ThreeDrosophilaEXT genes shape morphogen gradients through synthesis of heparan sulfate proteoglycans

Drosophila glypican Dally-like acts in FGF-receiving cells to modulate FGF signaling during tracheal morphogenesis

Previous studies in Drosophila have shown that heparan sulfate proteoglycans (HSPGs) are involved in both breathless (btl)- and heartless (htl)-mediated FGF signaling during embryogenesis. However, the mechanism(s) by which HSPGs control Btl and Htl signaling is unknown. Here we show that dally-like (dlp, a Drosophila glypican) mutant embryos exhibit severe defects in tracheal morphogenesis and show a reduction in btl-mediated FGF signaling activity. However, htl-dependent mesodermal cell migration is not affected in dlp mutant embryos. Furthermore, expression of Dlp, but not other Drosophila HSPGs, can restore effectively the tracheal morphogenesis in dlp embryos. Rescue experiments in dlp embryos demonstrate that Dlp functions only in Bnl/FGF receiving cells in a cell-autonomous manner, but is not essential for Bnl/FGF expression cells. To further dissect the mechanism(s) of Dlp in Btl signaling, we analyzed the role of Dlp in Btl-mediated air sac tracheoblast formation in wing discs. Mosaic analysis experiments show that removal of HSPG activity in FGF-producing or other surrounding cells does not affect tracheoblasts migration, while HSPG mutant tracheoblast cells fail to receive FGF signaling. Together, our results argue strongly that HSPGs regulate Btl signaling exclusively in FGF-receiving cells as co-receptors, but are not essential for the secretion and distribution of the FGF ligand. This mechanism is distinct from HSPG functions in morphogen distribution, and is likely a general paradigm for HSPG functions in FGF signaling in Drosophila.

Contribution of EXT1, EXT2, and EXTL3 to heparan sulfate chain elongation

The exostosin (EXT) family of genes encodes glycosyltransferases involved in heparan sulfate biosynthesis. Five human members of this family have been cloned to date: EXT1, EXT2, EXTL1, EXTL2, and EXTL3. EXT1 and EXT2 are believed to form a Golgi-located hetero-oligomeric complex that catalyzes the chain elongation step in heparan sulfate biosynthesis, whereas the EXTL proteins exhibit overlapping glycosyl-transferase activities in vitro, so that it is not apparent what reactions they catalyze in vivo. We used gene-silencing strategies to investigate the roles of EXT1, EXT2, and EXTL3 in heparan sulfate chain elongation. Small interfering RNAs (siRNAs) directed against the human EXT1, EXT2, or EXTL3 mRNAs were introduced into human embryonic kidney 293 cells. Compared with cells transfected with control siRNA, those transfected with EXT1 or EXT2 siRNA synthesized shorter heparan sulfate chains, and those transfected with EXTL3 siRNA synthesized longer chains. We also generated human cell lines overexpressing the EXT proteins. Overexpression of EXT1 resulted in increased HS chain length, which was even more pronounced in cells coexpressing EXT2, whereas overexpression of EXT2 alone had no detectable effect on heparan sulfate chain elongation. Mutations in either EXT1 or EXT2 are associated with hereditary multiple exostoses, a human disorder characterized by the formation of cartilage-capped bony outgrowths at the epiphyseal growth plates. To further investigate the role of EXT2, we generated human cell lines overexpressing mutant EXT2. One of the mutations, EXT2-Y419X, resulted in a truncated protein. Interestingly, the capacity of wild type EXT2 to enhance HS chain length together with EXT1 was not shared by the EXT2-Y419X mutant.

Lipoprotein-heparan sulfate interactions in the Hh pathway

The Drosophila lipoprotein particle, Lipophorin, bears lipid-linked morphogens on its surface and is required for long-range signaling activity of Wingless and Hedgehog. Heparan sulfate proteoglycans are also critical for trafficking and signaling of these morphogens. Here we show that Lipophorin interacts with the heparan sulfate moieties of the glypicans Dally and Dally-like. Membrane-associated glypicans can recruit Lipophorin to disc tissue, and remain associated with these particles after they are released from the membrane by cleavage of their gpi anchors. The released form of Dally colocalizes with Patched, Hedgehog, and Lipophorin in endosomes and increases Hedgehog signaling efficiency without affecting its distribution. These data suggest that heparan sulfate proteoglycans may influence lipid-linked morphogen signaling, at least in part, by binding to Lipophorin. They further suggest that the complement of proteins present on lipoprotein particles can regulate the activity of morphogens.

Functional conservation of the human EXT1 tumor suppressor gene and its Drosophila homolog tout velu

Heparan sulfate proteoglycans play a vital role in signaling of various growth factors in both Drosophila and vertebrates. In Drosophila, mutations in the tout velu (ttv) gene, a homolog of the mammalian EXT1 tumor suppressor gene, leads to abrogation of glycosaminoglycan (GAG) biosynthesis. This impairs distribution and signaling activities of various morphogens such as Hedgehog (Hh), Wingless (Wg), and Decapentaplegic (Dpp). Mutations in members of the exostosin (EXT) gene family lead to hereditary multiple exostosis in humans leading to bone outgrowths and tumors. In this study, we provide genetic and biochemical evidence that the human EXT1 (hEXT1) gene is conserved through species and can functionally complement the ttv mutation in Drosophila. The hEXT1 gene was able to rescue a ttv null mutant to adulthood and restore GAG biosynthesis.

NDST-1 modulates BMPR and PTHrP signaling during endochondral bone formation in a gene knockout model

GlcNAc N-deacetylase/N-sulfotransferase-1 (NDST-1), a member of the enzyme family catalyzing the first modification step in the biosynthesis of heparan sulfate (HS), was knocked out in mice to investigate its role in embryonic development. NDST-1 null mice exhibited delayed endochondral bone formation including shortened calcified zones in limbs, delayed chondrocyte and osteogenetic differentiation, and increased chondrocyte proliferation. In situ HS binding assay revealed that the binding ability of bone morphogenetic protein (BMP) -2, -4, and -6 to endogenous HS was decreased in mutant phalanges, while that of fibroblast growth factor-1 (FGF-1) was not affected. Up-regulation of BMPR-IA, Phospho-Smad1 (P-Smad1) and parathyroid-hormone related protein (PTHrP), but not the Indian hedgehog, Gli1, Gli3, Patched, and FGFR-3, was observed. Furthermore, block of BMPR signaling with noggin rescued the delayed chondrocyte hypertrophic differentiation in NDST-1 (-/-) mice and recovered the expression of both P-Smad1 and PTHrP proteins. These results suggested that NDST-1-dependent heparan sulfate might negatively modulate BMP and its downstream PTHrP signaling, and thus affect endochondral bone development.

Quantitative analysis of Hedgehog gradient formation using an inducible expression system

Background

The Hedgehog (Hh) family of secreted growth factors are morphogens that act in development to direct growth and patterning. Mutations in human Hh and other Hh pathway components have been linked to human diseases. Analysis of Hh distribution during development indicates that cholesterol modification and receptor mediated endocytosis affect the range of Hh signaling and the cellular localization of Hh.

Results

We have used an inducible, cell type-specific expression system to characterize the three-dimensional distribution of newly synthesized, GFP-tagged Hh in the developing Drosophila wing. Following induction of Hh-GFP expression in posterior producing cells, punctate structures containing Hh-GFP were observed in the anterior target cells. The distance of these particles from the expressing cells was quantified to determine the shape of the Hh gradient at different time points following induction. The majority of cholesterol-modified Hh-GFP was found associated with cells near the anterior/posterior (A/P) boundary, which express high levels of Hh target genes. Without cholesterol, the Hh gradient was flatter, with a lower percentage of particles near the source and a greater maximum distance. Inhibition of Dynamin-dependent endocytosis blocked formation of intracellular Hh particles, but did not prevent movement of newly synthesized Hh to the apical or basolateral surfaces of target cells. In the absence of both cholesterol and endocytosis, Hh particles accumulated in the extracellular space. Staining for the Hh receptor Ptc revealed four categories of Hh particles: cytoplasmic with and without Ptc, and cell surface with and without Ptc. Interestingly, mainly cholesterol-modified Hh is detected in the cytoplasmic particles lacking Ptc.

Conclusion

We have developed a system to quantitatively analyze Hh distribution during gradient formation. We directly demonstrate that inhibition of Dynamin-dependent endocytosis is not required for movement of Hh across target cells, indicating that transcytosis is not required for Hh gradient formation. The localization of Hh in these cells suggests that Hh normally moves across both apical and basolateral regions of the target cells. We also conclude that cholesterol modification is required for formation of a specific subset of Hh particles that are both cytoplasmic and not associated with the receptor Ptc.

Novel 'phage display antibodies identify distinct heparan sulfate domains in developing mammalian lung

Heparan sulfate proteoglycans (HSPGs) are essential to respiratory morphogenesis in species as diverse as Drosophila and mice; they play a role in the regulation of numerous HS-binding growth factors, e.g. fibroblast growth factors. Moreover, an HS analogue, heparin, modulates lung growth in vitro. However, it has been difficult to assess the roles of specific HS structures in lung development due to technical barriers to their spatial localisation. Lungs from Sprague-Dawley rats were harvested between E15.5 and E19.5 and immediately fixed in 4 % (w/v) paraformaldehyde (in 0.1 M phosphate-buffered saline (PBS), pH 7.4). Lungs were washed in PBS, cryoprotected with 20% (w/v) sucrose (in PBS), gelatin embedded [7.5% (w/v) gelatin, 15% (w/v) sucrose in PBS], before being covered in Cryo-M-Bed (Bright, Huntingdon, UK) and snap frozen at -40 degrees C. Cryosections were cut at 8 microm and stained with the HSPG core protein specific antibody 3G10 and a HS 'phage display antibody, EW4G2V. 3G10 and EW4G2V immunohistochemistry highlighted the presence of specific HS structures in lungs at all gestational ages examined. 3G10 strongly labelled airway basement membranes and the surrounding mesenchyme and showed weak staining of airway epithelial cells. EW4G2V, however, was far more selective, labelling the airway basement membranes only. Mesenchymal and epithelial cells did not appear to possess the HS epitope recognised by EW4G2V at these gestational ages. Novel 'phage display antibodies allow the spatial distribution of tissue HS to be analysed, and demonstrate in situ that distinct cellular compartments of a tissue possess different HS structures, possibly on the same proteoglycan core protein. These probes offer a new opportunity to determine the role of HS in the pathogenesis of congenital defects such as congenital diaphragmatic hernia (CDH), where lung development is aberrant, and the resulting pulmonary hypoplasia and hypertension are a primary cause of mortality.

toutvelu, a regulator of heparan sulfate proteoglycan biosynthesis, controls guidance cues for germ-cell migration

The primitive embryonic gonad in Drosophila melanogaster is composed of germ cells and somatic gonadal precursor cells (SGPs). The assembly of a functional gonad involves a complex series of germ-cell migration events, which are thought to be guided by attractive and repulsive cues. Here, we demonstrate a novel role for toutvelu (ttv), a regulator of heparan sulfate proteoglycan biosynthesis during this process. Germline clonal analysis suggests that maternal deposition of ttv is required for proper germ-cell migration. Conversely, ectopic expression of ttv in early embryos results in severe germ-cell migration defects and inappropriate spreading of Hh protein. Moreover, overexpression of ttv in only the receiving cells, rather than in the sending cells, leads to phenotypic consequences. Finally, supporting the claim that the signaling molecule Hedgehog (Hh) may function as a chemoattractant to guide germ cells, errant germ cells are found localized near pockets containing high concentrations of Hh protein.

Tinkering with heparan sulfate sulfation to steer development

Heparan sulfate (HS) proteoglycans, at the cell surface and extracellular matrix, facilitate ligand-receptor interactions crucial to many physiological processes. The distinct sulfation patterns of HS sugar chains presented by their protein core are key to HS proteoglycan activity. Tight regulation of several Golgi complex enzyme families is crucial to produce complex tissue-specific HS sequences. Several in vivo models deficient in HS biosynthesis enzymes demonstrate that developmental abnormalities result from modified HS structure. This review will discuss the plasticity of sulfation requirements on HS for activating protein ligands, which might reflect a flexible HS biosynthetic mechanism. In addition, the latest discovery of HS acting enzymes, the Sulfs, responsible for extracellular tweaking of HS sulfation levels subsequent to biosynthesis will be considered.

Hitch-hiking between cells on lipoprotein particles

Cell surface proteins containing covalently linked lipids associate with specialized membrane domains. Morphogens like Hedgehog and Wnt use their lipid anchors to bind to lipoprotein particles and employ lipoproteins to travel through tissues. Removal of their lipid anchors or decreasing lipoprotein levels give rise to adverse Hedgehog and Wnt signaling. Some parasites can also transfer their glycosylphosphatidylinositol-anchored surface proteins to host lipoprotein particles. These antigen-loaded lipoproteins spread throughout the circulation, and probably hamper an adequate immune response by killing neutrophils. Together, these findings imply a widespread role for lipoproteins in intercellular transfer of lipid-anchored surface proteins, and may have various physiological consequences. Here, we discuss how lipid-modified proteins may be transferred to and from lipoproteins at the cellular level.

Expression of rib-1, a Caenorhabditis elegans homolog of the human tumor suppressor EXT genes, is indispensable for heparan sulfate synthesis and embryonic morphogenesis

The proteins encoded by all of the five cloned human EXT family genes (EXT1, EXT2, EXTL1, EXTL2, and EXTL3), members of the hereditary multiple exostoses gene family of tumor suppressors, are glycosyltransferases required for the biosynthesis of heparan sulfate. In the Caenorhabditis elegans genome, only two genes, rib-1 and rib-2, homologous to the mammalian EXT genes have been identified. Although rib-2 encodes an N-acetylglucosaminyltransferase involved in initiating the biosynthesis and elongation of heparan sulfate, the involvement of the protein encoded by rib-1 in the biosynthesis of heparan sulfate remains unclear. Here we report that RIB-1 is indispensable for the biosynthesis and for embryonic morphogenesis. Despite little individual glycosyltransferase activity by RIB-1, the polymerization of heparan sulfate chains was demonstrated when RIB-1 was coexpressed with RIB-2 in vitro. In addition, RIB-1 and RIB-2 were demonstrated to interact by pulldown assays. To investigate the functions of RIB-1 in vivo, we depleted the expression of rib-1 by deletion mutagenesis. The null mutant worms showed reduced synthesis of heparan sulfate and embryonic lethality. Notably, the null mutant embryos showed abnormality at the gastrulation cleft formation stage or later and arrested mainly at the 1-fold stage. Nearly 100% of the embryos died before L1 stage, although the differentiation of some of the neurons and muscle cells proceeded normally. Similar phenotypes have been observed in rib-2 null mutant embryos. Thus, RIB-1 in addition to RIB-2 is indispensable for the biosynthesis of heparan sulfate in C. elegans, and the two cooperate to synthesize heparan sulfate in vivo. These findings also show that heparan sulfate is essential for post-gastrulation morphogenic movement of embryonic cells and is indispensable for ensuring the normal spatial organization of differentiated tissues and organs.

Detecting tagged Hedgehog with intracellular and extracellular immunocytochemistry for functional analysis

In this chapter, we explain different strategies to analyze the extracellular Hedgehog (Hh) morphogen distribution and Hh intracellular trafficking by immunohistochemistry techniques. For this purpose, it has been very useful to have a transgenic fly line that expresses a Hh-green fluorescent protein (GFP) fusion protein. These flies can be used to study the way Hh spreads through the anterior compartment where it signals, and analyze in detail how Hh is internalized by its receptor Patched. In addition, this Hh-GFP fusion made without lipid modifications (cholesterol or palmitic acid) can be used to investigate the function of these lipids on Hh in terms of spreading, internalization, and signaling abilities.

Heparan Sulfate Proteoglycans (HSPGs) Modulate BMP2 Osteogenic Bioactivity in C2C12 Cells

Cell surface heparan sulfate proteoglycans (HSPGs) have been implicated in bone morphogenetic protein (BMP)-mediated morphogenesis by regulating BMP activity and gradient formation. However, the direct role of HSPGs in BMP signaling is poorly understood. Here we show that HSPGs directly regulate BMP2-mediated transdifferentiation of C2C12 myoblasts into osteoblasts. HSPGs sequester BMP2 at the cell surface and mediate BMP2 internalization. Depletion of cell surface HSPGs by heparinase III treatment or decreased glycosaminoglycan chain sulfation with sodium chlorate enhances BMP2 morpho-genetic bioactivity. The addition of exogenous heparin, a widely used anticoagulant, reduced BMP2 signaling. Our results suggest that cell surface HSPGs mediate BMP2 internalization and modulate BMP2 osteogenic activity.

The molecular diversity of glycosaminoglycans shapes animal development

Proteoglycans (PGs), molecules in which glycosaminoglycans (GAGs) are covalently linked to a protein core, are components of the extracellular matrix of all multicellular organisms. Sugar moieties in GAGs are often extensively modified, which make these molecules enormously complex. We discuss here the role of PGs during animal development, emphasizing the in vivo significance of sugar modifications. We explore a model in which the modification patterns of GAG chains may provide a specific code that contributes to the correct development of a multicellular organism.

How does cholesterol affect the way Hedgehog works?

Members of the Hedgehog (Hh) family of proteins are conserved morphogens that spread and modulate cell fates in target tissue. Mature Hh carries two lipid adducts, a palmitoyl group at the N terminus and cholesterol at the C terminus. Recent findings have addressed how these lipid modifications affect the function and transport of Hh in Drosophila. In contrast to the palmitoyl adduct, cholesterol appears not to be essential for signalling. However, the absence of the cholesterol adduct affects the spread of Hh within tissues. As we discuss here, the exact nature of this effect is controversial.

The function of a Drosophila glypican does not depend entirely on heparan sulfate modification

Division abnormally delayed (Dally) is one of two glycosylphosphatidylinositol (GPI)-linked heparan sulfate proteoglycans in Drosophila. Numerous studies have shown that it influences Decapentaplegic (Dpp) and Wingless signaling. It has been generally assumed that Dally affects signaling by directly interacting with these growth factors, primarily through its heparan sulfate (HS) chains. To understand the functional contributions of HS chains and protein core we have (1) assessed the growth factor binding properties of purified Dally using surface plasmon resonance, (2) generated a form of Dally that is not HS modified and evaluated its signaling capacity in vivo. Purified Dally binds directly to FGF2, FGF10, and the functional Dpp homolog BMP4. FGF binding is abolished by preincubation with HS, but BMP4 association is partially HS-resistant, suggesting the Dally protein core contributes to binding. Cell binding and co-immunoprecipitation studies suggest that non-HS-modified Dally retains some ability to bind Dpp or BMP4. Expression of HS-deficient Dally in vivo showed it does not promote signaling as well as wild-type Dally, yet it can rescue several dally mutant phenotypes. These data reveal that heparan sulfate modification of Dally is not required for all in vivo activities and that significant functional capacity resides in the protein core.

Identification and characterization of a novel Drosophila 3'-phosphoadenosine 5'-phosphosulfate transporter

Sulfation of macromolecules requires the translocation of a high energy form of nucleotide sulfate, i.e. 3'-phosphoadenosine 5'-phosphosulfate (PAPS), from the cytosol into the Golgi apparatus. In this study, we identified a novel Drosophila PAPS transporter gene dPAPST2 by conducting data base searches and screening the PAPS transport activity among the putative nucleotide sugar transporter genes in Drosophila. The amino acid sequence of dPAPST2 showed 50.5 and 21.5% homology to the human PAPST2 and SLALOM, respectively. The heterologous expression of dPAPST2 in yeast revealed that the dPAPST2 protein is a PAPS transporter with an apparent K(m) value of 2.3 microm. The RNA interference of dPAPST2 in cell line and flies showed that the dPAPST2 gene is essential for the sulfation of cellular proteins and the viability of the fly. In RNA interference flies, an analysis of the genetic interaction between dPAPST2 and genes that contribute to glycosaminoglycan synthesis suggested that dPAPST2 is involved in the glycosaminoglycan synthesis and the subsequent signaling. The dPAPST2 and sll genes showed a similar ubiquitous distribution. These results indicate that dPAPST2 may be involved in Hedgehog and Decapentaplegic signaling by controlling the sulfation of heparan sulfate.

The ihog cell-surface proteins bind Hedgehog and mediate pathway activation

The ihog gene (interference hedgehog), identified by RNA interference in Drosophila cultured cells, encodes a type 1 membrane protein shown here to bind and to mediate response to the active Hedgehog (Hh) protein signal. ihog mutations produce defects characteristic of Hh signaling loss in embryos and imaginal discs, and epistasis analysis places ihog action at or upstream of the negatively acting receptor component, Patched (Ptc). The first of two extracellular fibronectin type III (FNIII) domains of the Ihog protein mediates a specific interaction with Hh protein in vitro, but the second FNIII domain is additionally required for in vivo signaling activity and for Ihog-enhanced binding of Hh protein to cells coexpressing Ptc. Other members of the Ihog family, including Drosophila Boi and mammalian CDO and BOC, also interact with Hh ligands via a specific FNIII domain, thus identifying an evolutionarily conserved family of membrane proteins that function in Hh signal response.

A C-terminal motif targets Hedgehog to axons, coordinating assembly of the Drosophila eye and brain

The developmental signal Hedgehog is distributed to two receptive fields by the photoreceptor neurons of the developing Drosophila retina. Delivery to the retina propagates ommatidial development across a precursor field. Transport along photoreceptor axons induces the development of postsynaptic neurons in the brain. Hedgehog is composed of N-terminal and C-terminal domains that dissociate in an autoproteolytic reaction that attaches cholesterol to the N-terminal cleavage product. Here, we show that the N-terminal domain is targeted to the retina when synthesized in the absence of the C-terminal domain. In contrast to studies that have focused on cholesterol as a determinant of subcellular localization, we find that the C-terminal domain harbors a conserved motif that overrides retinal localization, sending most of the autocleavage products into vesicles bound for growth cones or synapses. Competition between targeting signals at the opposite ends of Hedgehog apparently controls the match between eye and brain development.

Shedding light on the distinct functions of proteoglycans

Heparan sulfate proteoglycans (HSPGs) are ubiquitous molecules that are critical for signaling mediated by many growth factors, including members of the Wnt, transforming growth factor-beta, Hedgehog, and fibroblast growth factor families, and are essential for cell specification, axon guidance, and the establishment of morphogen gradients. Although the heparan sulfate modifications of HSPGs are critical, there is much to learn about how the protein cores contribute to the specific signaling functions of these cell-surface and matrix molecules. Recent work has demonstrated that glypican-1 and syndecan-1 expressed by tumor cells have specific roles in FGF2 signaling, affecting their responses to this mitogenic stimulus.

Mummy encodes an UDP-N-acetylglucosamine-dipohosphorylase and is required during Drosophila dorsal closure and nervous system development

Throughout development cell-cell interactions are of pivotal importance. Cells bind to each other or share information via secreted signaling molecules. To a large degree, these processes are modulated by post-translational modifications of membrane proteins. Glycan-chains are frequently added to membrane proteins and assist their exact function at the cell surface. In addition, the glycosylation pathway is required to generate GPI-linkage in the endoplasmatic reticulum. Here, we describe the analysis of the cabrio/mummy gene, which encodes an UDP-N-acetylglucosamine diphosphorylase. This is a well-conserved and central enzyme in the glycosylation pathway. As expected from this central role in glycosylation, cabrio/mummy mutants show many phenotypic traits ranging from CNS fasciculation defects to defects in dorsal closure and eye development. These phenotypes correlate well with specific glycosylation and GPI-anchorage defects in mummy mutants.

Signal dynamics in Sonic hedgehog tissue patterning

During development, secreted signaling factors, called morphogens, instruct cells to adopt specific mature phenotypes. However, the mechanisms that morphogen systems employ to establish a precise concentration gradient for patterning tissue architecture are highly complex and are typically analyzed only at long times after secretion (i.e. steady state). We have developed a theoretical model that analyzes dynamically how the intricate transport and signal transduction mechanisms of a model morphogen, Sonic hedgehog (Shh),cooperate in modular fashion to regulate tissue patterning in the neural tube. Consistent with numerous recent studies, the model elucidates how the dynamics of gradient formation can be a key determinant of cell response. In addition,this work yields several novel insights into how different transport mechanisms or `modules' control pattern formation. The model predicts that slowing the transport of a morphogen, such as by lipid modification of the ligand Shh, by ligand binding to proteoglycans, or by the moderate upregulation of dedicated transport molecules like Dispatched, can actually increase the signaling range of the morphogen by concentrating it near the secretion source. Furthermore, several transcriptional targets of Shh, such as Patched and Hedgehog-interacting protein, significantly limit its signaling range by slowing transport and promoting ligand degradation. This modeling approach elucidates how individual modular elements that operate dynamically at various times during patterning can shape a tissue pattern.

Heparan sulfate proteoglycans exert positive and negative effects in Shh activity

Hedgehog (Hh) proteins are morphogens involved in short- and long-range effects during early embryonic development. Genetic analysis in fly and vertebrate embryos showed that heparan sulfate proteoglycans (HSPGs) are required for Hh transport and signaling. To further understand how HSPGs regulate Sonic hedgehog (Shh), we performed experiments using cell culture and biochemical assays. When the synthesis of HSPGs was reduced, a decrease in Shh activity was observed. Contrary to that, addition of a peptide that competes the binding of Shh to HSPGs resulted in augmentation of Shh activity. From these results, we concluded that HSPGs exert positive and negative effects in Shh activity. This dual effect correlates with the finding that Shh interacts preferentially with two HSPGs. The current model for the role of HSPGs in Shh diffusion is discussed in view of our findings.

Release and trafficking of lipid-linked morphogens

Wnt and Hedgehog family proteins are secreted morphogens that act on surrounding cells to pattern many different tissues in both vertebrates and invertebrates. The discovery that these proteins are covalently linked to lipids has raised the puzzling problem of how they come to be released from cells and move through tissue. A synergistic combination of biochemical, cell biological and genetic approaches over the past several years is beginning to illuminate both the forms in which lipid-linked morphogens are released from cells and the variety of molecular and cell biological mechanisms that control their dispersal.

The highly ordered assembly of retinal axons and their synaptic partners is regulated by Hedgehog/Single-minded in the Drosophila visual system

During development of the Drosophila visual center, photoreceptor cells extend their axons (R axons) to the lamina ganglion layer, and trigger proliferation and differentiation of synaptic partners (lamina neurons) by delivering the inductive signal Hedgehog (Hh). This inductive mechanism helps to establish an orderly arrangement of connections between the R axons and lamina neurons, termed a retinotopic map because it results in positioning the lamina neurons in close vicinity to the corresponding R axons. We found that the bHLH-PAS transcription factor Single-minded (Sim) is induced by Hh in the lamina neurons and is required for the association of lamina neurons with R axons. In sim mutant brains, lamina neurons undergo the first step of differentiation but fail to associate with R axons. As a result, lamina neurons are set aside from R axons. The data reveal a novel mechanism for regulation of the interaction between axons and neuronal cell bodies that establishes precise neuronal networks.

Global and local mechanisms of forebrain and midbrain patterning

During the past years, major advances have been made in understanding the sequential events involved in neural plate patterning. Positional information is already conferred to cells of the neural plate at the time of its induction in the ectoderm. The interplay between the BMP- and the Fgf- signaling pathways leads to the induction of neural cell fates. Thus, neural induction and neural plate patterning are overlapping processes. Later, at the end of gastrulation, positional cell identities within the neural plate are refined and maintained by the action of several neural plate organizers. By locally emitting signaling molecules, they influence the fate of the developing nervous system with high regional specificity. Recent advances have been made both in understanding the mechanisms that dictate the relative position of these organizers and in how signaling molecules spread from them with high spatial and temporal resolution.

Cholesterol modification is necessary for controlled planar long-range activity of Hedgehog in Drosophila epithelia

The Hedgehog morphogen is a major developmental regulator that acts at short and long range to direct cell fate decisions in invertebrate and vertebrate tissues. Hedgehog is the only known metazoan protein to possess a covalently linked cholesterol moiety. Although the role of the cholesterol group of Hedgehog remains unclear, it has been suggested to be dispensable for the its long-range activity in Drosophila. Here, we provide data in three different epithelia - ventral and dorsal embryonic ectoderm, and larval imaginal disc tissue - showing that cholesterol modification is in fact necessary for the controlled long-range activity of Drosophila Hedgehog. We provide an explanation for the discrepancy between our results and previous reports by showing that unmodified Hh can act at long range, albeit in an uncontrolled manner, only when expressed in squamous cells. Our data show that cholesterol modification controls long-range Hh activity at multiple levels. First, cholesterol increases the affinity of Hh for the plasma membrane, and consequently enhances its apparent intrinsic activity, both in vitro and in vivo. In addition, multimerisation of active Hh requires the presence of cholesterol. These multimers are correlated with the assembly of Hh into apically located, large punctate structures present in active Hh gradients in vivo. By comparing the activity of cholesterol-modified Hh in columnar epithelial cells and peripodial squamous cells, we show that epithelial cells provide the machinery necessary for the controlled planar movement of Hh, thereby preventing the unrestricted spreading of the protein within the three-dimensional space of the epithelium. We conclude that, as in vertebrates, cholesterol modification is essential for controlled long-range Hh signalling in Drosophila.

Hedgehog lipid modifications are required for Hedgehog stabilization in the extracellular matrix

The Hedgehog (Hh) family of morphogenetic proteins has important instructional roles in metazoan development. Despite Hh being modified by Ct-cholesterol and Nt-palmitate adducts, Hh migrates far from its site of synthesis and programs cellular outcomes, depending on its local concentrations. We show that in the receiving cells of the Drosophila wing imaginal disc, lipid-unmodified Hh spreads across many more cell diameters than the wild type and this spreading leads to the activation of low but not high threshold responses. Unlipidated Hh forms become internalized through the apical plasma membrane, while wild-type Hh enters through the basolateral cell surface - in all cases via a dynamin-dependent mechanism. Full activation of the Hh pathway and the spread of Hh throughout the extracellular matrix depend on the ability of lipid-modified Hh to interact with heparan sulfate proteoglycans (HSPG). However, neither Hh-lipid modifications nor HSPG function are required to activate the targets that respond to low levels of Hh. All these data show that the interaction of lipid-modified Hh with HSPG is important both for precise Hh spreading through the epithelium surface and for correct Hh reception.

Heparan Sulfate Polymerization in Drosophila

The formation of heparan sulfate (HS) chains is catalyzed by glycosyltransferases encoded by EXT (hereditary multiple exostosin gene) family members. Genetic screening for mutations affecting morphogen signaling pathways in Drosophila has identified three genes, tout-velu (ttv), sister of tout-velu (sotv), and brother of toutvelu (botv), which encode homologues of human EXT1, EXT2, and EXTL3, respectively. So far, in vitro glycosyltransferase activities have been demonstrated only for BOTV/DEXTL3, which harbors both N-acetylglucosaminyltransferase-I (GlcNAcT-I) and N-acetylglucosaminyltransferase-II (GlcNAcT-II) activities responsible for the chain initiation and elongation of HS, and no glucuronyltransferase-II (GlcAT-II) activity. Here we demonstrated that TTV/DEXT1 and SOTV/DEXT2 had GlcNAcT-II and GlcAT-II activities required for the biosynthesis of repeating disaccharide units of the HS backbone, and the coexpression of TTV with SOTV markedly augmented both glycosyltransferase activities when compared with the expression of TTV or SOTV alone. Moreover, the polymerization of HS was demonstrated on a linkage region analogue as an acceptor substrate by BOTV and an enzyme complex composed of TTV and SOTV (TTV-SOTV). In contrast to human, TTV-SOTV exhibited no GlcNAcT-I activity, indicating that BOTV/DEXT3, which is an EXT-Like gene and possesses GlcNAcT-I activity required for the initiation of HS, is indispensable for the biosynthesis of HS chains in Drosophila. Thus, all three EXT members in Drosophila, TTV, SOTV, and BOTV, are required for the biosynthesis of full-length HS in Drosophila.

Expression of a secreted form of Dally, a Drosophila glypican, induces overgrowth phenotype by affecting action range of Hedgehog

Glypicans, a family of heparan sulfate proteoglycans attached to the cell surface via a glycosylphosphatidylinositol (GPI)-anchor, play essential roles in morphogen signaling and distributions. A Drosophila glypican, Dally, regulates the gradient formation of Decapentaplegic (Dpp) in the developing wing. To gain insights into the function of glypicans in morphogen signaling, we examined the activities of two mutant forms of Dally: a transmembrane form (TM-Dally) and a secreted form (Sec-Dally). Misexpression of tm-dally in the wing disc had a similar yet weaker effect in enhancing Dpp signaling compared to that of wild-type dally. In contrast, Sec-Dally shows a weak dominant negative activity on Dpp signal transduction. Furthermore, sec-dally expression led to patterning defects as well as a substantial overgrowth of tissues and animals through the expansion of the action range of Hh. These findings support the recently proposed model that secreted glypicans have opposing and/or distinct effects on morphogen signaling from the membrane-tethered forms.

DWnt4 regulates the dorsoventral specificity of retinal projections in the Drosophila melanogaster visual system

In Drosophila melanogaster, the axons of retinal photoreceptor cells extend to the first optic ganglion, the lamina, forming a topographic representation. Here we show that DWnt4, a secreted protein of the Wnt family, is the ventral cue for the lamina. In DWnt4 mutants, ventral retinal axons misprojected to the dorsal lamina. DWnt4 was normally expressed in the ventral half of the developing lamina and DWnt4 protein was detected along ventral retinal axons. Dfrizzled2 and dishevelled, respectively, encode a receptor and a signaling molecule required for Wnt signaling. Mutations in both genes caused DWnt4-like defects, and both genes were autonomously required in the retina, suggesting a direct role of DWnt4 in retinal axon guidance. In contrast, iroquois homeobox genes are the dorsal cues for the retina. Dorsal axons accumulated DWnt4 and misprojected to the ventral lamina in iroquois mutants; the phenotype was suppressed in iroquois Dfrizzled2 mutants, suggesting that iroquois may attenuate the competence of Dfrizzled2 to respond to DWnt4.

Fibroblast growth factors share binding sites in heparan sulphate

HS (heparan sulphate) proteoglycans bind secreted signalling proteins, including FGFs (fibroblast growth factors) through their HS side chains. Such chains contain a wealth of differentially sulphated saccharide epitopes. Whereas specific HS structures are commonly believed to modulate FGF-binding and activity, selective binding of defined HS epitopes to FGFs has generally not been demonstrated. In the present paper, we have identified a series of sulphated HS octasaccharide epitopes, derived from authentic HS or from biosynthetic libraries that bind with graded affinities to FGF4, FGF7 and FGF8b. These HS species, along with previously identified oligosaccharides that interact with FGF1 and FGF2, constitute the first comprehensive survey of FGF-binding HS epitopes based on carbohydrate sequence analysis. Unexpectedly, our results demonstrate that selective modulation of FGF activity cannot be explained in terms of binding of individual FGFs to specific HS target epitopes. Instead, different FGFs bind to identical HS epitopes with similar relative affinities and low selectivity, such that the strength of these interactions increases with increasing saccharide charge density. We conclude that FGFs show extensive sharing of binding sites in HS. This conclusion challenges the current notion of specificity in HS-FGF interactions, and instead suggests that a set of common HS motifs mediates cellular targeting of different FGFs.

Mice deficient in Ext2 lack heparan sulfate and develop exostoses

Hereditary multiple exostoses (HME) is a genetically heterogeneous human disease characterized by the development of bony outgrowths near the ends of long bones. HME results from mutations in EXT1 and EXT2, genes that encode glycosyltransferases that synthesize heparan sulfate chains. To study the relationship of the disease to mutations in these genes, we generated Ext2-null mice by gene targeting. Homozygous mutant embryos developed normally until embryonic day 6.0, when they became growth arrested and failed to gastrulate, pointing to the early essential role for heparan sulfate in developing embryos. Heterozygotes had a normal lifespan and were fertile; however, analysis of their skeletons showed that about one-third of the animals formed one or more ectopic bone growths (exostoses). Significantly, all of the mice showed multiple abnormalities in cartilage differentiation, including disorganization of chondrocytes in long bones and premature hypertrophy in costochondral cartilage. These changes were not attributable to a defect in hedgehog signaling, suggesting that they arise from deficiencies in other heparan sulfate-dependent pathways. The finding that haploinsufficiency triggers abnormal cartilage differentiation gives insight into the complex molecular mechanisms underlying the development of exostoses.

Mechanisms of Hedgehog gradient formation and interpretation

Morphogens are molecules that spread from localized sites of production, specifying distinct cell outcomes at different concentrations. Members of the Hedgehog (Hh) family of signaling molecules act as morphogens in different developmental systems. If we are to understand how Hh elicits multiple responses in a temporally and spatially specific manner, the molecular mechanism of Hh gradient formation needs to be established. Moreover, understanding the mechanisms of Hh signaling is a central issue in biology, not only because of the role of Hh in morphogenesis, but also because of its involvement in a wide range of human diseases. Here, we review the mechanisms affecting the dynamics of Hh gradient formation, mostly in the context of Drosophila wing development, although parallel findings in vertebrate systems are also discussed.

Regulation of Wnt gene expression

Members of the Wnt gene family play important roles in the regulation of a number of basic developmental processes. Because Wnt is such a potent morphogen, its expression must be controlled tightly and precisely. While many review papers focused on Wnt signaling downstream of the receptor, this review addresses regulations of Wnt itself on several levels, including the transcriptional level, RNA splicing, the post-transcriptional level, the translational level, and the post-translational level. It is these multiple, precise and tight regulations that guarantee that Wnts function correctly both temporally and spatially.

HSPG synthesis by zebrafish Ext2 and Extl3 is required for Fgf10 signalling during limb development

Heparan sulphate proteoglycans (HSPGs) are known to be crucial for signalling by the secreted Wnt, Hedgehog, Bmp and Fgf proteins during invertebrate development. However, relatively little is known about their effect on developmental signalling in vertebrates. Here, we report the analysis of daedalus, a novel zebrafish pectoral fin mutant. Positional cloning identified fgf10 as the gene disrupted in daedalus. We find that fgf10 mutants strongly resemble zebrafish ext2 and extl3 mutants, which encode glycosyltransferases required for heparan sulphate biosynthesis. This suggests that HSPGs are crucial for Fgf10 signalling during limb development. Consistent with this proposal, we observe a strong genetic interaction between fgf10 and extl3 mutants. Furthermore, application of Fgf10 protein can rescue target gene activation in fgf10, but not in ext2 or extl3 mutants. By contrast, application of Fgf4 protein can activate target genes in both ext2 and extl3 mutants, indicating that ext2 and extl3 are differentially required for Fgf10, but not Fgf4, signalling during limb development. This reveals an unexpected specificity of HSPGs in regulating distinct vertebrate Fgfs.

DPP signaling controls development of the lamina glia required for retinal axon targeting in the visual system of Drosophila

The Drosophila visual system consists of the compound eyes and the optic ganglia in the brain. Among the eight photoreceptor (R) neurons, axons from the R1-R6 neurons stop between two layers of glial cells in the lamina, the most superficial ganglion in the optic lobe. Although it has been suggested that the lamina glia serve as intermediate targets of R axons, little is known about the mechanisms by which these cells develop. We show that DPP signaling plays a key role in this process. dpp is expressed at the margin of the lamina target region, where glial precursors reside. The generation of clones mutant for Medea, the DPP signal transducer, or inhibition of DPP signaling in this region resulted in defects in R neuron projection patterns and in the lamina morphology, which was caused by defects in the differentiation of the lamina glial cells. glial cells missing/glial cells deficient (gcm; also known as glide) is expressed shortly after glia precursors start to differentiate and migrate. Its expression depends on DPP; gcm is reduced or absent in dpp mutants or Medea clones, and ectopic activation of DPP signaling induces ectopic expression of gcm and REPO. In addition, R axon projections and lamina glia development were impaired by the expression of a dominant-negative form of gcm, suggesting that gcm indeed controls the differentiation of lamina glial cells. These results suggest that DPP signaling mediates the maturation of the lamina glia required for the correct R axon projection pattern by controlling the expression of gcm.

Distinct functional units of the Golgi complex in Drosophila cells

A striking variety of glycosylation occur in the Golgi complex in a protein-specific manner, but how this diversity and specificity are achieved remains unclear. Here we show that stacked fragments (units) of the Golgi complex dispersed in Drosophila imaginal disk cells are functionally diverse. The UDP-sugar transporter FRINGE-CONNECTION (FRC) is localized to a subset of the Golgi units distinct from those harboring SULFATELESS (SFL), which modifies glucosaminoglycans (GAGs), and from those harboring the protease RHOMBOID (RHO), which processes the glycoprotein SPITZ (SPI). Whereas the glycosylation and function of NOTCH are affected in imaginal disks of frc mutants, those of SPI and of GAG core proteins are not, even though FRC transports a broad range of glycosylation substrates, suggesting that Golgi units containing FRC and those containing SFL or RHO are functionally separable. Distinct Golgi units containing FRC and RHO in embryos could also be separated biochemically by immunoisolation techniques. We also show that Tn-antigen glycan is localized only in a subset of the Golgi units distributed basally in a polarized cell. We propose that the different localizations among distinct Golgi units of molecules involved in glycosylation underlie the diversity of glycan modification.

Heparan sulphate proteoglycans: the sweet side of development

Pattern formation during development is controlled to a great extent by a small number of conserved signal transduction pathways that are activated by extracellular ligands such as Hedgehog, Wingless or Decapentaplegic. Genetic experiments have identified heparan sulphate proteoglycans (HSPGs) as important regulators of the tissue distribution of these extracellular signalling molecules. Several recent reports provide important new insights into the mechanisms by which HSPGs function during development.

Hedgehog signaling and congenital malformations

The Hedgehog (Hh)-signaling pathway is essential for numerous developmental processes in Drosophila and vertebrate embryos. Hh signal transduction encompasses a complex series of regulatory events, including the generation of the mature Hh ligand, propagation of the ligand from source of production as well as the reception and interpretation of the signal in Hh-receiving cells. Many congenital malformations in humans are known to involve mutations in various components of the Hh-signaling pathway. This mini review summarizes some recent findings about the regulation of Hh signal transduction and describes the spectrum of human congenital malformations that are associated with aberrant Hh signaling. Based on a comparison of mouse-mutant phenotypes and human syndromes, we discuss how Hh-dependent Gli activator and repressor functions contribute to some of the congenital malformations.

Lipoprotein particles are required for Hedgehog and Wingless signalling

Wnt and Hedgehog family proteins are secreted signalling molecules (morphogens) that act at both long and short range to control growth and patterning during development. Both proteins are covalently modified by lipid, and the mechanism by which such hydrophobic molecules might spread over long distances is unknown. Here we show that Wingless, Hedgehog and glycophosphatidylinositol-linked proteins copurify with lipoprotein particles, and co-localize with them in the developing wing epithelium of Drosophila. In larvae with reduced lipoprotein levels, Hedgehog accumulates near its site of production, and fails to signal over its normal range. Similarly, the range of Wingless signalling is narrowed. We propose a novel function for lipoprotein particles, in which they act as vehicles for the movement of lipid-linked morphogens and glycophosphatidylinositol-linked proteins.

Communicating with Hedgehogs

Signalling by secreted Hedgehog (Hh) proteins is important for the development of many tissues and organs. Damage to components of the Hh signal-transduction pathway can lead to birth defects and cancer. The Hh proteins are distributed in tissues in a gradient, and cells respond to different thresholds of Hh with distinct responses. The cellular machinery that is responsible for the unique molecular mechanisms of Hh signal transduction has been largely conserved during metazoan evolution.

A genetic screen in Drosophila for identifying novel components of the hedgehog signaling pathway

The Hedgehog signaling pathway plays an essential role in the pattern formation and development of metazoan animals. Misregulation of Hedgehog signaling has also been associated with the formation of multiple types of cancer. For these reasons, the Hedgehog pathway has attracted considerable interest. Many proteins required in the Hedgehog pathway have been identified, and while much has been learned about their function in signal transduction, it is clear that this complement of proteins does not comprise the full set necessary for Hedgehog signal transduction. Because significant gaps remain in our knowledge of the molecules required for Hedgehog signaling, we performed an enhancer/suppressor screen in Drosophila melanogaster to identify novel components of the pathway. In addition to the isolation of new alleles of the known pathway components patched and smoothened, this screen identified 14 novel complementation groups and a larger number of loci represented by single alleles. These groups include mutations in the genes encoding the translation factors eRF1 and eIF1A and the kinesin-like protein Pavarotti. It also identified mutations in a gene whose product is necessary for the movement of Hedgehog protein through tissues.

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.

The Drosophila ortholog of the human Wnt inhibitor factor Shifted controls the diffusion of lipid-modified Hedgehog

The Hedgehog (Hh) family of morphogenetic proteins has important instructional roles in metazoan development and human diseases. Lipid modified Hh is able to migrate to and program cells far away from its site of production despite being associated with membranes. To investigate the Hh spreading mechanism, we characterized Shifted (Shf) as a component in the Drosophila Hh pathway. We show that Shf is the ortholog of the human Wnt inhibitory factor (WIF), a secreted antagonist of the Wingless pathway. In contrast, Shf is required for Hh stability and for lipid-modified Hh diffusion. Shf colocalizes with Hh in the extracellular matrix and interacts with the heparan sulfate proteoglycans (HSPG), leading us to suggest that Shf could provide HSPG specificity for Hh. We also show that human WIF inhibits Wg signaling in Drosophila without affecting the Hh pathway, indicating that different WIF family members might have divergent functions in each pathway.

Interpreting clone-mediated perturbations of morphogen profiles

Generating clones of mutated cells within a wild-type tissue is a powerful experimental paradigm for elucidating gene function. Recently, this approach was employed for identifying genes that shape morphogen profiles in the Drosophila wing-imaginal disc. Interpreting such experiments poses a theoretical challenge. We present a general framework that links specific features of the morphogen profile in the clone vicinity to three basic morphogen properties: diffusion, degradation, and binding to immobile elements. Our results provide rigorous criteria to examine existing data and can facilitate the design and interpretation of future clone experiments.

Incredible journey: how do developmental signals travel through tissue?

How developmental signaling proteins traverse tissue during animal development, through or around tightly packed cells, remains an incompletely resolved mystery. Signaling protein movement is regulated to create gradients, control amounts, impose barriers, or provide direction. Signaling can be controlled by the rate of signal production, modification, active transport, trapping along the path, or by the properties of the receptor apparatus. Signals may move by diffusion outside cells, attached to migrating cells, attached to carrier molecules, through cells by transcytosis, along cell extensions, or in released membrane packets. Recent findings about the movement of Hedgehog, Wingless (Wnt), and TGF-beta signaling proteins have helped to clarify the molecular mechanisms used to ensure that developmental signals carry only good news.

Functions of heparan sulfate proteoglycans in cell signaling during development

Heparan sulfate proteoglycans (HSPGs) are cell-surface and extracellular matrix macromolecules that are composed of a core protein decorated with covalently linked glycosaminoglycan (GAG) chains. In vitro studies have demonstrated the roles of these molecules in many cellular functions, and recent in vivo studies have begun to clarify their essential functions in development. In particular, HSPGs play crucial roles in regulating key developmental signaling pathways, such as the Wnt, Hedgehog, transforming growth factor-beta, and fibroblast growth factor pathways. This review highlights recent findings regarding the functions of HSPGs in these signaling pathways during development.

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.