Wingless signaling generates pattern through two distinct mechanisms

The Wnt Co-Receptor PTK7/Otk and Its Homolog Otk-2 in Neurogenesis and Patterning

Wnt signaling is a highly conserved metazoan pathway that plays a crucial role in cell fate determination and morphogenesis during development. Wnt ligands can induce disparate cellular responses. The exact mechanism behind these different outcomes is not fully understood but may be due to interactions with different receptors on the cell membrane. PTK7/Otk is a transmembrane receptor that is implicated in various developmental and physiological processes including cell polarity, cell migration, and invasion. Here, we examine two roles of Otk-1 and Otk-2 in patterning and neurogenesis. We find that Otk-1 is a positive regulator of signaling and Otk-2 functions as its inhibitor. We propose that PTK7/Otk functions in signaling, cell migration, and polarity contributing to the diversity of cellular responses seen in Wnt-mediated processes.

Wnt Signaling Rescues Amyloid Beta-Induced Gut Stem Cell Loss

Patients with Alzheimer’s disease suffer from a decrease in brain mass and a prevalence of amyloid-β plaques. These plaques are thought to play a role in disease progression, but their exact role is not entirely established. We developed an optogenetic model to induce amyloid-β intracellular oligomerization to model distinct disease etiologies. Here, we examine the effect of Wnt signaling on amyloid in an optogenetic, Drosophila gut stem cell model. We observe that Wnt activation rescues the detrimental effects of amyloid expression and oligomerization. We analyze the gene expression changes downstream of Wnt that contribute to this rescue and find changes in aging related genes, protein misfolding, metabolism, and inflammation. We propose that Wnt expression reduces inflammation through repression of Toll activating factors. We confirm that chronic Toll activation reduces lifespan, but a decrease in the upstream activator Persephone extends it. We propose that the protective effect observed for lithium treatment functions, at least in part, through Wnt activation and the inhibition of inflammation.

Destruction complex dynamics: Wnt/β-catenin signaling alters Axin-GSK3β interactions in vivo

The central regulator of the Wnt/β-catenin pathway is the Axin/APC/GSK3β destruction complex (DC), which, under unstimulated conditions, targets cytoplasmic β-catenin for degradation. How Wnt activation inhibits the DC to permit β-catenin-dependent signaling remains controversial, in part because the DC and its regulation have never been observed in vivo Using bimolecular fluorescence complementation (BiFC) methods, we have now analyzed the activity of the DC under near-physiological conditions in Drosophila By focusing on well-established patterns of Wnt/Wg signaling in the developing Drosophila wing, we have defined the sequence of events by which activated Wnt receptors induce a conformational change within the DC, resulting in modified Axin-GSK3β interactions that prevent β-catenin degradation. Surprisingly, the nucleus is surrounded by active DCs, which principally control the degradation of β-catenin and thereby nuclear access. These DCs are inactivated and removed upon Wnt signal transduction. These results suggest a novel mechanistic model for dynamic Wnt signal transduction in vivo.

Wingless Signaling: A Genetic Journey from Morphogenesis to Metastasis

This FlyBook chapter summarizes the history and the current state of our understanding of the Wingless signaling pathway. Wingless, the fly homolog of the mammalian Wnt oncoproteins, plays a central role in pattern generation during development. Much of what we know about the pathway was learned from genetic and molecular experiments in Drosophila melanogaster, and the core pathway works the same way in vertebrates. Like most growth factor pathways, extracellular Wingless/Wnt binds to a cell surface complex to transduce signal across the plasma membrane, triggering a series of intracellular events that lead to transcriptional changes in the nucleus. Unlike most growth factor pathways, the intracellular events regulate the protein stability of a key effector molecule, in this case Armadillo/β-catenin. A number of mysteries remain about how the "destruction complex" destabilizes β-catenin and how this process is inactivated by the ligand-bound receptor complex, so this review of the field can only serve as a snapshot of the work in progress.

Supramolecular assembly of the beta-catenin destruction complex and the effect of Wnt signaling on its localization, molecular size, and activity in vivo

Wnt signaling provides a paradigm for cell-cell signals that regulate embryonic development and stem cell homeostasis and are inappropriately activated in cancers. The tumor suppressors APC and Axin form the core of the multiprotein destruction complex, which targets the Wnt-effector beta-catenin for phosphorylation, ubiquitination and destruction. Based on earlier work, we hypothesize that the destruction complex is a supramolecular entity that self-assembles by Axin and APC polymerization, and that regulating assembly and stability of the destruction complex underlie its function. We tested this hypothesis in Drosophila embryos, a premier model of Wnt signaling. Combining biochemistry, genetic tools to manipulate Axin and APC2 levels, advanced imaging and molecule counting, we defined destruction complex assembly, stoichiometry, and localization in vivo, and its downregulation in response to Wnt signaling. Our findings challenge and revise current models of destruction complex function. Endogenous Axin and APC2 proteins and their antagonist Dishevelled accumulate at roughly similar levels, suggesting competition for binding may be critical. By expressing Axin:GFP at near endogenous levels we found that in the absence of Wnt signals, Axin and APC2 co-assemble into large cytoplasmic complexes containing tens to hundreds of Axin proteins. Wnt signals trigger recruitment of these to the membrane, while cytoplasmic Axin levels increase, suggesting altered assembly/disassembly. Glycogen synthase kinase3 regulates destruction complex recruitment to the membrane and release of Armadillo/beta-catenin from the destruction complex. Manipulating Axin or APC2 levels had no effect on destruction complex activity when Wnt signals were absent, but, surprisingly, had opposite effects on the destruction complex when Wnt signals were present. Elevating Axin made the complex more resistant to inactivation, while elevating APC2 levels enhanced inactivation. Our data suggest both absolute levels and the ratio of these two core components affect destruction complex function, supporting models in which competition among Axin partners determines destruction complex activity.

SoxNeuro and Shavenbaby act cooperatively to shape denticles in the embryonic epidermis of Drosophila

During development, extracellular signals are integrated by cells to induce the transcriptional circuitry that controls morphogenesis. In the fly epidermis, Wingless (Wg)/Wnt signaling directs cells to produce either a distinctly shaped denticle or no denticle, resulting in a segmental pattern of denticle belts separated by smooth, or 'naked', cuticle. Naked cuticle results from Wg repression of shavenbaby (svb), which encodes a transcription factor required for denticle construction. We have discovered that although the svb promoter responds differentially to altered Wg levels, Svb alone cannot produce the morphological diversity of denticles found in wild-type belts. Instead, a second Wg-responsive transcription factor, SoxNeuro (SoxN), cooperates with Svb to shape the denticles. Co-expressing ectopic SoxN with svb rescued diverse denticle morphologies. Conversely, removing SoxN activity eliminated the residual denticles found in svb mutant embryos. Furthermore, several known Svb target genes are also activated by SoxN, and we have discovered two novel target genes of SoxN that are expressed in denticle-producing cells and that are regulated independently of Svb. We conclude that proper denticle morphogenesis requires transcriptional regulation by both SoxN and Svb.

Wingless/Wnt signaling in Drosophila: the pattern and the pathway

Wnt signaling generates pattern in all animal embryos, from flies and worms to humans, and promotes the undifferentiated, proliferative state critical for stem cells in adult tissues. Inappropriate Wnt pathway activation is the major cause of colorectal cancers, a leading cause of cancer death in humans. Although this pathway has been studied extensively for years, large gaps remain in our understanding of how it switches on and off, and how its activation changes cellular behaviors. Much of what is known about the pathway comes from genetic studies in Drosophila, where a single Wnt molecule, encoded by wingless (wg), directs an array of cell-fate decisions similar to those made by the combined activities of all 19 Wnt family members in vertebrates. Although Wg specifies fate in many tissues, including the brain, limbs, and major organs, the fly embryonic epidermis has proven to be a very powerful system for dissecting pathway activity. It is a simple, accessible tissue, with a pattern that is highly sensitive to small changes in Wg pathway activity. This review discusses what we have learned about Wnt signaling from studying mutations that disrupt epidermal pattern in the fly embryo, highlights recent advances and controversies in the field, and sets these issues in the context of questions that remain about how this essential signaling pathway functions.

crinkled reveals a new role for Wingless signaling in Drosophila denticle formation

The specification of the body plan in vertebrates and invertebrates is controlled by a variety of cell signaling pathways, but how signaling output is translated into morphogenesis is an ongoing question. Here, we describe genetic interactions between the Wingless (Wg) signaling pathway and a nonmuscle myosin heavy chain, encoded by the crinkled (ck) locus in Drosophila. In a screen for mutations that modify wg loss-of-function phenotypes, we isolated multiple independent alleles of ck. These ck mutations dramatically alter the morphology of the hook-shaped denticles that decorate the ventral surface of the wg mutant larval cuticle. In an otherwise wild-type background, ck mutations do not significantly alter denticle morphology, suggesting a specific interaction with Wg-mediated aspects of epidermal patterning. Here, we show that changing the level of Wg activity changes the structure of actin bundles during denticle formation in ck mutants. We further find that regulation of the Wg target gene, shaven-baby (svb), and of its transcriptional targets, miniature (m) and forked (f), modulates this ck-dependent process. We conclude that Ck acts in concert with Wg targets to orchestrate the proper shaping of denticles in the Drosophila embryonic epidermis.

Wg signaling via Zw3 and mad restricts self-renewal of sensory organ precursor cells in Drosophila

It is well known that the Dpp signal transducer Mad is activated by phosphorylation at its carboxy-terminus. The role of phosphorylation on other regions of Mad is not as well understood. Here we report that the phosphorylation of Mad in the linker region by the Wg antagonist Zw3 (homolog of vertebrate Gsk3-β) regulates the development of sensory organs in the anterior-dorsal quadrant of the wing. Proneural expression of Mad-RNA interference (RNAi) or a Mad transgene with its Zw3/Gsk3-β phosphorylation sites mutated (MGM) generated wings with ectopic sensilla and chemosensory bristle duplications. Studies with pMad-Gsk (an antibody specific to Zw3/Gsk3-β-phosphorylated Mad) in larval wing disks revealed that this phosphorylation event is Wg dependent (via an unconventional mechanism), is restricted to anterior-dorsal sensory organ precursors (SOP) expressing Senseless (Sens), and is always co-expressed with the mitotic marker phospho-histone3. Quantitative analysis in both Mad-RNAi and MGM larval wing disks revealed a significant increase in the number of Sens SOP. We conclude that the phosphorylation of Mad by Zw3 functions to prevent the self-renewal of Sens SOP, perhaps facilitating their differentiation via asymmetric division. The conservation of Zw3/Gsk3-β phosphorylation sites in vertebrate homologs of Mad (Smads) suggests that this pathway, the first transforming growth factor β-independent role for any Smad protein, may be widely utilized for regulating mitosis during development.

Cytokinesis proteins Tum and Pav have a nuclear role in Wnt regulation

Wg/Wnt signals specify cell fates in both invertebrate and vertebrate embryos and maintain stem-cell populations in many adult tissues. Deregulation of the Wnt pathway can transform cells to a proliferative fate, leading to cancer. We have discovered that two Drosophila proteins that are crucial for cytokinesis have a second, largely independent, role in restricting activity of the Wnt pathway. The fly homolog of RacGAP1, Tumbleweed (Tum)/RacGAP50C, and its binding partner, the kinesin-like protein Pavarotti (Pav), negatively regulate Wnt activity in fly embryos and in cultured mammalian cells. Unlike many known regulators of the Wnt pathway, these molecules do not affect stabilization of Arm/beta-catenin (betacat), the principal effector molecule in Wnt signal transduction. Rather, they appear to act downstream of betacat stabilization to control target-gene transcription. Both Tum and Pav accumulate in the nuclei of interphase cells, a location that is spatially distinct from their cleavage-furrow localization during cytokinesis. We show that this nuclear localization is essential for their role in Wnt regulation. Thus, we have identified two modulators of the Wnt pathway that have shared functions in cell division, which hints at a possible link between cytokinesis and Wnt activity during tumorigenesis.

Multiple Wnt genes are required for segmentation in the short-germ embryo of Tribolium castaneum

wingless (wg)/Wnt family are essential to development in virtually all metazoans. In short-germ insects, including the red flour beetle (Tribolium castaneum), the segment-polarity function of wg is conserved [1]. Wnt signaling is also implicated in posterior patterning and germband elongation [2-4], but despite its expression in the posterior growth zone, Wnt1/wg alone is not responsible for these functions [1-3]. Tribolium contains additional Wnt family genes that are also expressed in the growth zone [5]. After depleting Tc-WntD/8 we found a small percentage of embryos lacking abdominal segments. Additional removal of Tc-Wnt1 significantly enhanced the penetrance of this phenotype. Seeking alternative methods to deplete Wnt signal, we performed RNAi with other components of the Wnt pathway including wntless (wls), porcupine (porc), and pangolin (pan). Tc-wls RNAi caused segmentation defects similar to Tc-Wnt1 RNAi, but not Tc-WntD/8 RNAi, indicating that Tc-WntD/8 function is Tc-wls independent. Depletion of Tc-porc and Tc-pan produced embryos resembling double Tc-Wnt1,Tc-WntD/8 RNAi embryos, suggesting that Tc-porc is essential for the function of both ligands, which signal through the canonical pathway. This is the first evidence of functional redundancy between Wnt ligands in posterior patterning in short-germ insects. This Wnt function appears to be conserved in other arthropods [6] and vertebrates [7-9].

Wnt, Hedgehog and junctional Armadillo/beta-catenin establish planar polarity in the Drosophila embryo

To generate specialized structures, cells must obtain positional and directional information. In multi-cellular organisms, cells use the non-canonical Wnt or planar cell polarity (PCP) signaling pathway to establish directionality within a cell. In vertebrates, several Wnt molecules have been proposed as permissible polarity signals, but none has been shown to provide a directional cue. While PCP signaling components are conserved from human to fly, no PCP ligands have been reported in Drosophila. Here we report that in the epidermis of the Drosophila embryo two signaling molecules, Hedgehog (Hh) and Wingless (Wg or Wnt1), provide directional cues that induce the proper orientation of Actin-rich structures in the larval cuticle. We further find that proper polarity in the late embryo also involves the asymmetric distribution and phosphorylation of Armadillo (Arm or β-catenin) at the membrane and that interference with this Arm phosphorylation leads to polarity defects. Our results suggest new roles for Hh and Wg as instructive polarizing cues that help establish directionality within a cell sheet, and a new polarity-signaling role for the membrane fraction of the oncoprotein Arm.

Fate map of the distal portion of Drosophila proboscis as inferred from the expression and mutations of basic patterning genes

The late-third-instar labial disc is comprised of two disc-proper cell layers, one representing mainly the ventral half of the anterior compartment (L-layer) and the other, the dorsal half of the anterior compartment and most, if not all, of the posterior compartment (M-layer). In the L-layer, Distal-less represses homothorax whereas no Distal-less-dependent homothorax repression occurs in the M-layer where Distal-less is coexpressed with homothorax. In wild-type labial discs, clawless, one of the two homeobox genes expressed in distal cells receiving maximum (Decapentaplegic+Wingless) signaling activity in leg and antennal discs, is specifically repressed by proboscipedia. A fate map, inferred from data on basic patterning gene expression in larval and pupal stages and mutant phenotypes, indicates the inner surface of the labial palpus, which includes the pseudotracheal region, to be a derivative of the distal portion of the M-layer expressing wingless, patched, Distal-less and homothorax. The outer surface of the labial palpus with more than 30 taste bristles derives from an L-layer area consisting of dorsal portions of the anterior and posterior compartments, each expressing Distal-less. Our analysis also indicates that, in adults and pupae, the anterior-posterior boundary, dividing roughly equally the outer surface of the distiproboscis, runs along the outer circumference of the inner surface of distiproboscis.

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.

Mutations in eukaryotic release factors 1 and 3 act as general nonsense suppressors in Drosophila

In a screen for suppressors of the Drosophila wingless(PE4) nonsense allele, we isolated mutations in the two components that form eukaryotic release factor. eRF1 and eRF3 comprise the translation termination complex that recognizes stop codons and catalyzes the release of nascent polypeptide chains from ribosomes. Mutations disrupting the Drosophila eRF1 and eRF3 show a strong maternal-effect nonsense suppression due to readthrough of stop codons and are zygotically lethal during larval stages. We tested nonsense mutations in wg and in other embryonically acting genes and found that different stop codons can be suppressed but only a subset of nonsense alleles are subject to suppression. We suspect that the context of the stop codon is significant: nonsense alleles sensitive to suppression by eRF1 and eRF3 encode stop codons that are immediately followed by a cytidine. Such suppressible alleles appear to be intrinsically weak, with a low level of readthrough that is enhanced when translation termination is disrupted. Thus the eRF1 and eRF3 mutations provide a tool for identifying nonsense alleles that are leaky. Our findings have important implications for assigning null mutant phenotypes and for selecting appropriate alleles to use in suppressor screens.

Wg/Wnt signal can be transmitted through arrow/LRP5,6 and Axin independently of Zw3/Gsk3beta activity

Activation of the Wnt signaling cascade provides key signals during development and in disease. Here we provide evidence, by designing a Wnt receptor with ligand-independent signaling activity, that physical proximity of Arrow (LRP) to the Wnt receptor Frizzled-2 triggers the intracellular signaling cascade. We have uncovered a branch of the Wnt pathway in which Armadillo activity is regulated concomitantly with the levels of Axin protein. The intracellular pathway bypasses Gsk3beta/Zw3, the kinase normally required for controlling beta-catenin/Armadillo levels, suggesting that modulated degradation of Armadillo is not required for Wnt signaling. We propose that Arrow (LRP) recruits Axin to the membrane, and that this interaction leads to Axin degradation. As a consequence, Armadillo is no longer bound by Axin, resulting in nuclear signaling by Armadillo.

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.

Drosophila segment polarity gene product porcupine stimulates the posttranslational N-glycosylation of wingless in the endoplasmic reticulum

Wnt is a family of cysteine-rich secreted glycoproteins, which controls the fate and behavior of the cells in multicellular organisms. In the absence of Drosophila segment polarity gene porcupine (porc), which encodes an endoplasmic reticulum (ER) multispanning transmembrane protein, the N-glycosylation of Wingless (Wg), one of Drosophila Wnt family, is impaired. In contrast, the ectopic expression of porc stimulates the N-glycosylation of both endogenously and exogenously expressed Wg. The N-glycosylation of Wg in the ER occurs posttranslationally, while in the presence of dithiothreitol, it efficiently occurs cotranslationally. Thus, the cotranslational disulfide bond formation of Wg competes with the N-glycosylation by an oligosaccharyl transferase complex. Porc binds the N-terminal 24-amino acid domain (residues 83-106) of Wg, which is highly conserved in the Wnt family and stimulates the N-glycosylation at surrounding sites. Porc is also necessary for the processing of Drosophila Wnt-3/5 in both embryos and cultured cells. Thus, Porc binds the N-terminal specific domain of the Wnt family and stimulates its posttranslational N-glycosylation by anchoring them at the ER membrane possibly through acylation.

Bioengineering models of cell signaling

Strategies for rationally manipulating cell behavior in cell-based technologies and molecular therapeutics and understanding effects of environmental agents on physiological systems may be derived from a mechanistic understanding of underlying signaling mechanisms that regulate cell functions. Three crucial attributes of signal transduction necessitate modeling approaches for analyzing these systems: an ever-expanding plethora of signaling molecules and interactions, a highly interconnected biochemical scheme, and concurrent biophysical regulation. Because signal flow is tightly regulated with positive and negative feedbacks and is bidirectional with commands traveling both from outside-in and inside-out, dynamic models that couple biophysical and biochemical elements are required to consider information processing both during transient and steady-state conditions. Unique mathematical frameworks will be needed to obtain an integrated perspective on these complex systems, which operate over wide length and time scales. These may involve a two-level hierarchical approach wherein the overall signaling network is modeled in terms of effective "circuit" or "algorithm" modules, and then each module is correspondingly modeled with more detailed incorporation of its actual underlying biochemical/biophysical molecular interactions.

Cubitus interruptus acts to specify naked cuticle in the trunk of Drosophila embryos

One function of the Wingless signaling pathway is to determine the naked, cuticle cell fate choice in the trunk epidermis of Drosophila larvae. The zinc finger protein Teashirt binds to the transactivator domain of Armadillo to modulate Wingless signaling output in the embryonic trunk and contributes to the naked cell fate choice. The Hedgehog pathway is also necessary for the correct specification of larval epidermal cell fate, which signals via the zinc finger protein, Cubitus interruptus. Here, we show that Cubitus interruptus also has a Wingless-independent function, which is required for the specification of the naked cell fate; previously, it had been assumed that Ci induces naked cuticle exclusively by regulation of wg. Wg and Hh signaling pathways may be acting combinatorially in the same, or individually in different, cells for this process, by regulating common sets of target genes. First, the loss of the naked cuticular phenotype in embryos lacking cubitus interruptus activity is very similar to that induced by a late loss of Wingless function. Second, overexpression of Cubitus interruptus causes the suppression of denticles (as Wingless does) in absence of Wingless activity in the anterior trunk. Using epistasis experiments, we conclude that different combinations of the three proteins Teashirt, Cubitus interruptus, and Armadillo are employed for the specification of naked cuticle at distinct positions both along the antero-posterior axis and within individual trunk segments. Finally, biochemical approaches suggest the existence of protein complexes consisting of Teashirt, Cubitus interruptus, and Armadillo.

Induction of ureter branching as a response to Wnt-2b signaling during early kidney organogenesis

Epithelial-mesenchymal tissue interactions play a central role in vertebrate organogenesis, but the molecular mediators and mechanisms of these morphogenetic interactions are still not well characterized. We report here on the expression pattern of Wnt-2b during mouse organogenesis and on tests of its function in epithelial- mesenchymal interactions during kidney development. Wnt-2b is expressed in numerous developing organs in the mouse embryo, including the kidney, lung, salivary gland, gut, pancreas, adrenal gland, and genital tubercle. Additional sites of expression include the branchial arches and craniofacial placodes such as the eye and ear. The data suggest that the expression of Wnt-2b is associated with organs regulated by epithelial-mesenchymal interactions. It is typically localized in the capsular epithelium or peripheral mesenchymal cells of organ rudiments, e.g., the perinephric mesenchymal cells in the region of the presumptive renal stroma in the developing kidney at E11.5. Functional studies of the kidney demonstrate that cells expressing Wnt-2b are not capable of inducing tubule formation but instead stimulate ureter development. Incubation of isolated ureteric buds on such cells supports bud growth and branching. In addition, recombination of Wnt-2b-pretreated ureteric bud tissue with isolated nephrogenic mesenchyme results in a recovery of organogenesis and the expression of epithelial genes within the reconstituted organ explant. Lithium, a known activator of Wnt signaling (Hedgepeth et al. [1997] Dev Biol 185:82-91), is also sufficient to promote ureter branching in the reconstituted kidney in a comparable manner to Wnt-2b signaling, whereas Wnt-4, which induces tubules, neither supports the growth of a ureteric bud nor leads to reconstitution of the ureteric bud with the kidney mesenchyme. We conclude that Wnt-2b may act in the mouse kidney as an early mesenchymal signal controlling morphogenesis of epithelial tissue, and that the Wnt pathway may regulate ureter branching directly. In addition, Wnt signals in the kidney differ qualitatively and are specific to either the epithelial ureteric bud or the kidney mesenchyme.

Apical localization of wingless transcripts is required for wingless signaling

Many developing and adult tissues are comprised of polarized epithelia. Proteins that are asymmetrically distributed in these cells are thought to be localized by protein trafficking. Here we show that the distribution and function of the signaling protein Wingless is predetermined by the subcellular localization of its mRNA. High-resolution in situ hybridization reveals apical transcript localization in the majority of tissues examined. This localization is mediated by two independently acting elements in the 3' UTR. Replacement of these elements with non- or basolaterally localizing elements yields proteins with altered intracellular and extracellular distributions and reduced signaling activities. This novel aspect of the wingless signaling pathway is conserved and may prove to be a mechanism used commonly for establishing epithelial cell polarity.

The WNT antagonist cSFRP2 modulates programmed cell death in the developing hindbrain

In the avian hindbrain, the loss of premigratory neural crest cells from rhombomeres 3 and 5 (r3, r5) through programmed cell death contributes to the patterning of emigrant crest cells into three discrete streams. Programmed cell death is induced by the upregulation of Bmp4 and Msx2 in r3 and r5. We show that cSFRP2, a WNT antagonist, is expressed in the even-numbered rhombomeres and that over-expression of cSfrp2 inhibits Bmp4 expression in r3 and r5, preventing programmed cell death. By contrast, depleting cSFRP2 function in r4 results in elevated levels of Msx2 expression and ectopic programmed cell death, as does overexpression of Wnt1. We propose that programmed cell death in the rhombencephalic neural crest is modulated by pre-patterned cSfrp2 expression and a WNT-BMP signalling loop.

Non-equivalent roles of Drosophila Frizzled and Dfrizzled2 in embryonic wingless signal transduction

The highly conserved Wnt family of growth factors is essential for generating embryonic pattern in many animal species [1]. In the fruit fly Drosophila, most Wnt-mediated patterning is performed by a single family member, Wingless (Wg), acting through its receptors Frizzled (Fz) and DFrizzled2 (Dfz2). In the ventral embryonic epidermis, Wg signaling generates two different cell-fate decisions: the production of diverse denticle types and the specification of naked cuticle separating the denticle belts. Mutant alleles of wg disrupt these cellular decisions separately [2], suggesting that some aspect of ligand-receptor affinity influences cell-fate decisions, or that different receptor complexes mediate the distinct cellular responses. Here, we report that overexpression of Dfz2, but not Fz, rescues the mutant phenotype of wgPE2, an allele that produces denticle diversity but no naked cuticle. Fz was able to substitute for Dfz2 only under conditions where the Wg ligand was present in excess. The wgPE2 mutant phenotype was also sensitive to the dosage of glycosaminoglycans, suggesting that the mutant ligand is excluded from the receptor complex when proteoglycans are present. We conclude that wild-type Wg signaling requires efficient interaction between ligand and the Dfz2-proteoglycan receptor complex to promote the naked cuticle cell fate.

Restricted patterning of vestigial expression in Drosophila wing imaginal discs requires synergistic activation by both Mad and the drifter POU domain transcription factor

The Drosophila Vestigial protein has been shown to play an essential role in the regulation of cell proliferation and differentiation within the developing wing imaginal disc. Cell-specific expression of vg is controlled by two separate transcriptional enhancers. The boundary enhancer controls expression in cells near the dorsoventral (DV) boundary and is regulated by the Notch signal transduction pathway, while the quadrant enhancer responds to the Decapentaplegic and Wingless morphogen gradients emanating from cells near the anteroposterior (AP) and DV boundaries, respectively. MAD-dependent activation of the vestigial quadrant enhancer results in broad expression throughout the wing pouch but is excluded from cells near the DV boundary. This has previously been thought to be due to direct repression by a signal from the DV boundary; however, we show that this exclusion of quadrant enhancer-dependent expression from the DV boundary is due to the absence of an additional essential activator in those cells. The Drosophila POU domain transcriptional regulator, Drifter, is expressed in all cells within the wing pouch expressing a vgQ-lacZ transgene and is also excluded from the DV boundary. Viable drifter hypomorphic mutations cause defects in cell proliferation and wing vein patterning correlated with decreased quadrant enhancer-dependent expression. Drifter misexpression at the DV boundary using the GAL4/UAS system causes ectopic outgrowths at the distal wing tip due to induction of aberrant Vestigial expression, while a dominant-negative Drifter isoform represses expression of vgQ-lacZ and causes severe notching of the adult wing. In addition, we have identified an essential evolutionarily conserved sequence element bound by the Drifter protein with high affinity and located adjacent to the MAD binding site within the quadrant enhancer. Our results demonstrate that Drifter functions along with MAD as a direct activator of Vestigial expression in the wing pouch.

The evolutionarily conserved porcupine gene family is involved in the processing of the Wnt family

The Drosophila segment polarity gene product Porcupine (Porc) was first identified as being necessary for processing Wingless (Wg), a Drosophila Wnt (Wnt) family member. Mouse and Xenopus homologs of porc (Mporc and Xporc) were identified and found to encode endoplasmic reticulum (ER) proteins with multiple transmembrane domains. In contrast with porc, four different types of Mporc and Xporc mRNA (A-D) are generated from a single gene by alternative splicing. Mporc mRNA is differentially expressed during embryogenesis and in various adult tissues, demonstrating that the alternative splicing is regulated to synthesize the specific types of Mporc. In transfected mammalian cells, all Mporc types affect the processing of mouse Wnt 1, 3A, 4, 6, and 7B but not 5A. Furthermore, all Mporc types are co-immunoprecipitated with various Wnt proteins. These results suggest that Mporc may function as a chaperone-like molecule for Wnt. Interestingly, all Mporc types can substitute for Porc, as they are able to rescue the phenotypes of Drosophila porc embryos. Consistent with this observation, Mporc, like Porc, modifies the processing of Wg expressed in mammalian cells. These results demonstrate that the porc gene family encodes the multitransmembrane ER proteins, which are evolutionarily well conserved and involved in processing the Wnt family.

BMP, Wnt and Hedgehog signals: how far can they go?

Wnt, Hedgehog and bone morphogenetic proteins function as either short-range or long-range signaling molecules depending on the tissue in which they are expressed. In the past year, filapodia-like cytoplasmic extensions, cell-surface proteogylcans and/or extracellular binding proteins have been identified that may enable these molecules to signal at a distance. Furthermore, recent studies suggest that variations in the signaling range of these molecules may be due to tissue-specific differences in intracellular processing or tissue-restricted expression of binding proteins.

Wingless transduction by the Frizzled and Frizzled2 proteins of Drosophila

Wingless (Wg) protein is a founding member of the Wnt family of secreted proteins which have profound organizing roles in animal development. Two members of the Frizzled (Fz) family of seven-pass transmembrane proteins, Drosophila Fz and Fz2, can bind Wg and are candidate Wg receptors. However, null mutations of the fz gene have little effect on Wg signal transduction and the lack of mutations in the fz2 gene has thus far prevented a rigorous examination of its role in vivo. Here we describe the isolation of an amber mutation of fz2 which truncates the coding sequence just after the amino-terminal extracellular domain and behaves genetically as a loss-of-function allele. Using this mutation, we show that Wg signal transduction is abolished in virtually all cells lacking both Fz and Fz2 activity in embryos as well as in the wing imaginal disc. We also show that Fz and Fz2 are functionally redundant: the presence of either protein is sufficient to confer Wg transducing activity on most or all cells throughout development. These results extend prior evidence of a ligand-receptor relationship between Wnt and Frizzled proteins and suggest that Fz and Fz2 are the primary receptors for Wg in Drosophila.

Directionality of wingless protein transport influences epidermal patterning in the Drosophila embryo

Active endocytotic processes are required for the normal distribution of Wingless (Wg) protein across the epidermal cells of each embryonic segment. To assess the functional consequences of this broad Wg distribution, we have devised a means of perturbing endocytosis in spatially restricted domains within the embryo. We have constructed a transgene expressing a dominant negative form of shibire (shi), the fly dynamin homologue. When this transgene is expressed using the GAL4-UAS system, we find that Wg protein distribution within the domain of transgene expression is limited and that Wg-dependent epidermal patterning events surrounding the domain of expression are disrupted in a directional fashion. Our results indicate that Wg transport in an anterior direction generates the normal expanse of naked cuticle within the segment and that movement of Wg in a posterior direction specifies diverse denticle cell fates in the anterior portion of the adjacent segment. Furthermore, we have discovered that interfering with posterior movement of Wg rescues the excessive naked cuticle specification observed in naked (nkd) mutant embryos. We propose that the nkd segment polarity phenotype results from unregulated posterior transport of Wg protein and therefore that wild-type Nkd function may contribute to the control of Wg movement within the epidermal cells of the segment.

Frizzled and Dfrizzled-2 function as redundant receptors for Wingless during Drosophila embryonic development

In cell culture assays, Frizzled and Dfrizzled2, two members of the Frizzled family of integral membrane proteins, are able to bind Wingless and transduce the Wingless signal. To address the role of these proteins in the intact organism and to explore the question of specificity of ligand-receptor interactions in vivo, we have conducted a genetic analysis of frizzled and Dfrizzled2 in the embryo. These experiments utilize a small gamma-ray-induced deficiency that uncovers Dfrizzled2. Mutants lacking maternal frizzled and zygotic frizzled and Dfrizzled2 exhibit defects in the embryonic epidermis, CNS, heart and midgut that are indistinguishable from those observed in wingless mutants. Epidermal patterning defects in the frizzled, Dfrizzled2 double-mutant embryos can be rescued by ectopic expression of either gene. In frizzled, Dfrizzled2 mutant embryos, ectopic production of Wingless does not detectably alter the epidermal patterning defect, but ectopic production of an activated form of Armadillo produces a naked cuticle phenotype indistinguishable from that produced by ectopic production of activated Armadillo in wild-type embryos. These experiments indicate that frizzled and Dfrizzled2 function downstream of wingless and upstream of armadillo, consistent with their proposed roles as Wingless receptors. The lack of an effect on epidermal patterning of ectopic Wingless in a frizzled, Dfrizzled2 double mutant argues against the existence of additional Wingless receptors in the embryo or a model in which Frizzled and Dfrizzled2 act simply to present the ligand to its bona fide receptor. These data lead to the conclusion that Frizzled and Dfrizzled2 function as redundant Wingless receptors in multiple embryonic tissues and that this role is accurately reflected in tissue culture experiments. The redundancy of Frizzled and Dfrizzled2 explains why Wingless receptors were not identified in earlier genetic screens for mutants defective in embryonic patterning.

Mutual antagonism between signals secreted by adjacent wingless and engrailed cells leads to specification of complementary regions of the Drosophila parasegment

Specialized groups of cells known as organizers govern the establishment of cell type diversity across cellular fields. Segmental patterning within the Drosophila embryonic epidermis is one paradigm for organizer function. Here cells differentiate into smooth cuticle or distinct denticle types. At parasegment boundaries, cells expressing Wingless confront cells co-expressing Engrailed and Hedgehog. While Wingless is essential for smooth cell fates, the signals that establish denticle diversity are unknown. We show that wg mutants have residual mirror-symmetric pattern that is due to an Engrailed-dependent signal specifying anterior denticle fates. The Engrailed-dependent signal acts unidirectionally and Wg activity imposes this asymmetry. Reciprocally, the Engrailed/Hedgehog interface imposes asymmetry on Wg signaling. Thus, a bipartite organizer, with each signal acting essentially unidirectionally, specifies segmental pattern.

Patterning of Drosophila leg sensory organs through combinatorial signaling by hedgehog, decapentaplegic and wingless

During development, global patterning events initiate signal transduction cascades which gradually establish an array of individual cell fates. Many of the genes which pattern Drosophila are expressed throughout development and specify diverse cell types by creating unique local environments which establish the expression of locally acting genes. This process is exemplified by the patterning of leg microchaete rows. hairy (h) is expressed in a spatially restricted manner in the leg imaginal disc and functions to position adult leg bristle rows by negatively regulating the proneural gene achaete, which specifies sensory cell fates. While much is known about the events that partition the leg imaginal disc and about sensory cell differentiation, the mechanisms that refine early patterning events to the level of individual cell fate specification are not well understood. We have investigated the regulation of h expression along the dorsal/ventral (D/V) axis of the leg adjacent to the anterior/posterior (A/P) compartment boundary and have found that it requires input from both D/V and A/P patterning mechanisms. Expression of the D/V axis h stripe (D/V-h) is controlled by dorsal- and ventral-specific enhancer elements which are targets of Decapentaplegic (Dpp) and Wingless (Wg) signaling, respectively, but which are also dependent on Hedgehog (Hh) signaling for activation. D/V-h expression is lost in smoothened mutant clones and is specifically activated by exogenously supplied Cubitus interruptus (Ci). D/V-h expression is also lost in clones deficient for Dpp and Wg signaling, but ectopic activation of D/V-h by Dpp and Wg is limited to the A/P compartment boundary where endogenous levels of full-length Ci are high. We propose that D/V-h expression is regulated in a non-linear pathway in which Ci plays a dual role. In addition to serving as an upstream activator of Dpp and Wg, Ci acts combinatorially with them to activate D/V-h expression.

Antagonist activity of DWnt-4 and wingless in the Drosophila embryonic ventral ectoderm and in heterologous Xenopus assays

Wnt genes encode secreted signalling molecules involved in a number of basic developmental processes. In Drosophila, wingless and DWnt-4 are two physically clustered Wnt genes, which are transcribed in overlapping patterns during embryogenesis and, in several instances, are controlled by the same regulatory molecules. To address the question of the functional relationship of wingless and DWnt-4, we analysed how embryonic cells respond when they are exposed, simultaneously or not, to the encoded Wnt signals. We show that DWnt-4 has the capacity to antagonise Wingless signalling both in the Drosophila ventral epidermis and in a heterologous system, the Xenopus embryo. We provide evidence that DWnt-4 inhibits the Wingless/Wnt-1 signalling pathway upstream of the activation of transcriptional targets. This is the first report that antagonising Wnt signals exist in Drosophila.

Signalling at a distance: transport of Wingless in the embryonic epidermis of Drosophila

Secreted signalling molecules affect the behavior of cells at a distance. Here we discuss how the Wnt family member Wingless reaches distant cells within the embryonic epidermis of Drosophila. We consider three possible mechanisms: free diffusion, restricted diffusion and active transport. We argue that free diffusion is unlikely to occur. However, a variant of restricted diffusion may account for Wingless transport. It may be that Wingless is carried from one side of a cell to the other by a drifting transmembrane protein such as a specific receptor or a glycosaminoglycan. Transfer from cell-to-cell would involve release from the donor cell and recapture in an adjacent cell. Alternatively, Wingless might be transported by a mechanism akin to transcytosis. This would involve the packaging of Wingless in specialized vesicles at one end of a cell, active transport across the cell, and vesicle fusion and Wingless release on the other side. We describe the evidence in favor and against these two alternatives.

Wingless signaling in the Drosophila embryo: zygotic requirements and the role of the frizzled genes

Wingless signaling plays a central role during epidermal patterning in Drosophila. We have analyzed zygotic requirements for Wingless signaling in the embryonic ectoderm by generating synthetic deficiencies that uncover more than 99% of the genome. We found no genes required for initial wingless expression, other than previously identified segmentation genes. In contrast, maintenance of wingless expression shows a high degree of zygotic transcriptional requirements. Besides known genes, we have identified at least two additional genomic regions containing new genes involved in Wingless maintenance. We also assayed for the zygotic requirements for Wingless response and found that no single genomic region was required for the cytoplasmic accumulation of Armadillo in the receiving cells. Surprisingly, embryos homozygously deleted for the candidate Wingless receptor, Dfrizzled2, showed a normal Wingless response. However, the Armadillo response to Wingless was strongly reduced in double mutants of both known members of the frizzled family in Drosophila, frizzled and Dfrizzled2. Based on their expression pattern during embryogenesis, different Frizzled receptors may play unique but overlapping roles in development. In particular, we suggest that Frizzled and Dfrizzled2 are both required for Wingless autoregulation, but might be dispensable for late Engrailed maintenance. While Wingless signaling in embryos mutant for frizzled and Dfrizzled2 is affected, Wingless protein is still internalized into cells adjacent to wingless-expressing cells. Incorporation of Wingless protein may therefore involve cell surface molecules in addition to the genetically defined signaling receptors of the frizzled family.

Mechanisms of Wnt signaling in development

Wnt genes encode a large family of secreted, cysteine-rich proteins that play key roles as intercellular signaling molecules in development. Genetic studies in Drosophila and Caenorhabditis elegans, ectopic gene expression in Xenopus, and gene knockouts in the mouse have demonstrated the involvement of Wnts in processes as diverse as segmentation, CNS patterning, and control of asymmetric cell divisions. The transduction of Wnt signals between cells proceeds in a complex series of events including post-translational modification and secretion of Wnts, binding to transmembrane receptors, activation of cytoplasmic effectors, and, finally, transcriptional regulation of target genes. Over the past two years our understanding of Wnt signaling has been substantially improved by the identification of Frizzled proteins as cell surface receptors for Wnts and by the finding that beta-catenin, a component downstream of the receptor, can translocate to the nucleus and function as a transcriptional activator. Here we review recent data that have started to unravel the mechanisms of Wnt signaling.

Cellular mechanisms of wingless/Wnt signal transduction

Wg/Wnt signaling regulates cell proliferation and differentiation in species as divergent as nematodes, flies, frogs, and humans. Many components of this highly conserved process have been characterized and work from a number of laboratories is beginning to elucidate the mechanism by which this class of secreted growth factor triggers cellular decisions. The Wg/Wnt ligand apparently binds to Frizzled family receptor molecules to initiate a signal transduction cascade involving the novel cytosolic protein Dishevelled and the serine/threonine kinase Zeste-white 3/GSK3. Antagonism of Zw3 activity leads to stabilization of Armadillo/beta-catenin, which provides a transactivation domain when complexed with the HMG box transcription factor dTCF/LEF-1 and thereby activates expression of Wg/Wnt-responsive genes. The Wg/Wnt ligands pass through the secretory pathway and associate with extracellular matrix components; recent work shows that sulfated glycosaminoglycans are essential for proper transduction of the signal. Mutant forms of Wg in Drosophila reveal separable aspects of Wg function and suggest that proper transport of the protein across cells is essential for cell fate specification. Complex interactions with the Notch and EGF/Ras signaling pathways also play a role in cell fate decisions during different phases of Drosophila development. These many facets of Wg/Wnt signaling have been elucidated through studies in a variety of species, each with powerful and unique experimental approaches. The remarkable conservation of this pathway suggests that Wg/Wnt signal transduction represents a fundamental mechanism for the generation of diverse cell fates in animal embryos.

Use of dsRNA-mediated genetic interference to demonstrate that frizzled and frizzled 2 act in the wingless pathway

We investigated the potential of double-stranded RNA to interfere with the function of genes in Drosophila. Injection of dsRNA into embryos resulted in potent and specific interference of several genes that were tested. In contrast, single-stranded RNA weakly interfered with gene activity. The method was used to determine the reception mechanism of the morphogen Wingless. Interference of the frizzled and Drosophila frizzled 2 genes together produced defects in embryonic patterning that mimic loss of wingless function. Interference of either gene alone had no effect on patterning. Epistasis analysis indicates that frizzled and Drosophila frizzled 2 act downstream of wingless and upstream of zeste-white3 in the Wingless pathway. Our results demonstrate that dsRNA interference can be used to analyze many aspects of gene function.

frizzled and frizzled 2 play a partially redundant role in wingless signaling and have similar requirements to wingless in neurogenesis

The Drosophila Frizzled (Fz) and Frizzled2 (DFz2) proteins function as receptors for Wingless (Wg) in tissue culture cells. While previous results indicate that loss of function for fz has tissue polarity defects, loss-of-function effects of Dfz2 are not known. Here, we have examined the requirements of fz and Dfz2 during neurogenesis. Our results indicate that both Fz and DFz2 function in Wg signaling, and loss of either of the two affects the same subset of neuroblasts as those affected by loss of wg. While these defects are partially penetrant in embryos lacking either fz or Dfz2, the penetrance is significantly enhanced in embryos lacking both. Since the penetrance of the CNS phenotypes is not complete in double mutants, additional components that allow some degree of Wg signaling must exist in vivo.

Functional analysis of Wingless reveals a link between intercellular ligand transport and dorsal-cell-specific signaling

The Drosophila segment polarity gene wingless (wg) is essential for cell fate decisions in the developing embryonic epidermis. Wg protein is produced in one row of cells near the posterior of every segment and is secreted and distributed throughout the segment to generate wild-type pattern elements. Ventrally, epidermal cells secrete a diverse array of anterior denticle types and a posterior expanse of naked cuticle; dorsally, a stereotyped pattern of fine hairs is secreted. We describe three new wg alleles that promote naked cuticle cell fate but show reduced denticle diversity and dorsal patterning. These mutations cause single amino acid substitutions in a cluster of residues that are highly conserved throughout the Wnt family. By manipulating expression of transgenic proteins, we demonstrate that all three mutant molecules retain the intrinsic capacity to signal ventrally but fail to be distributed across the segment. Thus, movement of Wg protein through the epidermal epithelium is essential for proper ventral denticle specification and this planar movement is distinct from the apical-basal transcytosis previously described in polarized epithelia. Furthermore, ectopic overexpression of the mutant proteins fails to rescue dorsal pattern elements. Thus we have identified a region of Wingless that is required for both the transcytotic process and signal transduction in dorsal cell populations, revealing an unexpected link between these two aspects of Wg function.

Frizzled signaling and the developmental control of cell polarity

Within the last three years, Frizzled receptors have risen from obscurity to celebrity status owing to their functional identification as receptors for the ubiquitous family of secreted WNT signaling factors. However, the founding member of the Frizzled family, Drosophila Frizzled (FZ), was cloned almost a decade ago because of its role in regulating cell polarity within the plane of an epithelium. In this review, we consider the role of FZ in this intriguing context. We discuss recent progress towards elucidating mechanisms for the intracellular specification of planar polarity, and further review evidence for models of global polarity regulation at the tissue level. The data suggest that a genetic 'cassette', encoding a set of core signaling components, could pattern hair, bristle and ommatidial planar polarity in Drosophila, and that additional tissue-specific factors might explain the diversity of signal responses. Recently described examples from the nematode and frog suggest that the developmental control of cell polarity by FZ receptors might represent a functionally conserved signaling mechanism.

The Drosophila LIM-only gene, dLMO, is mutated in Beadex alleles and might represent an evolutionarily conserved function in appendage development

The process of wing patterning involves precise molecular mechanisms to establish an organizing center at the dorsal-ventral boundary, which functions to direct the development of the Drosophila wing. We report that misexpression of dLMO, a Drosophila LIM-only protein, in specific patterns in the developing wing imaginal disc, disrupts the dorsal-ventral (D-V) boundary and causes errors in wing patterning. When dLMO is misexpressed along the anterior-posterior boundary, extra wing outgrowth occurs, similar to the phenotype seen when mutant clones lacking Apterous, a LIM homeodomain protein known to be essential for normal D-V patterning of the wing, are made in the wing disc. When dLMO is misexpressed along the D-V boundary in third instar larvae, loss of the wing margin is observed. This phenotype is very similar to the phenotype of Beadex, a long-studied dominant mutation that we show disrupts the dLMO transcript in the 3' untranslated region. dLMO normally is expressed in the wing pouch of the third instar wing imaginal disc during patterning. A mammalian homolog of dLMO is expressed in the developing limb bud of the mouse. This indicates that LMO proteins might function in an evolutionarily conserved mechanism involved in patterning the appendages.

Van Gogh: a new Drosophila tissue polarity gene

Mutations in the Van Gogh gene result in the altered polarity of adult Drosophila cuticular structures. On the wing, Van Gogh mutations cause an altered polarity pattern that is typical of mutations that inactivate the frizzled signaling/signal transduction pathway. The phenotype however, differs from those seen previously, as the number of wing cells forming more than one hair is intermediate between that seen previously for typical frizzled-like or inturned-like mutations. Consistent with Van Gogh being involved in the function of the frizzled signaling/signal transduction pathway, Van Gogh mutations show strong interactions with mutations in frizzled and prickle. Mitotic clones of Van Gogh display domineering cell nonautonomy. In contrast to frizzled clones, Van Gogh clones alter the polarity of cells proximal (and in part anterior and posterior) but not distal to the clone. In further contrast to frizzled clones, Van Gogh clones cause neighboring wild-type hairs to point away from rather than toward the clone. This anti-frizzled type of domineering nonautonomy and the strong genetic interactions seen between frizzled and Van Gogh suggested the possibility that Van Gogh was required for the noncell autonomous function of frizzled. As a test of this possibility we induced frizzled clones in a Van Gogh mutant background and Van Gogh clones in a frizzled mutant background. In both cases the domineering nonautonomy was suppressed consistent with Van Gogh being essential for frizzled signaling.

Differential recruitment of Dishevelled provides signaling specificity in the planar cell polarity and Wingless signaling pathways

In Drosophila, planar cell polarity (PCP) signaling is mediated by the receptor Frizzled (Fz) and transduced by Dishevelled (Dsh). Wingless (Wg) signaling also requires Dsh and may utilize DFz2 as a receptor. Using a heterologous system, we show that Dsh is recruited selectively to the membrane by Fz but not DFz2, and this recruitment depends on the DEP domain but not the PDZ domain in Dsh. A mutation in the DEP domain impairs both membrane localization and the function of Dsh in PCP signaling, indicating that translocation is important for function. Further genetic and molecular analyses suggest that conserved domains in Dsh function differently during PCP and Wg signaling, and that divergent intracellular pathways are activated. We propose that Dsh has distinct roles in PCP and Wg signaling. The PCP signal may selectively result in focal Fz activation and asymmetric relocalization of Dsh to the membrane, where Dsh effects cytoskeletal reorganization to orient prehair initiation.

Propagation and localization of Wnt signaling

Cellular mechanisms for the transport and localization of Wnt signaling components are important for the propagation, distribution, and polarization of Wnt signals in embryonic tissues. Wnt signals are distributed through tissues by vesicular transport of Wnt proteins, localized in embryos by directed transport of cytoplasmic Wnt-signaling components, and propagated asymmetrically during cell division.