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

Temporal pattern of the posterior expression of Wingless in Drosophila blastoderm

In most animals, the antero-posterior (A-P) axis requires a gradient of Wnt signaling. Wnts are expressed posteriorly in many vertebrate and invertebrate embryos, forming a gradient of canonical Wnt/β-Catenin activity that is highest in the posterior and lowest in the anterior. One notable exception to this evolutionary conservation is in the Drosophila embryo, in which the A-P axis is established by early transcription factors of maternal origin. Despite this initial axial establishment, Drosophila still expresses Wingless (Wg), the main Drosophila Wnt homologue, in a strong posterior band early in embryogenesis. Since its discovery 30 years ago this posterior band of Wg has been largely ignored. In this study, we re-examined the onset of expression of the Wg posterior band in relation to the expression of Wg in other segments, and compared the timing of its expression to that of axial regulators such as gap and pair-rule genes. It was found that the posterior band of Wg is first detected in blastoderm at mid nuclear cycle 14, before the segment-polarity stripes of Wg are formed in other segments. The onset of the posterior band of Wg expression was preceded by that of the gap gene products Hunchback (hb) and Krüppel (Kr), and the pair-rule protein Even-skipped (Eve). Although the function of the posterior band of Wg was not analyzed in this study, we note that in temperature-sensitive Wg mutants, in which Wg is not properly secreted, the posterior band of Wg expression is diminished in strength, indicating a positive feedback loop required for Wg robust expression at the cellular blastoderm stage. We propose that this early posterior expression could play a role in the refinement of A-P patterning.

Endocytic Relay as a Potential Means for Enhancing Ligand Transport through Cellular Tissue Matrices: Analysis and Possible Implications for Drug Delivery

The transport of peptide ligands, such as cytokines, through tissue is complicated by resistances due to cell multilayers and holdup in extracellular matrix. To determine whether it is possible for receptor-mediated endocytic trafficking to enhance ligand transport, we have developed a mathematical model of ligand flux through tissue containing cells possessing complementary receptors. Tissue is considered as two phases: the cell phase and the matrix phase; thus tissue is modeled as analogous to a packed bed reactor. This model allows calculation of steady-state flux of intact and degraded peptide through a one-dimensional cell/tissue matrix. Both environmental and molecular parameters were considered in this study. Results predict that three quantities should have a major influence on growth factor flux: the ratio of matrix diffusivity to intracellular "diffusivity" (D(m)/D(i)), the extracellular matrix proteolysis rate constant (k (prot)), and the fraction of internalized growth factor degraded (f(1)). For basal levels of intracellular degradation (0 < f(1) >/= 0.05) but no extracellular proteolysis, significant enhancement is possible only for D(m)/D(i) >/=1. f(1) increases, enhancement is only possible up to f(1)= 0.07 even for D(m)/D(i) < 1. For significant levels of extracellular proteolysis (k (prot) > 0), the requirements for D(m)/D(i) and f(1) to permit transport enhancement encompass a broader range with the exact values dependent on k (prot). These insights may be helpful for delivery of ligands generated from controlled-release devices or genetically modified autocrine cells, and may also provide better understanding of cytokine transport in embryonic development.

Disrupted dorsal neural tube BMP signaling in the cilia mutant Arl13b hnn stems from abnormal Shh signaling

In the embryonic neural tube, multiple signaling pathways work in concert to create functional neuronal circuits in the adult spinal cord. In the ventral neural tube, Sonic hedgehog (Shh) acts as a graded morphogen to specify neurons necessary for movement. In the dorsal neural tube, bone morphogenetic protein (BMP) and Wnt signals cooperate to specify neurons involved in sensation. Several signaling pathways, including Shh, rely on primary cilia in vertebrates. In this study, we used a mouse mutant with abnormal cilia, Arl13b(hnn), to study the relationship between cilia, cell signaling, and neural tube patterning. Arl13b(hnn) mutants have abnormal ventral neural tube patterning due to disrupted Shh signaling; in addition, dorsal patterning defects occur, but the cause of these is unknown. Here we show that the Arl13b(hnn) dorsal patterning defects result from abnormal BMP signaling. In addition, we find that Wnt ligands are abnormally expressed in Arl13b(hnn) mutants; surprisingly, however, downstream Wnt signaling is normal. We demonstrate that Arl13b is required non-autonomously for BMP signaling and Wnt ligand expression, indicating that the abnormal Shh signaling environment in Arl13b(hnn) embryos indirectly causes dorsal defects.

Sexually dimorphic regulation of the Wingless morphogen controls sex-specific segment number in Drosophila

Sexual dimorphism is widespread throughout the metazoa and plays important roles in mate recognition and preference, sex-based niche partitioning, and sex-specific coadaptation. One notable example of sex-specific differences in insect body morphology is presented by the higher diptera, such as Drosophila, in which males develop fewer abdominal segments than females. Because diversity in segment number is a distinguishing feature of major arthropod clades, it is of fundamental interest to understand how different numbers of segments can be generated within the same species. Here we show that sex-specific and segment-specific regulation of the Wingless (Wg) morphogen underlies the development of sexually dimorphic adult segment number in Drosophila. Wg expression is repressed in the developing terminal male abdominal segment by the combination of the Hox protein Abdominal-B (Abd-B) and the sex-determination regulator Doublesex (Dsx). The subsequent loss of the terminal male abdominal segment during pupation occurs through a combination of developmental processes including segment compartmental transformation, apoptosis, and suppression of cell proliferation. Furthermore, we show that ectopic expression of Wg is sufficient to rescue this loss. We propose that dimorphic Wg regulation, in concert with monomorphic segment-specific programmed cell death, are the principal mechanisms of sculpting the sexually dimorphic abdomen of Drosophila.

Wnt signaling requires sequestration of glycogen synthase kinase 3 inside multivesicular endosomes

Canonical Wnt signaling requires inhibition of Glycogen Synthase Kinase 3 (GSK3) activity, but the molecular mechanism by which this is achieved remains unclear. Here, we report that Wnt signaling triggers the sequestration of GSK3 from the cytosol into multivesicular bodies (MVBs), so that this enzyme becomes separated from its many cytosolic substrates. Endocytosed Wnt colocalized with GSK3 in acidic vesicles positive for endosomal markers. After Wnt addition, endogenous GSK3 activity decreased in the cytosol, and GSK3 became protected from protease treatment inside membrane-bounded organelles. Cryoimmunoelectron microscopy showed that these corresponded to MVBs. Two proteins essential for MVB formation, HRS/Vps27 and Vps4, were required for Wnt signaling. The sequestration of GSK3 extended the half-life of many other proteins in addition to β-Catenin, including an artificial Wnt-regulated reporter protein containing GSK3 phosphorylation sites. We conclude that multivesicular endosomes are essential components of the Wnt signal-transduction pathway.

Wnt signalling and cancer stem cells

Intracellular signalling mediated by secreted Wnt proteins is essential for the establishment of cell fates and proper tissue patterning during embryo development and for the regulation of tissue homeostasis and stem cell function in adult tissues. Aberrant activation of Wnt signalling pathways has been directly linked to the genesis of different tumours. Here, the components and molecular mechanisms implicated in the transduction of Wnt signal, along with important results supporting a central role for this signalling pathway in stem cell function regulation and carcinogenesis will be briefl y reviewed.

Trans-synaptic transmission of vesicular Wnt signals through Evi/Wntless

Wnts play pivotal roles during development and in the mature nervous system. However, the mechanism by which Wnts traffic between cells has remained elusive. Here we demonstrate a mechanism of Wnt transmission through release of exosome-like vesicles containing the Wnt-binding protein Evenness Interrupted/Wntless/Sprinter (Evi/Wls/Srt). We show that at the Drosophila larval neuromuscular junction (NMJ), presynaptic vesicular release of Evi is required for the secretion of the Wnt, Wingless (Wg). We also show that Evi acts cell-autonomously in the postsynaptic Wnt-receiving cell to target dGRIP, a Wg-receptor-interacting protein, to postsynaptic sites. Upon Evi loss of function, dGRIP is not properly targeted to synaptic sites, interfering with postsynaptic Wnt signal transduction. These findings uncover a previously unknown cellular mechanism by which a secreted Wnt is transported across synapses by Evi-containing vesicles and reveal trafficking functions of Evi in both the Wnt-producing and the Wnt-receiving cells. For a video summary of this article, see the PaperFlick file with the Supplemental Data available online.

In vivo analysis in Drosophila reveals differential requirements of contact residues in Axin for interactions with GSK3beta or beta-catenin

Proper regulation of the Wingless/Wnt signaling pathway is essential for normal development. The scaffolding protein Axin plays a key role in this process through interactions with Drosophila Shaggy and Armadillo. In the current studies, we used a yeast two-hybrid assay to identify ten amino acids in Axin that are critical for in vitro interaction with Shaggy and two for interaction with Armadillo. We then generated five Axin variants in which individual putative contact amino acids were mutated and compared their activity, as assayed by rescue of axin null mutant flies, to that of Axin lacking the entire Shaggy (AxinDeltaSgg) or Armadillo (AxinDeltaArm) binding domain. Although we expected these mutants to function identically to Axin in which the entire binding domain was deleted, we instead observed a spectrum of phenotypic rescue. Specifically, two point mutants within the Shaggy binding domain showed loss of activity similar to that of AxinDeltaSgg and dominantly interfered with complex function, whereas a third mutant allele, AxinK446E, retained most function. Two Axin point mutants within the Armadillo binding domain were weak alleles and retained most function. These findings demonstrate the importance of in vivo verification of the role of specific amino acids within a protein.

Specification and positioning of parasegment grooves in Drosophila

Developmental boundaries ensure that cells fated to participate in a particular structure are brought together or maintained at the appropriate locale within developing embryos. Parasegment grooves mark the position of boundaries that separate every segment of the Drosophila embryo into anterior and posterior compartments. Here, we dissect the genetic hierarchy that controls the formation of this morphological landmark. We report that primary segment polarity genes (engrailed, hedgehog and wingless) are not involved in specifying the position of parasegment grooves. Wingless signalling plays only a permissive role by triggering the formation of grooves at cellular interfaces defined by the ON/OFF state of expression of the earlier acting pair-rule genes eve and ftz. We suggest that the transcription factors encoded by these genes activate two programmes in parallel: a cell fate programme mediated by segment polarity genes and a boundary/epithelial integrity programme mediated by unknown target genes.

Cellular trafficking of the glypican Dally-like is required for full-strength Hedgehog signaling and wingless transcytosis

Hedgehog (Hh) and Wingless (Wg) morphogens specify cell fate in a concentration-dependent manner in the Drosophila wing imaginal disc. Proteoglycans, components of the extracellular matrix, are involved in Hh and Wg stability, spreading, and reception. In this study, we demonstrate that the glycosyl-phosphatidyl-inositol (GPI) anchor of the glypican Dally-like (Dlp) is required for its apical internalization and its subsequent targeting to the basolateral compartment of the epithelium. Dlp endocytosis from the apical surface of Hh-receiving cells catalyzes the internalization of Hh bound to its receptor Patched (Ptc). The cointernalization of Dlp with the Hh/Ptc complex is dynamin dependent and necessary for full-strength Hh signaling. We also demonstrate that Wg is secreted apically in the disc epithelium and that apicobasal trafficking of Dlp allows Wg transcytosis to favor Wg spreading along the basolateral compartment. Thus, Dlp endocytosis is a common regulatory mechanism of both Hh and Wg morphogen action.

A combinatorial enhancer recognized by Mad, TCF and Brinker first activates then represses dpp expression in the posterior spiracles of Drosophila

A previous genetic analysis of a reporter gene carrying a 375-bp region from a dpp intron (dppMX-lacZ) revealed that the Wingless and Dpp pathways are required to activate dpp expression in posterior spiracle formation. Here we report that within the dppMX region there is an enhancer with binding sites for TCF and Mad that are essential for activating dppMX expression in posterior spiracles. There is also a binding site for Brinker likely employed to repress dppMX expression. This combinatorial enhancer may be the first identified with the ability to integrate temporally distinct positive (TCF and Mad) and negative (Brinker) inputs in the same cells. Cuticle studies on a unique dpp mutant lacking this enhancer showed that it is required for viability and that the Filzkorper are U-shaped rather than straight. Together with gene expression data from these mutants and from brk mutants, our results suggest that there are two rounds of Dpp signaling in posterior spiracle development. The first round is associated with dorsal-ventral patterning and is necessary for designating the posterior spiracle field. The second is governed by the combinatorial enhancer and begins during germ band retraction. The second round appears necessary for proper spiracle internal morphology and fusion with the remainder of the tracheal system. Intriguingly, several aspects of dpp posterior spiracle expression and function are similar to demonstrated roles for Wnt and BMP signaling in proximal-distal outgrowth of the mammalian embryonic lung.

Function of the wingless signaling pathway in Drosophila

Signaling by the wingless pathway has been shown to govern numerous developmental processes. Much of our current understanding of wingless signaling mechanisms comes from studies conducted in Drosophila melanogaster, which offers superior experimental tractability for genetic and developmental studies. Wingless signaling is highly consequential during normal development and patterning of Drosophila. Its earliest identifiable role during development of Drosophila is in the embryonic segmentation cascade, wherein wingless functions as a segment polarity gene and serves to pattern each individual segment along the antero-posterior axis of the developing embryo. Subsequent developmental roles fulfilled by wingless include patterning the developing wings, legs, eyes, CNS, heart, and muscles. Each of these developmental contexts offers excellent systems to query mechanisms regulating different aspects of wingless signal transduction such as synthesis, secretion, reception, and transcription. This chapter presents a brief overview on the functions of wingless signaling during development of Drosophila melanogaster.

Cell-autonomous, myristyl-independent activity of the Drosophila Wnt/Wingless antagonist Naked cuticle (Nkd)

Robust animal development, tissue homeostasis, and stem cell renewal requires precise control of the Wnt/beta-catenin signaling axis. In the embryo of the fruit fly Drosophila melanogaster, the naked cuticle (nkd) gene attenuates signaling by the Wnt ligand Wingless (Wg) during segmentation. nkd mutants have been reported to exhibit abnormalities in wg transcription, Wg protein distribution and/or transport, and the intracellular response to Wg, but the relationship between each alteration and the molecular mechanism of Nkd action remains unclear. In addition, whether Nkd acts in a cell-autonomous or nonautonomous fashion in the embryo is not known. Mammalian Nkd homologs have N-terminal consensus sequences that direct the post-translational addition of a lipophilic myristoyl moiety, but fly and mosquito Nkd, while sharing N-terminal sequence homology, lack a myristoylation consensus sequence. Here we provide evidence that fly Nkd acts cell-autonomously in the embryo, with its N-terminus able to confer unique functional properties and membrane association that cannot be mimicked in vivo by heterologous myristoylation consensus sequences. In conjunction with our recent observation that Nkd requires nuclear localization for function, our data suggest that Nkd acts at more than one subcellular location within signal-receiving cells to attenuate Wg signaling.

The Wnt family of cell signalling molecules in postimplantation development of the mouse

The mammalian Wnt gene family consists of at least ten members, all of which share a common structure. The N-terminus encodes a putative signal peptide sequence, suggesting that Wnt proteins are secreted. A number of absolutely conserved cysteine residues imply that inter- or intramolecular disulphide bonding is important to Wnt protein function. Wnt RNAs are localized to discrete regions of the postimplantation embryo and fetus, particularly within the developing central nervous system. Studies on Wnt gene expression strongly suggest that Wnt-mediated signalling is likely to be an important aspect of mouse development. One member of the family, Wnt-1, has been studied in some detail. By generating mutant alleles, we have demonstrated that Wnt-1 regulates regional development of the central nervous system at early somite stages. There is circumstantial evidence that some aspects of the pathway through which Wnt-1 action is mediated may be evolutionarily conserved. We propose that the Wnt family plays a major role in cell-cell interactions in the mouse.

The int genes in mouse mammary tumorigenesis and in normal development

In mice, the mouse mammary tumour virus causes tumours by insertional activation of host cell oncogenes. By the application of transposon tagging techniques, several cellular oncogenes, called int, have been discovered. The int-1 gene encodes a cysteine-rich protein with a signal peptide, suggesting that it may act as an extracellular growth or differentiation factor. Normally, the int-1 gene is expressed in early embryogenesis of the mouse, in particular in the developing nervous system. The essential role of int-1 in embryogenesis is underscored by its high degree of homology with the Drosophila segment polarity gene wingless, a gene involved in pattern formation in segments of the developing fly. In Drosophila, the int-1/wingless gene appears to encode a secreted factor, as concluded from antibody staining experiments. The int-4 gene is not yet fully characterized at the molecular level. From its expression pattern, however, we have concluded that int-4 may also act in the control of embryogenesis: the gene is expressed only during specific time intervals in mouse embryos and it is highly conserved in evolution.

Helping Wingless take flight: how WNT proteins are secreted

How functional WNT proteins are made and how their secretion is regulated is becoming a focal point for the WNT-signalling field. Recently, lipoprotein particles, WNT lipid modifications, the conserved transmembrane protein Wntless (WLS; also known as EVI and SRT) and the retromer complex have been implicated in WNT secretion. Our aim is to synthesize ideas from these new findings for the mechanisms that underlie WNT secretion.

Genetic regulation of patterned tubular branching in Drosophila

A common theme in organogenesis is the branching of epithelial tubes, for example in the lung, liver, or kidney. The later morphogenesis of these branched epithelia dictates the final form and function of the mature tissue. Epithelial branching requires the specification of branch cells, the eversion process itself, and, frequently, patterned morphogenesis to produce branches of specific shape and orientation. Using the branching of renal tubule primordia from the hindgut in Drosophila, we show that these aspects are coordinately regulated. Cell specification depends on Wnt signaling along the tubular gut and results in the spatially restricted coexpression of two transcription factors, Krüppel and Cut, in the hindgut, whose activity drives cells toward renal tubule fate. Significantly, these transcription factors also confer the competence to respond to a second signal; TGF-beta induces branching to form the four renal tubule buds. Differential activation of the TGF-beta pathway also patterns the tubules, resulting in the asymmetry in size and positioning that is characteristic of the two tubule pairs. High levels of TGF-beta promote the expression of Dorsocross1-3 and anterior tubule growth, whereas low levels allow the expression of the transcriptional repressor, Brinker, and thus promote posterior tubule identity. We show that patterning of the tubule primordium into two distinct pairs is critical for the eversion of tubule branches, as well as for their asymmetric morphogenesis.

Tracking down the "head blob": comparative analysis of wingless expression in the developing insect procephalon reveals progressive reduction of embryonic visual system patterning in higher insects

The evolution of larval head morphology in holometabolous insects is characterized by reduction of antennal appendages and the visual system components. Little insight has been gained into molecular developmental changes underlying this morphological diversification. Here we compare the expression of the segment polarity gene wingless (wg) in the pregnathal head of fruit fly, flour beetle and grasshopper embryos. We provide evidence that wg activity contributes to segment border formation, and, subsequently, the separation of the visual system and protocerebrum anlagen in the anterior procephalon. In directly developing insects like grasshopper, seven expression domains are formed during this process. The activation of four of these, which correspond to polar expression pairs in the optic lobe anlagen and the protocerebral ectoderm, has shifted to postembryonic stages in flour beetle and Drosophila. The remaining three domains map to the protocerebral neuroectoderm, and form by disintegration of a large precursor domain in flour beetle and grasshopper. In Drosophila, the precursor domain remains intact, constituting the previously described "head blob". These data document major changes in the expression of an early patterning gene correlated with the dramatic evolution of embryonic visual system development in the Holometabola.

Multiplicity of the interactions of Wnt proteins and their receptors

Wnts are secreted proteins that are essential for a wide array of developmental and physiological processes. They signal across the plasma membranes by interacting with serpentine receptors of the Frizzled (Fz) family and members of the low-density-lipoprotein receptor-related protein (LRP) family. Recent advances in the Wnt signaling field have revealed that Wnt-unrelated proteins activate or suppress Wnt signaling by binding to Fzs or LRP5/6 and that atypical receptor tyrosine kinases mediate Wnt signaling independently of Fz and/or function as a Fz co-receptor. This review highlights recent progress in our understanding of the multiplicity of Wnts and their receptors. We discuss how the interaction between the ligands and receptors activate distinct intracellular signaling pathways. We also discuss how intracellular trafficking of Wnt signaling components can regulate the sensitivity of cells to Wnts.

Wingless signaling modulates cadherin-mediated cell adhesion in Drosophila imaginal disc cells

Armadillo, the Drosophila homolog of beta-catenin, plays a crucial role in both the Wingless signal transduction pathway and cadherin-mediated cell-cell adhesion, raising the possibility that Wg signaling affects cell adhesion. Here, we use a tissue culture system that allows conditional activation of the Wingless signaling pathway and modulation of E-cadherin expression levels. We show that activation of the Wingless signaling pathway leads to the accumulation of hypophosphorylated Armadillo in the cytoplasm and in cellular processes, and to a concomitant reduction of membrane-associated Armadillo. Activation of the Wingless pathway causes a loss of E-cadherin from the cell surface, reduced cell adhesion and increased spreading of the cells on the substratum. After the initial loss of E-cadherin from the cell surface, E-cadherin gene expression is increased by Wingless. We suggest that Wingless signaling causes changes in Armadillo levels and subcellular localization that result in a transient reduction of cadherin-mediated cell adhesion, thus facilitating cell shape changes, division and movement of cells in epithelial tissues.

Secretion of Wnt Ligands Requires Evi, a Conserved Transmembrane Protein

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

Endocytic trafficking of Wingless and its receptors, Arrow and DFrizzled-2, in the Drosophila wing

During animal development, Wnt/Wingless (Wg) signaling is required for the patterning of multiple tissues. While insufficient signal transduction is detrimental to normal development, ectopic activation of the pathway can be just as devastating. Thus, numerous controls exist to precisely regulate Wg signaling levels. Endocytic trafficking of pathway components has recently been proposed as one such control mechanism. Here, we characterize the vesicular trafficking of Wg and its receptors, Arrow and DFrizzled-2 (DFz2), and investigate whether trafficking is important to regulate Wg signaling during dorsoventral patterning of the larval wing. We demonstrate a role for Arrow and DFz2 in Wg internalization. Subsequently, Wg, Arrow and DFz2 are trafficked through the endocytic pathway to the lysosome, where they are degraded in a hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs)-dependent manner. Surprisingly, we find that Wg signaling is not attenuated by lysosomal targeting in the wing disc. Rather, we suggest that signaling is dampened intracellularly at an earlier trafficking step. This is in contrast to patterning of the embryonic epidermis, where lysosomal targeting is required to restrict the range of Wg signaling. Thus, signal modulation by endocytic routing will depend on the tissue to be patterned and the goals during that patterning event.

The first deltex null mutant indicates tissue-specific deltex-dependent Notch signaling in Drosophila

Notch (N) is a single-pass transmembrane receptor. The N signaling pathway is an evolutionarily conserved mechanism that controls various cell-specification processes. Drosophila Deltex (Dx), a RING-domain E3 ubiquitin ligase, binds to the N intracellular domain, promotes N's endocytic trafficking to late endosomes, and was proposed to activate Suppressor of Hairless [Su(H)]-independent N signaling. However, it has been difficult to evaluate the importance of dx, because no null mutant of a dx family gene has been available in any organism. Here, we report the first null mutant allele of Drosophila dx. We found that dx was involved only in the subsets of N signaling, but was not essential for it in any developmental context. A strong genetic interaction between dx and Su(H) suggested that dx might function in Su(H)-dependent N signaling. Our epistatic analyses suggested that dx functions downstream of the ligands and upstream of activated Su(H). We also uncovered a novel dx activity that suppressed N signaling downstream of N.

Regulation of the Drosophila transcription factor, Cubitus interruptus, by two conserved domains

Hedgehog signaling is required for the development of many organisms, including Drosophila. In flies, Hh patterns the embryonic epidermis and larval imaginal discs by regulating the transcription factor, Cubitus interruptus (Ci). To date, three levels of regulation have been identified: proteolytic processing into a repressor, nuclear import, and activation. In this report, we characterize the function of two Ci domains that are conserved in the vertebrate homologues, GLI1, GLI2, and GLI3. One domain includes the first two of five C(2)-H(2) zinc-fingers. While conserved in all members of the GLI/Ci family, the first two fingers do not appear to make significant contacts with the DNA target sequence. Ci protein lacking this region is still able to interact with the cytoplasmic complex and activate transcription in embryos and wing imaginal discs, but it is no longer processed into the repressor form. The second domain, termed NR for "N-terminal Regulatory", binds Suppressor of Fused. Deletion of this region has little effect on embryonic patterning, but compromises cytoplasmic retention of Ci. Analysis of the amino acid sequence of this domain identifies 11 perfectly conserved serines and one tyrosine. We propose that this region may be modified, possibly by phosphorylation, to regulate Ci nuclear import.

Delivery of wingless to the ventral mesoderm by the developing central nervous system ensures proper patterning of individual slouch-positive muscle progenitors

During the development of any organism, care must be given to properly pattern gene expression in temporally and spatially regulated manners. This process becomes more complex when the signals that regulate a target tissue are produced in an adjacent tissue and must travel to the target tissue to affect gene expression. We have used the developing somatic mesoderm in Drosophila as a system in which to examine this problem. Our investigation uncovered a novel mechanism by which Wingless (Wg) can travel from its source in the ectoderm to regulate the expression of the somatic muscle founder identity gene, slouch, in the ventral mesoderm. Delivery of Wg to the mesoderm by the developing Central Nervous System (CNS) exploits the stereotypic formation of this tissue to provide high Wg levels to Slouch founder cell cluster II in a temporally specific manner. Coordinated development of these tissues provides a reliable mechanism for delivering high Wg levels to a subset of mesodermal cells. It also provides a means for one signaling pathway to be used reiteratively throughout development to impart unique positional and character information within a target field.

Activation of Rac1 or Cdc42 during early morphogenesis of eye discs induces ectopic antennae in Drosophila

The Rho family small guanosine triphosphatases (GTPases) play important roles in many cellular processes, especially in regulation of cytoskeletal organization. In this study, I examined the functions of Rac1 and Cdc42 for disc morphogenesis in Drosophila. I expressed either a constitutively active form or a dominant negative form of each protein during early morphogenesis of eye discs. Inactivation of Rac1 or Cdc42 resulted in small eye phenotypes. On the other hand, I found that activation of either Rac1 or Cdc42 induces ectopic antennae. In some cases, an almost complete antenna was observed instead of an eye, which was possibly transformation from an eye to an antenna. As a molecular evidence for the ectopic antennae, I found that the Distal-less protein, which is essential for the distalization process, was ectopically induced in the eye discs. I also found that the Decapentaplegic and Wingless proteins, which are upstream morphogenetic signaling proteins of the distalization process, could be ectopically induced by activation of Rac1 or Cdc42. My observations suggest novel functions of Rac1 and Cdc42 for disc morphogenesis.

A screen for genes regulating the wingless gradient in Drosophila embryos

During the development of the Drosophila embryonic epidermis, the secreted Wingless protein initially spreads symmetrically from its source. At later stages, Wingless becomes asymmetrically distributed in a Hedgehog-dependent manner, to control the patterning of the embryonic epidermis. When Wingless is misexpressed in engrailed cells in hedgehog heterozygous mutant embryos, larvae show a dominant phenotype consisting of patches of naked cuticle in denticle belts. This dose-sensitive phenotype is a direct consequence of a change in Wg protein distribution. We used this phenotype to carry out a screen for identifying genes regulating Wingless distribution or transport in the embryonic epidermis. Using a third chromosome deficiency collection, we found several genomic regions that showed a dominant interaction. After using a secondary screen to test for mutants and smaller deficiencies, we identified three interacting genes: dally, notum, and brahma. We confirmed that dally, as well as its homolog dally-like, and notum affect Wingless distribution in the embryonic epidermis, directly or indirectly. Thus, our assay can be used effectively to screen for genes regulating Wingless distribution or transport.

A homolog of the hydrolase Notum is expressed during segmentation and appendage formation in the Central American hunting spider Cupiennius salei

The hydrolase Notum (also known as Wingful) has been implicated in shaping the distribution gradient of the morphogen Wingless in Drosophila by modifying the Wingless-binding heparan sulfate proteoglycan (HSPG) core glypicans Dally and Dally-like. Here we report on the expression of a Notum homolog during the embryonic development of the spider Cupiennius salei. Notum is expressed in two to three stripes in the posterior region of the germband where new segments are formed. At this location no Wingless expression is present, suggesting that Notum may be involved in the regulation of another HSPG-binding morphogen, possibly Hedgehog. In older segments, however, expression of Notum and Wingless roughly coincides. In the appendages, Notum is expressed in ventral mesodermal cells, directly adjacent to the Wingless expressing ectodermal cells. This could indicate a role for the mesoderm in regulating morphogen gradient formation in the ectoderm.

Rho1 regulates signaling events required for proper Drosophila embryonic development

The Rho small GTPase has been implicated in many cellular processes, including actin cytoskeletal regulation and transcriptional activation. The molecular mechanisms underlying Rho function in many of these processes are not yet clear. Here we report that in Drosophila, reduction of maternal Rho1 compromises signaling pathways consistent with defects in membrane trafficking events. These mutants fail to maintain expression of the segment polarity genes engrailed (en), wingless (wg), and hedgehog (hh), contributing to a segmentation phenotype. Formation of the Wg protein gradient involves the internalization of Wg into vesicles. The number of these Wg-containing vesicles is reduced in maternal Rho1 mutants, suggesting a defect in endocytosis. Consistent with this, stripes of cytoplasmic beta-catenin that accumulate in response to Wg signaling are narrower in these mutants relative to wild type. Additionally, the amount of extracellular Wg protein is reduced in maternal Rho1 mutants, indicating a defect in secretion. Signaling pathways downregulated by endocytosis, such as the epidermal growth factor receptor (EGFR) and Torso pathways, are hyperactivated in maternal Rho1 mutants, consistent with a general role for Rho1 in regulating signaling events governing proper patterning during Drosophila development.

Sustained Wnt protein expression in chondral constructs from mesenchymal stem cells

Wnt genes encode a number of secreted glycoproteins which are closely associated with the cell surface and the extracellular matrix. Recently, members of Wnt family have been implicated in regulating chondrocyte differentiation, but their roles in the chondrogenic process are not fully understood. To contribute to an understanding of the roles of Wnts during chondrogenesis, we have analysed the spatiotemporal expression patterns of Wnt using in vitro models for chondrogenesis of human bone marrow-derived mesenchymal stem cells (hMSCs). In chondrogenic aggregate culture system, RT-PCR analysis revealed expression of Wnt5a and Wnt4 during late chondrogenesis (days 10 and 15). Immunohistochemical analysis showed widespread distribution of Wnt5a and Wnt4 throughout the aggregates at this late phase of culture (days 14 and 21). In addition, in this aggregate culture system, immunohistochemical staining of Wnt4 and Wnt5a showed similar spatiotemporal expression patterns to that of type II collagen or type X collagen. To confirm the results obtained by immunostaining, the specificity of the anti-Wnt4 or anti-Wnt5a antibody was assessed by Western blot analysis. Of Wnt4 and Wnt5a, only Wnt5a was immunodetectable by Western blot analysis. Western blot analysis showed that Wnt5a was expressed as two different molecular weight forms of 40 and 44 kDa. Treatment with PNGase F, which removes N-linked oligosaccharides, revealed that the mass difference between these two forms could be accounted for by the N-glycosylation status of the protein. When hMSCs were seeded on a porous gelatin sponge, immunolocalization studies showed that type II collagen and type X collagen were detected particularly at the periphery at day 7 of culture. In contrast, Wnt4 and Wnt5a showed even distribution throughout the hMSC/gelatin sponge constructs. Their different spatial expression patterns suggest that Wnt4 and Wnt5a proteins are not functionally linked to type II collagen and type X collagen synthesis in in vitro chondrogenic models of hMSCs.

DrosophilaDeltex mediates Suppressor of Hairless-independent and late-endosomal activation of Notch signaling

Notch (N) signaling is an evolutionarily conserved mechanism that regulates many cell-fate decisions. deltex (dx) encodes an E3-ubiquitin ligase that binds to the intracellular domain of N and positively regulates N signaling. However, the precise mechanism of Dx action is unknown. Here, we found that Dx was required and sufficient to activate the expression of gene targets of the canonical Su(H)-dependent N signaling pathway. Although Dx required N and a cis-acting element that overlaps with the Su(H)-binding site, Dx activated a target enhancer of N signaling, the dorsoventral compartment boundary enhancer of vestigial (vgBE), in a manner that was independent of the Delta (Dl)/Serrate (Ser) ligands- or Su(H). Dx caused N to be moved from the apical cell surface into the late-endosome, where it accumulated stably and co-localized with Dx. Consistent with this, the dx gene was required for the presence of N in the endocytic vesicles. Finally, blocking the N transportation from the plasma membrane to the late-endosome by a dominant-negative form of Rab5 inhibited the Dx-mediated activation of N signaling, suggesting that the accumulation of N in the late-endosome was required for the Dx-mediated Su(H)-independent N signaling.

Drosophila wnt-1 undergoes a hydrophobic modification and is targeted to lipid rafts, a process that requires porcupine

Wnt signaling pathways regulate many developmental responses; however, little is known about how Wnt ligands function on a biochemical level. Recent studies have shown that Wnt-3a is palmitoylated before secretion. Here we report that Drosophila Wnt-1 (Wingless) also undergoes a lipid modification. Lipidation occurs in the endoplasmic reticulum and is dependent on Porcupine, a putative O-acyltransferase. After modification, DWnt-1 partitions as a membrane-anchored protein and is sorted into lipid raft detergent-insoluble microdomains. Lipidation, raft targeting, and secretion can be blocked by the addition of 2-bromopalmitate, a competitive inhibitor of O-acyltransferase activity. Based on these results we propose a model whereby lipidation targets Wnt-1 to secretory vesicles that deliver the ligand to specialized microdomains at the cell surface where it can be packaged for secretion.

Abrogation of heparan sulfate synthesis in Drosophila disrupts the Wingless, Hedgehog and Decapentaplegic signaling pathways

Studies in Drosophila and vertebrate systems have demonstrated that heparan sulfate proteoglycans (HSPGs) play crucial roles in modulating growth factor signaling. We have isolated mutations in sister of tout velu (sotv), a gene that encodes a co-polymerase that synthesizes HSPG glycosaminoglycan (GAG) chains. Our phenotypic and biochemical analyses reveal that HS levels are dramatically reduced in the absence of Sotv or its partner co-polymerase Tout velu (Ttv), suggesting that both copolymerases are essential for GAG synthesis. Furthermore, we find that mutations in sotv and ttv impair Hh, Wg and Decapentaplegic(Dpp) signaling. This contrasts with previous studies that suggested loss of ttv compromises only Hh signaling. Our results may contribute to understanding the biological basis of hereditary multiple exostoses (HME), a disease associated with bone overgrowth that results from mutations in EXT1 and EXT2, the human orthologs of ttv and sotv.

The ins and outs of Wingless signaling

Signaling through the highly conserved Wingless/Wnt pathway plays a crucial role in a diverse array of developmental processes, many of which depend upon the precise regulation of Wingless/Wnt signaling levels. Recent evidence has indicated that the intracellular trafficking of Wingless/Wnt signaling components can result in significant changes in the level of signaling. Here, we examine three mechanisms through which intracellular trafficking might regulate Wingless signaling--the degradation of Wingless, its transport and the transduction of its signal. The intracellular trafficking of several Wingless/Wnt signaling components, including LRP5, LRP6, Dishevelled and Axin, as well as the functional implications of protein localization on Wingless/Wnt signaling, will be discussed.

Gene expression in spider appendages reveals reversal of exd/hth spatial specificity, altered leg gap gene dynamics, and suggests divergent distal morphogen signaling

Leg development in Drosophila has been studied in much detail. However, Drosophila limbs form in the larva as imaginal discs and not during embryogenesis as in most other arthropods. Here, we analyze appendage genes in the spider Cupiennius salei and the beetle Tribolium castaneum. Differences in decapentaplegic (dpp) expression suggest a different mode of distal morphogen signaling suitable for the specific geometry of growing limb buds. Also, expression of the proximal genes homothorax (hth) and extradenticle (exd) is significantly altered: in the spider, exd is restricted to the proximal leg and hth expression extends distally, while in insects, exd is expressed in the entire leg and hth is restricted to proximal parts. This reversal of spatial specificity demonstrates an evolutionary shift, which is nevertheless compatible with a conserved role of this gene pair as instructor of proximal fate. Different expression dynamics of dachshund and Distal-less point to modifications in the regulation of the leg gap gene system. We comment on the significance of this finding for attempts to homologize leg segments in different arthropod classes. Comparison of the expression profiles of H15 and optomotor-blind to the Drosophila patterns suggests modifications also in the dorsal-ventral patterning system of the legs. Together, our results suggest alterations in many components of the leg developmental system, namely proximal-distal and dorsal-ventral patterning, and leg segmentation. Thus, the leg developmental system exhibits a propensity to evolutionary change, which probably forms the basis for the impressive diversity of arthropod leg morphologies.

Endocytic trafficking during Drosophila development

During the last decade, many of the factors and mechanisms controlling membrane and protein trafficking in general and endocytic trafficking in particular have been uncovered. We have a detailed understanding of the different endocytic trafficking steps: plasma membrane budding, endocytic vesicle motility and fusion with the endosome, recycling, transcytosis and lysosomal degradation. The kinetics and trafficking pathway of many signaling receptors and the relevance of endocytic trafficking during signaling in many mammalian cultured cells are also well understood. However, only in recent years has the role of endocytic trafficking during cell-to-cell communication during development, i.e. during patterning, induction and lateral inhibition, begun to be explored. The contribution of Drosophila developmental genetics and cell biology has been fundamental in elucidating the essential role of endocytosis during these processes. Reviewed here are some of the recent developments on the role of endocytic trafficking during long- and short-range signaling and during lateral inhibition.

Smoothened translates Hedgehog levels into distinct responses

In the Drosophila wing, Hedgehog is made by cells of the posterior compartment and acts as a morphogen to pattern cells of the anterior compartment. High Hedgehog levels instruct L3/4 intervein fate, whereas lower levels instruct L3 vein fate. Transcriptional responses to Hedgehog are mediated by the balance between repressor and activator forms of Cubitus interruptus, CiR and CiA. Hedgehog regulates this balance through its receptor, Patched, which acts through Smoothened and thence a regulatory complex that includes Fused, Costal, Suppressor of Fused and Cubitus interruptus. It is not known how the Hedgehog signal is relayed from Smoothened to the regulatory complex nor how responses to different levels of Hedgehog are implemented. We have used chimeric and deleted forms of Smoothened to explore the signaling functions of Smoothened. A Frizzled/Smoothened chimera containing the Smo cytoplasmic tail (FFS) can induce the full spectrum of Hedgehog responses but is regulated by Wingless rather than Hedgehog. Smoothened whose cytoplasmic tail is replaced with that of Frizzled (SSF) mimics fused mutants, interfering with high Hedgehog responses but with no effect on low Hedgehog responses. The cytoplasmic tail of Smoothened with no transmembrane or extracellular domains (SmoC) interferes with high Hedgehog responses and allows endogenous Smoothened to constitutively initiate low responses. SmoC mimics costal mutants. Genetic interactions suggest that SSF interferes with high signaling by titrating out Smoothened, whereas SmoC drives constitutive low signaling by titrating out Costal. These data suggest that low and high signaling (1) are qualitatively different, (2) are mediated by distinct configurations of the regulatory complex and (3) are initiated by distinct activities of Smoothened. We present a model where low signaling is initiated when a Costal inhibitory site on the Smoothened cytoplasmic tail shifts the regulatory complex to its low state. High signaling is initiated when cooperating Smoothened cytoplasmic tails activate Costal and Fused, driving the regulatory complex to its high state. Thus, two activities of Smoothened translate different levels of Hedgehog into distinct intracellular responses.

Segment polarity and DV patterning gene expression reveals segmental organization of the Drosophila brain

The insect brain is traditionally subdivided into the trito-, deuto- and protocerebrum. However, both the neuromeric status and the course of the borders between these regions are unclear. The Drosophila embryonic brain develops from the procephalic neurogenic region of the ectoderm, which gives rise to a bilaterally symmetrical array of about 100 neuronal precursor cells, called neuroblasts. Based on a detailed description of the spatiotemporal development of the entire population of embryonic brain neuroblasts, we carried out a comprehensive analysis of the expression of segment polarity genes (engrailed, wingless, hedgehog, gooseberry distal, mirror) and DV patterning genes (muscle segment homeobox, intermediate neuroblast defective, ventral nervous system defective) in the procephalic neuroectoderm and the neuroblast layer (until stage 11, when all neuroblasts are formed). The data provide new insight into the segmental organization of the procephalic neuroectodem and evolving brain. The expression patterns allow the drawing of clear demarcations between trito-, deuto- and protocerebrum at the level of identified neuroblasts. Furthermore, we provide evidence indicating that the protocerebrum (most anterior part of the brain) is composed of two neuromeres that belong to the ocular and labral segment, respectively. These protocerebral neuromeres are much more derived compared with the trito- and deutocerebrum. The labral neuromere is confined to the posterior segmental compartment. Finally, similarities in the expression of DV patterning genes between the Drosophila and vertebrate brains are discussed.

[Morphogens and endocytosis]

During development, secreted signaling proteins of the Wingless/Wnt, Hedgehog and Decapentaplegic (Dpp)/Bone Morphogenic Protein (BMP) families act as morphogens. Previous work had shown that these molecules act directly on distant cells, although until recently nothing was known about how they reach those distant cells. During the past two years, work carried out on Drosophila using genetic and cell biology approaches have revealed that endocytosis plays a central part in the mechanisms that control the spread of morphogens.

Ligand receptor interactions in the Wnt signaling pathway in Drosophila

Secreted Wnt proteins have numerous signaling functions during development, mediated by Frizzled molecules that act as Wnt receptors on the cell surface. In the genome of Drosophila, seven Wnt genes (including wingless; wg), and five frizzled genes have been identified. Relatively little is known about signaling and binding specificities of different Wnt and Frizzled proteins. We have developed an assay to determine the strength of binding between membrane-tethered Wnts and ligand binding domains of the Frizzled receptors. We found a wide spectrum of binding affinities, reflecting known genetic interactions. Most Wnt proteins can bind to multiple Frizzleds and vice versa, suggesting redundancy in vivo. In an extension of these experiments, we tested whether two different subdomains of the Wg protein would by themselves bind to Frizzled and generate a biological response. Whereas these two separate domains are secreted from cells, suggesting that they form independently folded parts of the protein, they were only able to evoke a response when co-transfected, indicating that both are required for function. In addition to the Frizzleds, members of the LRP family (represented by the arrow gene in Drosophila) are also necessary for Wnt signal transduction and have been postulated to act as co-receptors. We have therefore examined whether a soluble form of the Arrow molecule can bind to Wingless and Frizzled, but no interactions were detected.

Design and constraints of the Drosophila segment polarity module: robust spatial patterning emerges from intertwined cell state switches

The Drosophila segment polarity genes constitute the last tier in the segmentation cascade; their job is to maintain the boundaries between parasegments and provide positional "read-outs" within each parasegment for the entire developmental history of the animal. These genes constitute a relatively well-defined network with a relatively well-understood patterning task. In a previous publication (von Dassow et al. 2000. Nature 406:188-192) we showed that a computer model predicts the segment polarity network to be a robust boundary-making device. Here we elaborate those findings. First, we explore the constraints among parameters that govern the network model. Second, we test architectural variants of the core network, and show that the network tolerates a wide variety of adjustments in design. Third, we evaluate several topologically identical models that incorporate more or less molecular detail, finding that more-complex models perform noticeably better than simplified ones. Fourth, we discuss two instances in which the failure of the network model to behave in a life-like fashion highlights mechanistic details that need further experimental investigation. We conclude with an explanation of how the segment polarity network can be understood as an interwoven conspiracy of simple dynamical elements, several bistable switches and a homeostat. The robustness with which the network as a whole maintains a spatial regime of stable cell state emerges from generic dynamical properties of these simple elements.