Drosophila glypicans Dally and Dally-like shape the extracellular Wingless morphogen gradient in the wing disc

Drosophila Wingless (Wg) is the founding member of the Wnt family of secreted proteins. During the wing development, Wg acts as a morphogen whose concentration gradient provides positional cues for wing patterning. The molecular mechanism(s) of Wg gradient formation is not fully understood. Here, we systematically analyzed the roles of glypicans Dally and Dally-like protein (Dlp), the Wg receptors Frizzled (Fz) and Fz2, and the Wg co-receptor Arrow (Arr) in Wg gradient formation in the wing disc. We demonstrate that both Dally and Dlp are essential and have different roles in Wg gradient formation. The specificities of Dally and Dlp in Wg gradient formation are at least partially achieved by their distinct expression patterns. To our surprise, although Fz2 was suggested to play an essential role in Wg gradient formation by ectopic expression studies, removal of Fz2 activity does not alter the extracellular Wg gradient. Interestingly, removal of both Fz and Fz2, or Arr causes enhanced extracellular Wg levels, which is mainly resulted from upregulated Dlp levels. We further show that Notum, a negative regulator of Wg signaling, downregulates Wg signaling mainly by modifying Dally. Last, we demonstrate that Wg movement is impeded by cells mutant for both dally and dlp. Together, these new findings suggest that the Wg morphogen gradient in the wing disc is mainly controlled by combined actions of Dally and Dlp. We propose that Wg establishes its concentration gradient by a restricted diffusion mechanism involving Dally and Dlp in the wing disc.     

Spatial regulation of Wingless morphogen distribution and signaling by Dally-like protein

Wingless (Wg) is a morphogen required for the patterning of many Drosophila tissues. Several lines of evidence implicate heparan sulfate-modified proteoglycans (HSPGs) such as Dally-like protein (Dlp) in the control of Wg distribution and signaling. We show that dlp is required to limit Wg levels in the matrix, contrary to the expectation from overexpression studies. dlp mutants show ectopic activation of Wg signaling at the presumptive wing margin and a local increase in extracellular Wg levels. dlp somatic cell clones disrupt the gradient of extracellular Wg, producing ectopic activation of high threshold Wg targets but reducing the expression of lower threshold Wg targets where Wg is limiting. Notum encodes a secreted protein that also limits Wg distribution, and genetic interaction studies show that dlp and Notum cooperate to restrict Wg signaling. These findings suggest that modification of an HSPG by a secreted hydrolase can control morphogen levels in the matrix.     

Dpp gradient formation by dynamin-dependent endocytosis: receptor trafficking and the diffusion model

Developing cells acquire positional information by reading the graded distribution of morphogens. In Drosophila, the Dpp morphogen forms a long-range concentration gradient by spreading from a restricted source in the developing wing. It has been assumed that Dpp spreads by extracellular diffusion. Under this assumption, the main role of endocytosis in gradient formation is to downregulate receptors at the cell surface. These surface receptors bind to the ligand and thereby interfere with its long-range movement. Recent experiments indicate that Dpp spreading is mediated by Dynamin-dependent endocytosis in the target tissue, suggesting that extracellular diffusion alone cannot account for Dpp dispersal. Here, we perform a theoretical study of a model for morphogen spreading based on extracellular diffusion, which takes into account receptor binding and trafficking. We compare profiles of ligand and surface receptors obtained in this model with experimental data. To this end, we monitored directly the pool of surface receptors and extracellular Dpp with specific antibodies. We conclude that current models considering pure extracellular diffusion cannot explain the observed role of endocytosis during Dpp long-range movement.     

The Wingless morphogen gradient is established by the cooperative action of Frizzled and Heparan Sulfate Proteoglycan receptors

We have examined the respective contribution of Heparan Sulfate Proteoglycans (HSPGs) and Frizzled (Fz) proteins in the establishment of the Wingless (Wg) morphogen gradient. From the analysis of mutant clones of sulfateless/N-deacetylase-sulphotransferase in the wing imaginal disc, we find that lack of Heparan Sulfate (HS) causes a dramatic reduction of both extracellular and intracellular Wg in receiving cells. Our studies, together with others [Kirkpatrick, C.A., Dimitroff, B.D., Rawson, J.M., Selleck, S.B., 2004. Spatial regulation of Wingless morphogen distribution and signalling by Dally-like protein. Dev. Cell (in press)], reveals that the Glypican molecule Dally-like Protein (Dlp) is associated with both negative and positive roles in Wg short- and long-range signaling, respectively. In addition, analyses of the two Fz proteins indicate that the Fz and DFz2 receptors, in addition to transducing the signal, modulate the slope of the Wg gradient by regulating the amount of extracellular Wg. Taken together, our analysis illustrates how the coordinated activities of HSPGs and Fz/DFz2 shape the Wg morphogen gradient.     

Formation and maintenance of morphogen gradients: an essential role for the endomembrane system in Drosophila melanogaster wing development

As early as 1964 it was suggested that simple diffusion of morphogens away from their secretion source did not provide an adequate explanation for the formation and maintenance of morphogen gradients. Involvement of the endosome in morphogen distribution models provides an explanation for the slow, directional movement of morphogens, as well as their ability to form intracellular and extracellular gradients independent of morphogen production rates. Drosophila melanogaster morphogens Wg and Dpp form stable, steep, long-range gradients that specify the polarity of the wing disc. The process of endocytosis is imperative to the two central themes in gradient formation: active transport facilitating long-range signaling and degradation of morphogen to sustain gradient shape. This review investigates the endomembrane-mediated processes of re-secretion, degradation and argosome transport of Wg and Dpp in the hope that a better understanding of the endomembrane system will contribute to a more accurate and comprehensive model for morphogen gradient formation and maintenance.     

Formation and maintenance of morphogen gradients: An essential role for the endomembrane system in Drosophila melanogaster wing development

As early as 1964 it was suggested that simple diffusion of morphogens away from their secretion source did not provide an adequate explanation for the formation and maintenance of morphogen gradients. Involvement of the endosome in morphogen distribution models provides an explanation for the slow, directional movement of morphogens, as well as their abilty to form intracellular and extracellular gradients independent of morphogen production rates. Drosophila melanogaster morphogens Wg and Dpp form stable, steep, long-range gradients that specify the polarity of the wing disc. The process of endocytosis is imparative to the two central themes in gradient formation; active transport facilitating long-range signalling, and degradation of morphogen to sustain gradient shape. This review investigates the endomembrane mediated processes of re-secretion, degradation, and argosome transport of Wg and Dpp in the hope that a better understanding of the endomembrane system will contribute to a more accurate and comprehensive model for morphogen gradient formation and maintenance.     

Opposing activities of Dally-like glypican at high and low levels of Wingless morphogen activity

The glypican family of heparan sulfate proteoglycans has been implicated in formation of morphogen gradients. Here, we examine the role of the glypican Dally-like protein (Dlp) in shaping the Wingless gradient in the Drosophila wing disc. Surprisingly, we find that Dlp has opposite effects at high and low levels of Wingless. Dlp promotes low-level Wingless activity but reduces high-level Wingless activity. We present evidence that the Wg antagonist Notum acts to induce cleavage of the Dlp glypican at the level of its GPI anchor, which leads to shedding of Dlp. Thus, spatially regulated modification of Dlp by Notum employs the ligand binding activity of Dlp to promote or inhibit signaling in a context-dependent manner. Notum-induced shedding of Dlp could convert Dlp from a membrane-tethered coreceptor to a secreted antagonist.     

Wingless gradient formation in the Drosophila wing

BACKGROUND: Secreted signaling proteins of the Wingless (Wg)/Wnt, Hedgehog and bone morphogenetic protein (BMP)/Decapentaplegic (Dpp) families function as morphogens to control growth and pattern formation during development. Although these proteins have been shown to act directly on distant cells in the developing limbs of the fruit fly Drosophila, little is known about how ligand gradients form in vivo. Wg protein is found in vesicles in Wg-responsive cells in the embryo and imaginal discs. It has been proposed that Wg may be transported by a vesicle-mediated mechanism. RESULTS: A novel method to visualize extracellular Wg protein was used to show that Wg forms an unstable gradient on the basolateral surface of the wing imaginal disc epithelium. Wg movement did not require internalization by dynamin-mediated endocytosis. Dynamin activity was, however, required for Wg secretion. By reversibly blocking Wg secretion, we found that Wg moves rapidly to form a long-range extracellular gradient. CONCLUSIONS: The Wg morphogen gradient forms by rapid movement of ligand through the extracellular space, and depends on continuous secretion and rapid turnover. Endocytosis is not required for Wg movement, but contributes to shaping the gradient by removing extracellular Wg. We propose that the extracellular Wg gradient forms by diffusion.     

Carrier of Wingless (Cow), a Secreted Heparan Sulfate Proteoglycan,Promotes Extracellular Transport of Wingless

Morphogens are signaling molecules that regulate growth and patterning during development by forming a gradient and activating different target genes at different concentrations. The extracellular distribution of morphogens is tightly regulated, with the Drosophila morphogen Wingless (Wg) relying on Dally-like (Dlp) and transcytosis for its distribution. However, in the absence of Dlp or endocytic activity, Wg can still move across cells along the apical (Ap) surface. We identified a novel secreted heparan sulfate proteoglycan (HSPG) that binds to Wg and promotes its extracellular distribution by increasing Wg mobility, which was thus named Carrier of Wg (Cow). Cow promotes the Ap transport of Wg, independent of Dlp and endocytosis, and this function addresses a previous gap in the understanding of Wg movement. This is the first example of a diffusible HSPG acting as a carrier to promote the extracellular movement of a morphogen.     

Self-enhanced ligand degradation underlies robustness of morphogen gradients

Morphogen gradients provide long-range positional information by extending across a developing field. To ensure reproducible patterning, their profile is invariable despite genetic or environmental fluctuations. Common models assume a morphogen profile that decays exponentially. Here, we show that exponential profiles cannot, at the same time, buffer fluctuations in morphogen production rate and define long-range gradients. To comply with both requirements, morphogens should decay rapidly close to their source but at a significantly slower rate over most of the field. Numerical search revealed two network designs that support robustness to fluctuations in morphogen production rate. In both cases, morphogens enhance their own degradation, leading to a higher degradation rate close to their source. This is achieved through reciprocal interactions between the morphogen and its receptor. The two robust networks are consistent with properties of the Wg and Hh morphogens in the Drosophila wing disc and provide novel insights into their function.     

A zebrafish Notum homolog specifically blocks the Wnt/β-catenin signaling pathway

Multiple developmental processes require tightly controlled Wnt signaling, and its misregulation leads to congenital abnormalities and diseases. Glypicans are extracellular proteins that modulate the Wnt pathway. In addition to interacting with Wnts, these glycosophosphotidylinositol (GPI)-anchored, heparan-sulfate proteoglycans bind ligands of several other signaling pathways in both vertebrates and invertebrates. In Drosophila, Notum, a secreted α/β-hydrolase, antagonizes the signaling of the prototypical Wnt Wingless (Wg), by releasing glypicans from the cell surface. Studies of mammalian Notum indicate promiscuous target specificity in cell culture, but the role of Notum in vertebrate development has not been studied. Our work shows that zebrafish Notum 1a, an ortholog of mammalian Notum, contributes to a self-regulatory loop that restricts Wnt/β-catenin signaling. Notum 1a does not interact with Glypican 4, an essential component of the Wnt/planar cell polarity (PCP) pathway. Our results suggest a surprising specific role of Notum in the developing vertebrate embryo.     

On the role of glypicans in the process of morphogen gradient formation

Glypicans are cell surface molecules that influence signaling and gradient formation of secreted morphogens and growth factors. Several distinct functions have been ascribed to glypicans including acting as co-receptors for signaling proteins. Recent data show that glypicans are also necessary for morphogen propagation in the tissue. In the present study, a model describing the interaction of a morphogen with glypicans is formulated, analyzed and compared with measurements of the effect of glypican Dally-like (Dlp) overexpression on Wingless (Wg) morphogen signaling in Drosophila melanogaster wing imaginal discs. The model explains the opposing effect that Dlp overexpression has on Wg signaling in the distal and proximal regions of the disc and makes a number of quantitative predictions for further experiments. In particular, our model suggests that Dlp acts by allowing Wg to diffuse on cell surface while protecting it from loss and degradation, and that Dlp rather than acting as Wg co-receptor competes with receptors for morphogen binding.     

Positional information and patterning revisited

The concept of positional information proposes that cells acquire positional values as in a coordinate system, which they interpret by developing in particular ways to give rise to spatial patterns. Some of the best evidence for positional information comes from regeneration experiments, and the patterning of the leg and antenna in Drosophila and the vertebrate limb. Central problems are how positional information is set up, how it is recorded, and then how it is interpreted by the cells. A number of models have been proposed for the setting up of positional gradients, and most are based on diffusion of a morphogen and its interactions with extracellular molecules. It is argued that diffusion may not be reliable mechanism. There are also mechanisms based on timing. There is no good evidence for the quantitative aspects of any of the gradients and details how they are set up. The way in which a signalling gradient regulates differential gene expression in a concentration-dependent manner also raises several mechanistic issues.     

Implications of diffusion and time-varying morphogen gradients for the dynamic positioning and precision of bistable gene expression boundaries

The earliest models for how morphogen gradients guide embryonic patterning failed to account for experimental observations of temporal refinement in gene expression domains. Following theoretical and experimental work in this area, dynamic positional information has emerged as a conceptual framework to discuss how cells process spatiotemporal inputs into downstream patterns. Here, we show that diffusion determines the mathematical means by which bistable gene expression boundaries shift over time, and therefore how cells interpret positional information conferred from morphogen concentration. First, we introduce a metric for assessing reproducibility in boundary placement or precision in systems where gene products do not diffuse, but where morphogen concentrations are permitted to change in time. We show that the dynamics of the gradient affect the sensitivity of the final pattern to variation in initial conditions, with slower gradients reducing the sensitivity. Second, we allow gene products to diffuse and consider gene expression boundaries as propagating wavefronts with velocity modulated by local morphogen concentration. We harness this perspective to approximate a PDE model as an ODE that captures the position of the boundary in time, and demonstrate the approach with a preexisting model for Hunchback patterning in fruit fly embryos. We then propose a design that employs antiparallel morphogen gradients to achieve accurate boundary placement that is robust to scaling. Throughout our work we draw attention to tradeoffs among initial conditions, boundary positioning, and the relative timescales of network and gradient evolution. We conclude by suggesting that mathematical theory should serve to clarify not just our quantitative, but also our intuitive understanding of patterning processes.     

HSPG modification by the secreted enzyme Notum shapes the Wingless morphogen gradient

The secreted signaling protein Wingless acts as a morphogen to pattern the imaginal discs of Drosophila. Here we report identification of a secreted repressor of Wingless activity, which we call Notum. Loss of Notum function leads to increased Wingless activity by altering the shape of the Wingless protein gradient. When overexpressed, Notum blocks Wingless activity. Notum encodes a member of the alpha/beta-hydrolase superfamily, with similarity to pectin acetylesterases. We present evidence that Notum influences Wingless protein distribution by modifying the heparan sulfate proteoglycans Dally-like and Dally. High levels of Wingless signaling induce Notum expression. Thus, Wingless contributes to shaping its own gradient by regulating expression of a protein that modifies its interaction with cell surface proteoglycans.     

When are cellular oscillators sufficient for sequential segmentation?

Sequential segmentation during embryogenesis involves the generation of a repeated pattern along the embryo, which is concurrently undergoing axial elongation by cell division. Most mathematical models of sequential segmentation involve inherent cellular oscillators, acting as a segmentation clock. The cellular oscillation is assumed to be governed by the cell's physiological age or by its interaction with an external morphogen gradient. Here, we address the issue of when cellular oscillators alone are sufficient for predicting segmentation, and when a morphogen gradient is required. The key to resolving this issue lies in how cells determine positional information in the model - this is directly related to the distribution of cell divisions responsible for axial elongation. Mathematical models demonstrate that if axial elongation occurs through cell divisions restricted to the posterior end of the unsegmented region, a cell can obtain its positional information from its physiological age, and therefore cellular oscillators will suffice. Alternatively, if axial elongation occurs through cell divisions distributed throughout the unsegmented region, then positional information can be obtained through another mechanism, such as a morphogen gradient. Two alternative ways to establish a morphogen gradient in tissue with distributed cell divisions are presented - one with diffusion and the other without diffusion. Our model produces segment polarity and a distribution of segment size from the anterior-to-posterior ends, as observed in some systems. Furthermore, the model predicts segment deletions when there is an interruption in cell division, just as seen in heat shock experiments, as well as the growth and final shrinkage of the presomitic mesoderm during somitogenesis.     

Interpretation of the FGF8 morphogen gradient is regulated by endocytic trafficking

Forty years ago, it was proposed that during embryonic development and organogenesis, morphogen gradients provide positional information to the individual cells within a tissue leading to specific fate decisions. Recently, much insight has been gained into how such morphogen gradients are formed and maintained; however, which cellular mechanisms govern their interpretation within target tissues remains debated. Here we used in vivo fluorescence correlation spectroscopy and automated image analysis to assess the role of endocytic sorting dynamics on fibroblast growth factor 8 (Fgf8) morphogen gradient interpretation. By interfering with the function of the ubiquitin ligase Cbl, we found an expanded range of Fgf target gene expression and a delay of Fgf8 lysosomal transport. However, the extracellular Fgf8 morphogen gradient remained unchanged, indicating that the observed signalling changes are due to altered gradient interpretation. We propose that regulation of morphogen signalling activity through endocytic sorting allows fast feedback-induced changes in gradient interpretation during the establishment of complex patterns.     

Distinct and collaborative roles of Drosophila EXT family proteins in morphogen signalling and gradient formation

Heparan sulfate proteoglycans (HSPG) have been implicated in regulating the signalling activities of secreted morphogen molecules including Wingless (Wg), Hedgehog (Hh) and Decapentaplegic (Dpp). HSPG consists of a protein core to which heparan sulfate (HS) glycosaminoglycan (GAG) chains are attached. The formation of HS GAG chains is catalyzed by glycosyltransferases encoded by members of the EXT family of putative tumor suppressors linked to hereditary multiple exostoses. Previous studies in Drosophila demonstrated that tout-velu (ttv), the Drosophila EXT1, is required for Hh movement. However, the functions of other EXT family members are unknown. We have identified and isolated the other two members of the Drosophila EXT family genes, which are named sister of tout-velu (sotv) and brother of tout-velu (botv), and encode Drosophila homologues of vertebrate EXT2 and EXT-like 3 (EXTL3), respectively. We show that both Hh and Dpp signalling activities, as well as their morphogen distributions, are defective in cells mutant for ttv, sotv or botv in the wing disc. Surprisingly, although Wg morphogen distribution is abnormal in ttv, sotv and botv, Wg signalling is only defective in botv mutants or ttv-sotv double mutants, and not in ttv nor sotv alone, suggesting that Ttv and Sotv are redundant in Wg signalling. We demonstrate further that Ttv and Sotv form a complex and are co-localized in vivo. Our results, along with previous studies on Ttv, provide evidence that all three Drosophila EXT proteins are required for the biosynthesis of HSPGs, and for the gradient formation of the Wg, Hh and Dpp morphogens. Our results also suggest that HSPGs have two distinct roles in Wg morphogen distribution and signalling.     

Evidence for Wg-independent tergite boundary formation in the millipede Glomeris marginata

The correlation between dorsal and ventral segmental units in diplopod myriapods is complex and disputed. Recent results with engrailed (en), hedgehog (hh), wingless (wg), and cubitus-interruptus (ci) have shown that the dorsal segments are patterned differently from the ventral segments. Ventrally, gene expression is compatible with the classical autoregulatory loop known from Drosophila to specify the parasegment boundary. In the dorsal segments, however, this Wg/Hh autoregulatory loop cannot be present because the observed gene expression patterns argue against the involvement of Wg signalling. In this paper, we present further evidence against an involvement of Wg signalling in dorsal segmentation and propose a hypothesis about how dorsal segmental boundaries may be controlled in a wg-independent way. We find that (1) the Notum gene, a modulator of the Wg gradient in Drosophila, is not expressed in the dorsal segments. (2) The H15/midline gene, a repressor of Wg action in Drosophila, is not expressed in the dorsal segments, except for future heart tissue. (3) The patched (ptc) gene, which encodes a Hh receptor, is strongly expressed in the dorsal segments, which is incompatible with Wg-Hh autoregulation. The available data suggest that anterior-posterior (AP) boundary formation in dorsal segments could instead rely on Dpp signalling rather than Wg signalling. We present a hypothesis that relies on Hh-mediated activation of Dpp signalling and optomotor-blind (omb) expression to establish the dorsal AP boundary (the future tergite boundary). The proposed mechanism is similar to the mechanism used to establish the AP boundary in Drosophila wings and ventral pleura.     

Smiling Gurken gradient: An expansion of the Gurken gradient

Morphogen gradients provide unique positional information within a tissue. Cells that are sensitive to the concentration of the morphogen integrate this signal and develop an appropriately distinct cell fate. A morphogen gradient is usually generated by a restricted source and shaped by the speed of diffusion and stability of the signaling molecule. In addition, the availability of receptor and Heparan Sulfate Proteoglycans (HSPGs) help to shape the gradient. We have shown that overexpression of Dally-like protein (Dlp) causes an expansion of Gurken distribution and a loss of cell fates which are specified by high levels of epidermal growth factor receptor (Egfr) signaling. In this article, we discuss how D-Cbl mediated Egfr endocytosis and the levels of Dlp affect the shape of the Gurken gradient.