respectively. The PGK-A genes of both Trypanosomatid species encode open reading frames related to PGK, which have most active site residues conserved, but contain an insert of 80 amino acids at approximately position 80 of the 420 amino acids average PGK sequence. The deduced amino acid sequence of these inserts is conserved between T. brucei and C. fasciculata (48% positional identity), indicating its functional importance. Although we have not been able to demonstrate PGK activity in the PGK-A gene product, we consider it likely that this gene codes for a minor PGK with special function.

Swaying is a mutant allele of the proto-oncogene Wnt-1

Mice homozygous for the recessive mutation swaying (sw) are characterized by ataxia and hypertonia, attributed to the malformation of anterior regions of the cerebellum. We show that sw is a deletion of a single base pair from the proto-oncogene Wnt-1. The deletion is predicted to cause premature termination of translation, eliminating the carboxy-terminal half of the Wnt-1 protein. Histological examination shows that sw is phenotypically identical to a previously described wnt-1 mutation introduced into mice by gene targeting. Although both mutations in Wnt-1 disrupt primarily the development of the anterior cerebellum, they also exhibit a variability in expressivity such that rostrally adjacent structures in the midbrain and caudally adjacent structures in the posterior cerebellum can also be affected.

The Drosophila dorsal-ventral patterning gene tolloid is related to human bone morphogenetic protein 1

Mutations in the Drosophila tolloid (tld) gene lead to a partial transformation of dorsal ectoderm into ventral ectoderm. The null phenotype of tld is similar to, but less severe than decapentaplegic (dpp), a TGF-beta family member required for the formation of all dorsal structures. We have cloned the tld locus by P element tagging. At the blastoderm stage, tld RNA is expressed dorsally, similar to that described for dpp. Analysis of a tld cDNA reveals three sequence motifs: an N terminal region of similarity to a metalloprotease, two EGF-like repeats, and five copies of a repeat found in human complement proteins C1r and C1s. tld sequence is 41% identical to human bone morphogenetic protein 1 (BMP-1); the closest members to dpp within the TGF-beta superfamily are BMP-2 and BMP-4, two other bone morphogenetic proteins. These findings suggest that these genes are members of a signal generating pathway that has been conserved between insects and mammals.

Generating lineage-specific markers to study Drosophila development

To generate cell- and tissue-specific expression patterns of the reporter gene lacZ in Drosophila, we have generated and characterized 1,426 independent insertion strains using four different P-element constructs. These four transposons carry a lacZ gene driven either by the weak promoter of the P-element transposase gene or by partial promoters from the even-skipped, fushi-tarazu, or engrailed genes. The tissue-specific patterns of beta-galactosidase expression that we are able to generate depend on the promoter utilized. We describe in detail 13 strains that can be used to follow specific cell lineages and demonstrate their utility in analyzing the phenotypes of developmental mutants. Insertion strains generated with P-elements that carry various sequences upstream of the lacZ gene exhibit an increased variety of expression patterns that can be used to study Drosophila development.

Role of the Drosophila patched gene in positional signalling

After cellularization of the Drosophila embryo, positional differences within each primordial segment are maintained and elaborated by processes that require cell interactions. The best-documented examples of such intercellular signalling are the mutual interactions between neighbouring cells expressing the homeodomain protein engrailed and the secreted glycoprotein encoded by wingless, the Drosophila homologue of the murine Wnt-1 gene. Little is known about the molecular basis of these signalling mechanisms but the activities of several other genes, notably patched and hedgehog, have been implicated in the process. Here we show that the role of patched in positional signalling is permissive rather than instructive, its activity being required to suppress wingless transcription in cells predisposed to express the latter. According to this view, expression of wingless is normally maintained only in those cells receiving an extrinsic signal, encoded by hedgehog, that antagonizes the repressive activity of patched. We suggest that the patched protein may itself be the receptor for this signal, implying that this is an unusual mechanism of ligand-dependent receptor inactivation.

Interactions between segment polarity genes and the generation of the segmental pattern in Drosophila

Although mutations in the segment polarity genes wingless, engrailed, hedgehog, gooseberry and cubitus-interruptusD all affect the region of naked cuticle within each segment of the Drosophila larva, subtle phenotypic differences suggest that these genes play different roles in segmental patterning. In this paper, the regulative interactions between these genes are analysed. They have revealed that the products of most of these genes accomplish more than one function during embryogenesis. Whereas early on a positive feed-back loop involving wg, en and hh maintains the expression of wg and en in the extremes of each parasegment, later on wg and en become independent from each other. en appears to regulate the expression of hh and ptc, while wg depends on gsb and ciD.

Segment polarity genes and cell patterning within the Drosophila body segment

The formation of complex structures in any multicellular organism requires cooperative behaviour within cell populations. Genetic and molecular analysis of pattern formation in the Drosophila embryo is providing us with new insights into the cellular basis of this process, implicating a diversity of molecules, some familiar, others novel, in intercellular communication.

Multiple modes of engrailed regulation in the progression towards cell fate determination

The engrailed gene product of Drosophila specifies the fate of a subset of cells in each segment. Our studies of engrailed regulation suggest that fate determination is an elaborate, multistep process. At the time in embryogenesis when the engrailed-dependent cell fate is probably determined, four modes of control act in an overlapping progression to govern engrailed expression. After activation by pair-rule genes, both an extracellular signal, wingless, and autoregulation are required for engrailed expression. Autoregulation graduates to wingless independence, but is transient, and is superseded by an engrailed-independent mode of maintenance.

Secretion and movement of wingless protein in the epidermis of the Drosophila embryo

The segment polarity gene wingless encodes a cysteine rich protein which is essential for pattern formation in Drosophila. Using polyclonal antibodies against the product of the wingless gene, we demonstrate that this protein is secreted in the embryo and that it is taken up by neighbouring cells. The protein can be found two or three cell diameters away from the cells in which it is synthesized. We discuss the possible mechanisms which are responsible for this spatial distribution and its regulation during embryogenesis.

Induction of a mesencephalic phenotype in the 2-day-old chick prosencephalon is preceded by the early expression of the homeobox gene en

The homeobox gene en, homologous to the gene en-grailed of Drosophila, is expressed in the metencephalic-mesencephalic segment of the vertebrate neural tube. Using quail-chick chimeras, an antibody against en proteins, and cytoarchitectonic techniques, we demonstrate that metencephalon transplanted to prosencephalon, at E2, maintains a high level of en proteins and its presumptive cerebellar fate. The ectopic metencephalon induces in the contiguous host prosencephalon the expression of en and, subsequently, a mesencephalic phenotype. These related genetic and phenotypic expressions indicate that the transcriptional regulatory en gene is involved in cerebellar and mesencephalic cyto-differentiation. The expression of en can also be induced in chick prosencephalon by a mammalian metencephalic graft, indicating that the factors regulating the transcription of en are phylogenetically well conserved.

Expression of two members of the Wnt family during mouse development--restricted temporal and spatial patterns in the developing neural tube

The Wnt gene family encodes a group of cysteine-rich proteins implicated in intercellular signaling during several stages of vertebrate development. This family includes Wnt-1 and Wnt-3, both discovered as activated oncogenes in mouse mammary tumors. Here we describe the molecular cloning of an additional member of the Wnt family, called Wnt-3A, and the spatial and temporal expression pattern of this gene as well as that of its close relative Wnt-3. The putative amino acid sequences of both proteins are almost 90% identical, but in situ hybridization to mouse embryo sections showed highly restricted patterns of expression of Wnt-3 and Wnt-3A, largely in separate areas in the developing nervous system. In the spinal cord Wnt-3 was expressed at low levels in the alar laminae and in the ventral horns, whereas Wnt-3A expression was confined to the roof plate. In the developing brain Wnt-3 was expressed broadly across the dorsal portion of the neural tube with a rostral boundary of expression at the diencephalon. In contrast, Wnt-3A was expressed in a narrow region very close to the midline; expression extended into the bifurcating telencephalon, in a highly localized fashion. Both Wnt-3 and Wnt-3A were expressed in the ectoderm, and Wnt-3A was also expressed in the periumbilical mesenchyme. Characteristic expression patterns of these two closely related genes suggest that Wnt-3 and Wnt-3A play distinct roles in cell-cell signaling during morphogenesis of the developing neural tube.

The segment polarity gene armadillo encodes a functionally modular protein that is the Drosophila homolog of human plakoglobin

The Drosophila segment polarity gene armadillo is required for pattern formation within embryonic segments and imaginal discs. We have found that armadillo is highly conserved during evolution; it is 63% identical to human plakoglobin, a protein found in adhesive junctions joining epithelial and other cells. We have examined arm protein localization in a number of larval tissues and found that arm protein accumulation within cells shares many features with the accumulation of plakoglobin. We have compared the phenotype and molecular lesions responsible for the different arm mutations. Surprisingly, severely truncated proteins retain some function; the degree of function is strictly correlated with the length of the truncated protein, suggesting that the internally repetitive arm protein is modular in function. We present a possible model for the cellular role of arm.

Spacing differentiation in the developing Drosophila eye: a fibrinogen-related lateral inhibitor encoded by scabrous

In the development of multicellular organisms a diversity of cell types differentiate at specific positions. Spacing patterns, in which an array of two or more cell types forms from a uniform field of cells, are a common feature of development. Identical precursor cells may adopt different fates because of competition and inhibition between them. Such a pattern in the developing Drosophila eye is the evenly spaced array of R8 cells, around which other cell types are subsequently recruited. Genetic studies suggest that the scabrous mutation disrupts a signal produced by R8 cells that inhibits other cells from also becoming R8 cells. The scabrous locus was cloned, and it appears to encode a secreted protein partly related to the beta and gamma chains of fibrinogen. It is proposed that the sca locus encodes a lateral inhibitor of R8 differentiation. The roles of the Drosophila EGF-receptor homologue (DER) and Notch genes in this process were also investigated.

Spatial expression of the Drosophila segment polarity gene armadillo is posttranscriptionally regulated by wingless

armadillo (arm) is one of a group of Drosophila segment polarity genes that are required for normal patterning within the embryonic segment. Although arm RNA is uniformly distributed in embryos, arm protein accumulates at higher levels in regions that contain wingless, another segment polarity gene which encodes a secreted protein that regulates patterning via cell-cell communication. These local increases in arm protein require wingless activity, and mutations that alter wingless distribution produce corresponding changes in the arm protein pattern. These results suggest that wingless regulates accumulation of arm protein by a posttranscriptional mechanism. Two other segment polarity genes, porcupine and dishevelled, are required for this effect. We also show that arm protein is closely associated with the plasma membrane in virtually all cell types and often colocalizes with F-actin.

Serine/threonine protein kinases in Drosophila

The study of serine/threonine kinases in Drosophila is coming of age. Recently several kinases have been identified and their role in cell determination has been established. This review discusses these recent findings and describes the potential for genetic analyses of kinase activity and signal transduction.

The Wnt-1 (int-1) proto-oncogene is required for development of a large region of the mouse brain

The Wnt-1 (int-1) proto-oncogene, which encodes a putative signaling molecule, is expressed exclusively in the developing central nervous system and adult testes. To examine the role of Wnt-1, we generated six independent embryonic stem cell lines in which insertion of a neoR gene by homologous recombination inactivated a Wnt-1 allele. Germline chimeras were generated from two lines, and progeny from matings between heterozygous parents were examined. In all day 9.5 fetuses homozygous for mutated Wnt-1 alleles, most of the midbrain and some rostral metencephalon were absent. The remainder of the neural tube and all other tissues were normal. In late-gestation homozygotes, there was virtually no midbrain and no cerebellum, while the rest of the fetus was normal. Homozygotes are born, but die within 24 hr. Thus the normal role of Wnt-1 is in determination or subsequent development of a specific region of the central nervous system.