A freely diffusible form of Sonic hedgehog mediates long-range signalling

The you gene encodes an EGF-CUB protein essential for Hedgehog signaling in zebrafish

Hedgehog signaling is required for many aspects of development in vertebrates and invertebrates. Misregulation of the Hedgehog pathway causes developmental abnormalities and has been implicated in certain types of cancer. Large-scale genetic screens in zebrafish have identified a group of mutations, termed you-class mutations, that share common defects in somite shape and in most cases disrupt Hedgehog signaling. These mutant embryos exhibit U-shaped somites characteristic of defects in slow muscle development. In addition, Hedgehog pathway mutations disrupt spinal cord patterning. We report the positional cloning of you, one of the original you-class mutations, and show that it is required for Hedgehog signaling in the development of slow muscle and in the specification of ventral fates in the spinal cord. The you gene encodes a novel protein with conserved EGF and CUB domains and a secretory pathway signal sequence. Epistasis experiments support an extracellular role for You upstream of the Hedgehog response mechanism. Analysis of chimeras indicates that you mutant cells can appropriately respond to Hedgehog signaling in a wild-type environment. Additional chimera analysis indicates that wild-type you gene function is not required in axial Hedgehog-producing cells, suggesting that You is essential for transport or stability of Hedgehog signals in the extracellular environment. Our positional cloning and functional studies demonstrate that You is a novel extracellular component of the Hedgehog pathway in vertebrates.

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.

Fast-tracking morphogen diffusion

The readout of morphogen concentrations has been proposed to be an essential mechanism allowing embryos to specify cell identities [Wolpert Trends Genet 12 (1996) 359], but theoretical and experimental results have led to conflicting ideas as to how useful concentration gradients can be established. In particular, it has been pointed out that some models of passive extracellular diffusion exhibit traveling waves of receptor saturation, inadequate for the establishment of positional information. Two alternative (but not mutually exclusive) models are proposed here, which are based on recent experimental results highlighting the roles of extracellular glycoproteins and morphogen oligomerization. In the first model, inspired from the interactions of Dally and Dally-like with Wingless and Decapentaplegic in the third-instar Drosophila wing disc, two morphogen populations are considered: one in a cell-membrane phase, and another one in an extracellular matrix phase, which does not interact with receptors; in the second model, inspired from biochemical studies of Sonic Hedgehog, morphogen oligomers are considered to diffuse freely without interacting with receptors. The existence of a dynamic sub-population of freely diffusing morphogen allows the system to establish a gradient of bound receptor that is suitable for the specification of positional information. Recent experimental results are discussed within the framework of these models, as well as further possible experiments. The role of Notum in the setup of the Wg gradient is also shown to be likely not to involve a gradient in Notum distribution, even though Notum is only expressed close to the source of Wg synthesis.

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.

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.

Making digit patterns in the vertebrate limb

The vertebrate limb has been a premier model for studying pattern formation - a striking digit pattern is formed in human hands, with a thumb forming at one edge and a little finger at the other. Classic embryological studies in different model organisms combined with new sophisticated techniques that integrate gene-expression patterns and cell behaviour have begun to shed light on the mechanisms that control digit patterning, and stimulate re-evaluation of the current models.

A highly conserved amino-terminal region of sonic hedgehog is required for the formation of its freely diffusible multimeric form

Although members of the Hedgehog (Hh) family were initially described as morphogens, many of these early conclusions were based on experiments that used non-physiologically relevant forms of Hh. Native Hh is modified by cholesterol (HhNp) and palmitate. These hydrophobic modifications are responsible for the ability of Hh to associate with cellular membranes, a property that initially appeared inconsistent with its ability to act far from its site of synthesis. Although it is now clear that Hh family members are capable of acting directly in long-range signaling, the form of Hh capable of this activity remains controversial. We have previously provided evidence for a freely diffusible multimeric form of Sonic Hedgehog (Shh) termed s-ShhNp, which is capable of accumulating in a gradient fashion through a morphogenic field. Here, we provide further evidence that s-ShhNp is the physiologically relevant form of Shh. We show that the biological activity of freely diffusible ShhNp resides in its multimeric form and that this multimeric form is exceedingly stable, even to high concentrations of salt and detergent. Furthermore, we now validate the Shh-Shh interactions previously observed in the crystal structure of human Shh, showing that a highly conserved amino-terminal domain of Shh is important for the formation of s-ShhNp. We also conclusively show that palmitoylation is required for s-ShhNp formation. Thus, our results identify both protein-protein and protein-lipid interactions that are required for s-ShhNp formation, and provide the first structural analyses supporting the existence of Shh multimers.

Gli2 and Gli3 localize to cilia and require the intraflagellar transport protein polaris for processing and function

Intraflagellar transport (IFT) proteins are essential for cilia assembly and have recently been associated with a number of developmental processes, such as left–right axis specification and limb and neural tube patterning. Genetic studies indicate that IFT proteins are required for Sonic hedgehog (Shh) signaling downstream of the Smoothened and Patched membrane proteins but upstream of the Glioma (Gli) transcription factors. However, the role that IFT proteins play in transduction of Shh signaling and the importance of cilia in this process remain unknown. Here we provide insights into the mechanism by which defects in an IFT protein, Tg737/Polaris, affect Shh signaling in the murine limb bud. Our data show that loss of Tg737 results in altered Gli3 processing that abrogates Gli3-mediated repression of Gli1 transcriptional activity. In contrast to the conclusions drawn from genetic analysis, the activity of Gli1 and truncated forms of Gli3 (Gli3R) are unaffected in Tg737 mutants at the molecular level, indicating that Tg737/Polaris is differentially involved in specific activities of the Gli proteins. Most important, a negative regulator of Shh signaling, Suppressor of fused, and the three full-length Gli transcription factors localize to the distal tip of cilia in addition to the nucleus. Thus, our data support a model where cilia have a direct role in Gli processing and Shh signal transduction.

Cilia are small projections extending from the surface of most cells. Research has shown that they are important in diseases such as cystic kidney diseases as well as during the development of many tissues including the limb. More recently, proteins such as Polaris, which is required to build cilia, have been demonstrated to be essential for the regulation of Sonic hedgehog (Shh) signaling, although the mechanism has remained elusive. Precise regulation of Shh signal transduction is important for the proper development of many tissues. Excessive activation of the Shh pathway results in severe developmental defects and has been implicated in certain types of cancer. In the limb, Shh signaling is involved in digit development, and excess signaling leads to the formation of extra digits. The main targets of Shh signaling are the Glioma (Gli) family of transcription factors, and Gli3 has been shown to be processed to a shortened repressor form when Shh signaling is repressed. The localization of the Gli transcription factors and Suppressor of fused, a protein involved in the regulation of Gli protein function, to cilia suggests that the cilia may be an important site for regulation of Shh signal transduction by modulating Gli protein function.

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.

Differential range and activity of various forms of the Hedgehog protein

Background

The Hedgehog (Hh) family of secreted proteins act as extracellular messengers to control and coordinate growth and differentiation. The mechanism by which Hh protein travels across a field of cells, and results in a range of specific effects relating to the distance from the source, has been the subject of much debate. It has been suggested that the range and activity of the pathway can be linked to modifications of the Hh protein, specifically the addition of lipid groups at N- and C-terminal sites.

Results

Here we have addressed the potency of different forms of Hh protein by expressing these in Drosophila, where we are able to precisely establish pathway activity and range in naïve but responsive tissues. As expected, a construct that can produce all forms of Hh recapitulates endogenous signaling potencies. In comparison, expression of a form that lacks the cholesterol moiety (HhN) leads to an extended range, but the product is less effective at inducing maximal Hh responses. Expression of a point mutant that lacks the N-terminal palmitate binding site shows that the palmitoylation of Hh is absolutely required for activity in this system.

Conclusion

We conclude that the addition of the cholesterol moiety limits the range of the protein and is required for maximal activity, while addition of palmitate is required for all activity. These findings have implications for understanding how Hedgehog proteins move, and thus their potential at influencing distant sites, and concomitantly, how modifications of the signaling protein can affect the efficacy of the response in exposed cells.

Sphedgehog is expressed by pigment cell precursors during early gastrulation in Strongylocentrotus purpuratus

We have sequenced the Sphedgehog (Sphh) gene from the sea urchin Strongylocentrotus purpuratus. Sphh transcripts are detected first at the mesenchyme blastula stage, and they accumulate throughout early embryogenesis. The Sphh protein is produced by precursor pigment cells during early and midgastrulation. NiCl2 inhibits pigment cell differentiation in sea urchins. Here, we show that, in S. purpuratus, nickel affects a process(es) between 17 and 24 hr of development, corresponding to the time period when Sphh mRNA is first detected. However, nickel treatment does not alter the early expression of Sphh.

Enhanced Efficacy of Cholesterol-Minus Sonic Hedgehog in Postnatal Skin

The role of Sonic hedgehog (Shh) as a morphogen in diverse developmental settings depends on covalent addition of cholesterol. In this study, expression of Shh lacking cholesterol stimulated hair growth with greater efficacy than native Shh, suggesting that Shh acts as a chemokine rather than a morphogen in this postnatal role. Thus, a structural modification that renders a morphogen ineffective in developmental models may have distinct advantages for postnatal applications.

Signaling interactions between squamous and columnar epithelia of the Drosophila wing disc

Understanding the interactions between distinct epithelial cells would help us to understand the development of tissues. Drosophila imaginal discs, which are made up of two types of epithelial cells, provide good model systems for such studies. The disc proper or the columnar epithelial cells are apposed to a layer of squamous epithelial cells (the peripodial membrane). We have examined organization of peripodial and disc proper cells vis-à-vis their polarity since cell polarity plays an important role in the polarized transport of signaling molecules. With the help of polarity-specific cell markers, we have observed that apical surfaces of peripodial and disc proper cells face each other. This provides the cellular basis for the recently demonstrated signaling interactions between peripodial and disc proper cells during disc patterning. We also report significant similarities as well as differences between peripodial and disc proper cells in Engrailed-dependent wingdisc-patterning events, which make them an appropriate model system for studying the mechanism of diffusion of signal molecules, such as Hedgehog. Results with wild-type and two mutant forms of Hedgehog suggest that direct cell-cell contact is a requirement for the movement of wild-type Hedgehog signal and reconfirm that cholesterol-modification of Hedgehog makes it a short-range signaling molecule by restricting its movement.

FGF-induced vesicular release of Sonic hedgehog and retinoic acid in leftward nodal flow is critical for left-right determination

The precise specification of left-right asymmetry is an essential process for patterning internal organs in vertebrates. In mouse embryonic development, the symmetry-breaking process in left-right determination is initiated by a leftward extraembryonic fluid flow on the surface of the ventral node. However, it is not known whether the signal transduction mechanism of this flow is chemical or mechanical. Here we show that fibroblast growth factor (FGF) signalling triggers secretion of membrane-sheathed objects 0.3-5 microm in diameter termed 'nodal vesicular parcels' (NVPs) that carry Sonic hedgehog and retinoic acid. These NVPs are transported leftward by the fluid flow and eventually fragment close to the left wall of the ventral node. The silencing effects of the FGF-receptor inhibitor SU5402 on NVP secretion and on a downstream rise in Ca2+ were sufficiently reversed by exogenous Sonic hedgehog peptide or retinoic acid, suggesting that FGF-triggered surface accumulation of cargo morphogens may be essential for launching NVPs. Thus, we propose that NVP flow is a new mode of extracellular transport that forms a left-right gradient of morphogens.

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.

Localization of the human hedgehog-interacting protein (Hip) in the normal and diseased pancreas

Pancreatic ductal adenocarcinoma (PDAC) is an aggressive malignancy with a poor prognosis. Previously, it has been shown that Indian hedgehog (Ihh) and its two signaling receptors patched (Ptc) and smoothened (Smo) are involved in the pathogenesis of chronic pancreatitis (CP) and PDAC. In the current study we analyzed the expression, distribution, and function of another component of this signaling pathway, the human hedgehog-interacting protein (Hip), in the normal pancreas, CP and PDAC utilizing real-time quantitative reverse transcription-polymerase chain reaction (QRT-PCR), immunohistochemistry, immunofluorescence, Hip siRNA transfection, cell growth assays, and cell cycle analysis. By QRT-PCR, Hip mRNA levels were fifteenfold and fourteenfold increased in CP (n = 22) and PDAC (n = 31) tissues, respectively, compared to normal pancreatic tissues (n=20) and correlated with glioma associated antigen (Gli1) but not Ptc or Protein kinase A (PKA) mRNA levels. Only SU-8686 and BxPC-3 pancreatic cancer cells expressed Hip mRNA, whereas expression was below the level of detection in the other six pancreatic cancer cell lines tested. As shown by immunohistochemistry, Hip was expressed in normal pancreatic tissues mainly in the cytoplasm of islet cells and in smooth muscle cells of blood vessels. In contrast, in CP and PDAC there was a different distribution and staining intensity within the islets. Moreover, Hip immunoreactivity was observed in the tubular complexes, PanIN 1-3 lesions, as well as in pancreatic cancer cells. Incubation of pancreatic cancer cell lines with recombinant Hip revealed a growth inhibitory effect in SU-8686 and Capan-1 pancreatic cancer cells and no effect on cell growth in the other tested cell lines. In addition, silencing of Hip expression using specific siRNA molecules increased the growth of SU-8686 cells. In conclusion, Hip is expressed in the normal pancreas, CP and PDAC tissues. The different pattern of Hip expression and abnormal localization in the diseased pancreas suggest that the enhanced activation of hedgehog signaling in CP and PDAC is-at least in part-due to the aberrant responsiveness and expression of Hip in these diseases.

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.

Sonic hedgehog is produced by follicular dendritic cells and protects germinal center B cells from apoptosis

The Hedgehog (Hh) signaling pathway is involved in the development of many tissues during embryogenesis, but has also been described to function in adult self-renewing tissues. In the immune system, Sonic Hedgehog (Shh) regulates intrathymic T cell development and modulates the effector functions of peripheral CD4(+) T cells. In this study we investigate whether Shh signaling is involved in peripheral B cell differentiation in mice. Shh is produced by follicular dendritic cells, mainly in germinal centers (GCs), and GC B cells express both components of the Hh receptor, Patched and Smoothened. Blockade of the Hh signaling pathway reduces the survival, and consequently the proliferation and Ab secretion, of GC B cells. Furthermore, Shh rescues GC B cells from apoptosis induced by Fas ligation. Taken together, our data suggest that Shh is one of the survival signals provided by follicular dendritic cells to prevent apoptosis in GC B cells.

Constitutive activation of the shh-ptc1 pathway by a patched1 mutation identified in BCC

Mutations in the transmembrane receptor patched1 (ptc1) are responsible for the majority of basal cell carcinoma (BCC) cases. Many of these mutations, including ptc1-Q688X, result in premature truncation of the ptc1 protein. ptc1-Q688X has been identified in patients with both BCC and nevoid basal cell carcinoma syndrome, an inheritable disorder causing a predisposition to cancer susceptibility. Here we describe a mechanism by which ptc1-Q688X causes constitutive cellular signaling. Cells expressing ptc1-Q688X demonstrate an increase in cell cycle progression and induce cell transformation. The ptc1-Q688X mutant enhances Gli1 activity, a downstream reporter of sonic hedgehog (shh)-ptc1 signaling, independent of shh stimulation. In contrast to wild-type ptc1, ptc1-Q688X fails to associate with endogenous cyclin B1. Expression of nuclear-targeted cyclin B1 derivatives promotes Gli1-dependent transcription, which correlates temporally with cyclin B1-cdk1 kinase activity. Coexpression of wild-type ptc1 with a nuclear-targeted cyclin B1 derivative, mutated to mimic constitutive phosphorylation, dramatically decreases Gli1 activity. In addition, the coexpression of this constitutively nuclear cyclin B1 derivative with ptc1-Q688X substantially enhances foci formation. These studies therefore describe a molecular mechanism for the aberrant activity of ptc1-Q688X that includes the premature activation of the transcription factor Gli1.

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.

Regulation and function of the sonic hedgehog signal transduction pathway in isolated gastric parietal cells

Shh (Sonic hedgehog) regulates gastric epithelial cell differentiation. We reported that incubation of purified canine parietal cells with epidermal growth factor (EGF) for 6-16 h, stimulates H(+)/K(+)-ATPase alpha-subunit gene expression through the activation of Akt. We explored if Shh mediates some of the actions of EGF in the parietal cells. EGF induced a 6-fold increase in Shh expression, measured by Western blots, after 5 h of incubation. This effect was inhibited by both the phosphatidylinositol 3-kinase inhibitor LY294002 and by transduction of the cells with an adenoviral vector expressing dominant negative Akt. EGF stimulated the release of Shh-like immunoreactivity from the parietal cells, after 16 h of incubation. Shh induced H(+)/K(+)-ATPase alpha-subunit gene expression, assessed by Northern blots, it stimulated a luciferase reporter plasmid containing the EGF-responsive sequence (ERE) of the canine H(+)/K(+)-ATPase alpha-subunit gene promoter, and it induced parietal cell nuclear protein binding to the ERE. Gli transcription factors mediate the intracellular actions of Shh. Co-transfection of the parietal cells with the H(+)/K(+)-luc plasmid together with one expressing Gli2, induced H(+)/K(+)-luciferase activity 5-fold, whereas co-transfection of the cells with the H(+)/K(+)-luc plasmid together with one expressing dominant negative Gli2, inhibited EGF induction of H(+)/K(+)-luciferase activity. Identical results were observed in the presence of the Shh signal transduction pathway inhibitor, cyclopamine. Transfection of the cells with dominant negative Akt inhibited EGF, but not Shh stimulation of H(+)/K(+)-ATPase-luciferase activity. Thus, EGF but not Shh signals through Akt. Preincubation of the cells for 16 h with either Shh or EGF enhanced histamine-stimulated [(14)C]aminopyrine uptake by 50%. In conclusions, some of the actions of EGF in the parietal cells are mediated by the sequential activation of the Akt and the Shh signal transduction pathways. These effects might represent novel mechanisms mediating the actions of growth factors on gastric epithelial cell differentiation.

Hedgehog Signaling: From the Drosophila Cuticle to Anti-Cancer Drugs

Hh signaling controls cell proliferation and differentiation in processes that range from insect segmentation and limb formation to vertebrate neural tube development and bone differentiation. Moreover, Hh signaling appears to regulate stem cell homeostasis in adult tissues, while persistent Hh pathway activity has pathological consequences in a number of cancers. Two recent meetings, a Karolinska Institute Nobel conference (August 22-24, 2004) and a joint EMBO and Juan March Institute workshop (October 25-27, 2004), provided the opportunity to take stock of the progress that has been made in understanding Hh signaling and also to remind us of the many questions that still remain unanswered.

Answering a century old riddle: brachydactyly type A1

In 1903, Farabee analyzed the heredity of the human digital malformation, brachydactyly, the first recorded disorder of the autosomal dominant Mendelian trait. In 1951, Bell classified this type of brachydactyly as type A1 (BDA1). Over 100 cases from different ethnic groups have so far been reported. However, the real breakthrough in identifying the cause of BDA1 has only taken place in the last few years with the progress of the mapping and identification of one of the genes responsible for this disorder, thus providing an answer for a century old riddle. In this article, we attempt to review the current state of knowledge on the genetic features of BDA1 with its century-old history and signalling pathway of IHH, and also discuss genotype-phenotype correlation not only of BDA1, but also of all types of brachydactyly.

Levels of Gli3 repressor correlate with Bmp4 expression and apoptosis during limb development

Removal of the posterior wing bud leads to massive apoptosis of the remaining anterior wing bud mesoderm. We show here that this finding correlates with an increase in the level of the repressor form of the Gli3 protein, due to the absence of the Sonic hedgehog (Shh) protein signaling. Therefore, we used the anterior wing bud mesoderm as a model system to analyze the relationship between the repressor form of Gli3 and apoptosis in the developing limb. With increased Gli3R levels, we demonstrate a concomitant increase in Bmp4 expression and signaling in the anterior mesoderm deprived of Shh signaling. Several experimental approaches show that the apoptosis can be prevented by exogenous Noggin, indicating that Bmp signaling mediates it. The analysis of Bmp4 expression in several mouse and chick mutations with defects in either expression or processing of Gli3 indicates a correlation between the level of the repressor form of Gli3 and Bmp4 expression in the distal mesoderm. Our analysis adds new insights into the way Shh differentially controls the processing of Gli3 and how, subsequently, BMP4 expression may mediate cell survival or cell death in the developing limb bud in a position-dependent manner.

Shh signaling in limb bud ectoderm: Potential role in teratogen-induced postaxial ectrodactyly

A variety of teratogens induce the loss of postaxial forelimb structures when administered during mid-gestation to the mouse. Previous studies demonstrated that teratogen exposure is associated with a reduction in zone of polarizing activity (ZPA) -related polarizing activity without a noticeable loss of Shh expression. Herein, we quantitatively confirm that expression of Shh, Ptch1, and Gli3 are unaltered by teratogen exposure and demonstrate that sonic hedgehog (Shh) translation is unaffected. Examination of the polarizing response of host chick wings to teratogen-exposed ZPA tissue revealed an induced growth response and ectopic induction of Fgf4, Bmp2, Ptch1, and Gli1 expression similar to control ZPA tissue. Control ZPA tissue altered the fate of cells destined to die in the anterior necrotic zone, whereas cell death ensued in hosts receiving teratogen-exposed grafts. Immunohistochemical studies localized Shh protein in the mouse limb to the posterior mesoderm and overlying ectoderm. We postulate that teratogen exposure alters the ability of Shh to signal to the ectoderm and present microarray and reverse transcriptase-polymerase chain reaction data, indicating that Shh signaling could occur in the limb bud ectoderm.

Region-specific expression ofmario reveals pivotal function of the anterior nondigit region on digit formation in chick wing bud

We report the region-specific expression of a novel gene, named mario, whose expression domain is in the distal tip of the presumptive and developing digit 2 region in the developing chick wing bud. The anterior region-specific expression of mario corresponds well with the presence of digit 2, and fate map analysis showed that mario expression at early stages represents the presumptive digit 2 region. Using mario expression as a region-specific marker for the digit 2 region, several surgical operations were performed to obtain insights into digit 2 development in the chick wing. Cell fate tracing concomitant with a zone of polarizing activity (ZPA) implantation revealed that an additional digit 2 in the ZPA implantation into the anterior or middle region of wing bud is derived from the original digit 2 region (mario-positive region). Surgical manipulations revealed that the anterior nondigit region has an inhibitory effect on digit 2 formation. Taken together, these results suggest that the most-anterior region, including the anterior necrotic zone, restricts the position of digit 2 region by limiting the anterior border of the digit 2 region and preventing its expansion.

Temporal modulation of the Hedgehog morphogen gradient by a patched-dependent targeting to lysosomal compartment

Morphogenetic gradient of Hh is tightly regulated for correct patterning in Drosophila and vertebrates. The Patched (Ptc) receptor is required for restricting Hh long-range activity in the imaginal discs. In this study, we investigate the different types of Hh accretion that can be observed in the Drosophila embryonic epithelial cells. We found that, in receiving cells, large apical punctate structures of Hh (Hh-LPSs) are not depending on the Ptc receptor-dependent internalization of Hh but rather reflect Hh gradient. By analyzing the dynamic of the Hh-LPS gradient formation, we demonstrate that Hh distribution is strongly restricted during late embryonic stages compared to earlier stages. We demonstrate that the up-regulation of Ptc is required for the temporal regulation of the Hh gradient. We further show that dynamin-dependent internalization of Hh is not regulating Hh spreading but is involved in shaping Hh gradient. We found that Hh gradient modulation is directly related with the dynamic expression of the ventral Hh target gene serrate (ser) and with the Hh-dependent dorsal cell fate determination. Finally, our study shows that, in vivo, the Hh/Ptc complex is internalized in the Rab7-enriched lysosomal compartment in a Ptc-dependent manner without the co-receptor Smoothened (Smo). We propose that controlled degradation is an active mechanism important for Hh gradient formation.

Neuroprotective properties of cultured neural progenitor cells are associated with the production of sonic hedgehog

Numerous studies have shown that abnormal motor behavior improves when neural progenitor cells (NPCs) are transplanted into animal models of neurodegeneration. The mechanisms responsible for this improvement are not fully understood. Indirect anatomical evidence suggests that attention of abnormal motor behavior is attributed, at least in part, to the secretion of trophic factors from the transplanted NPCs. However, there is little direct evidence supporting this hypothesis. Here we show that NPCs isolated from the subventricular zone (SVZ) of neonatal mice are highly teratogenic when transplanted into the neural tube of developing chick embryos and are neuroprotective for fetal dopaminergic neurons in culture because they release sonic hedgehog (Shh). In addition, the neuroprotective properties of NPCs can be exploited to promote better long-term survival of transplanted fetal neurons in an animal model of Parkinson's disease. Thus, cultured NPCs isolated from the SVZ can secrete at least one potent mitogen (Shh) that dramatically affects the fate of neighboring cells. This trait may account for some of the improvement in motor behavior often reported in animal models of neurodegeneration after transplantation of cultured NPCs that were isolated from the SVZ.

Role of morphogens in brain growth

During the last century, experiments on the chick embryo established that the ballooning expansion of the early forebrain and midbrain vesicles is dependent on the underlying axial (notochordal) mesoderm. Transient separation of the early midbrain primordium from the notochord causes subsequent collapse of both midbrain and forebrain (telencephalic) vesicles, accompanied by pronounced folding of the neural epithelium. More recent experiments have shown that vesicle collapse is caused by defective Sonic Hedgehog (Shh) signaling from the notochord and floor plate. Separation of the notochord from the brain causes loss of ventral Shh expression, resulting in reduced cell proliferation and increased cell death in the expanding neural epithelium, and culminating in vesicle collapse. These experiments are reviewed here, and set in the context of other studies illustrating the wide range of molecular and cellular processes that cause abnormal brain morphogenesis when perturbed. We also speculate that variation in the regulation of signaling pathways such as Hedgehog may have played a significant part in generating rapid morphogenetic changes during the evolution of the vertebrate brain.

Stanniocalcin: no longer just a fish tale

Stanniocalcin was originally described as a hormone with calcitonin-like actions in fish. During the last decade, mammalian forms of stanniocalcin have been identified, and this discovery has led to important advances in our understanding of this enigmatic polypeptide hormone. This review briefly covers some early studies on stanniocalcin in fish and then provides a more in-depth look at some of the more intriguing, new aspects of its functions in mammals. The roles of stanniocalcin in renal function, metabolism, angiogenesis, pregnancy and lactation, bone formation, and neural protection are discussed, along with new information relating to its receptor-mediated sequestration and accumulation in target cell organelles.

Mouse Disp1 is required in sonic hedgehog-expressing cells for paracrine activity of the cholesterol-modified ligand

Previous studies have demonstrated that Disp1 function is essential for Shh and Ihh signaling in the mouse, and Disp1 gene dose regulates the level of Shh signaling activity in vivo. To determine whether Disp1 activity is required in Shh-producing cells for paracrine signaling in Shh target fields, we used a ShhGFP-Cre (here shortened to ShhCre) knock-in allele and a Disp1 conditional allele to knock down Disp1 activity specifically within Shh-producing cells. The resulting facial and neural tube phenotypes support the conclusion that the primary and probably exclusive role for Disp1 is within hedgehog protein-producing cells. Furthermore, using an allele that produces N-Shh (a noncholesterol modified form of the Shh protein), we demonstrate that N-Shh is sufficient to rescue most of the early embryonic lethal defects in a Disp1-null mutant background. Thus, Disp1 activity is only required for paracrine hedgehog protein signaling by the cholesterol modified form of Shh (N-Shhp), the normal product generated by auto-processing of a Shh precursor protein. In both respects, Disp function is conserved from Drosophila to mice.

Sonic hedgehog signaling in basal cell carcinomas

The development of basal cell carcinoma, the commonest human cancer in fair skinned populations, is clearly associated with constitutive activation of sonic hedgehog signaling. Insight into the genesis of BCC came from the identification of germline mutations of the tumor suppressor gene, PATCHED, a key regulatory component of hedgehog signaling in the nevoid basal cell carcinoma syndrome. Analysis of sporadic basal cell carcinomas and those from repair deficient xeroderma pigmentosum patients has revealed mutational inactivation of PATCHED and gain of function mutations of the proto-oncogenes, SMOOTHENED and SONIC HEDGEHOG associated with solar UV exposure. The molecular mechanisms involved in alterations of the hedgehog signaling pathway that lead to the formation of basal cell carcinomas are being unraveled and has already allowed the investigation of future therapeutic strategies for treating these skin cancers.

Novel lipid modifications of secreted protein signals

Secreted signaling proteins function in a diverse array of essential patterning events during metazoan development, ranging from embryonic segmentation in insects to neural tube differentiation in vertebrates. These proteins generally are expressed in a localized manner, and they may elicit distinct concentration-dependent responses in the cells of surrounding tissues and structures, thus functioning as morphogens that specify the pattern of cellular responses by their tissue distribution. Given the importance of signal distribution, it is notable that the Hedgehog (Hh) and Wnt proteins, two of the most important families of such signals, are known to be covalently modified by lipid moieties, the membrane-anchoring properties of which are not consistent with passive models of protein mobilization within tissues. This review focuses on the mechanisms underlying biogenesis of the mature Hh proteins, which are dually modified by cholesteryl and palmitoyl adducts, as well as on the relationship between Hh proteins and the self-splicing proteins (i.e., proteins containing inteins) and the Hh-like proteins of nematodes. We further discuss the cellular mechanisms that have evolved to handle lipidated Hh proteins in the spatial deployment of the signal in developing tissues and the more recent findings that implicate palmitate modification as an important feature of Wnt signaling proteins.

Model organisms lead the way to protein palmitoyltransferases

The acylation of proteins with palmitate and related fatty acids has been known for over 30 years, but the molecular machinery that carries out palmitoylation has only recently emerged from studies in the model organisms Saccharomyces cerevisiae and Drosophila. Two classes of protein acyltransferases (PATs) have been proposed. In yeast, members of a family of integral membrane proteins harboring a cysteine-rich domain (CRD) containing a conserved DHHC (Asp-His-His-Cys) motif are PATs for cytoplasmic signaling molecules. The DHHC-CRD protein Erf2p, together with an associated subunit Erf4p, palmitoylates yeast Ras proteins, and Akr1p catalyzes the palmitoylation of the yeast casein kinase Yck2p. The existence of a second class of PATs that modify secreted signaling proteins has been suggested from work in Drosophila. Rasp is required in vivo for the production of functional Hedgehog and shares sequence identity with membrane-bound O-acyltransferases, which suggests that it catalyzes the palmitoylation of Hedgehog. With the identification of PATs in model genetic organisms, the field is now poised to uncover their mammalian counterparts and to understand the enzymology of protein palmitoylation.

Differentiation and transcription factor gene therapy in experimental parkinson's disease: sonic hedgehog and Gli-1, but not Nurr-1, protect nigrostriatal cell bodies from 6-OHDA-induced neurodegeneration

We tested the activity of the dopaminergic neuron differentiation factor sonic hedgehog, its downstream transcription factor target Gli-1, and an orphan nuclear receptor, Nurr-1, necessary for the induction of the dopaminergic phenotype of nigrostriatal neurons, in an in vivo model of nigrostriatal neurodegeneration. Our preliminary experiments demonstrated that all three constructs expressed the proper molecules and that these had the predicted biological activities in vitro. We expressed the N-terminal of sonic hedgehog (ShhN) and the Gli-1 and Nurr-1 entire coding regions from highly purified, and quality controlled, replication-defective adenoviral vectors injected into the brains of rats and used the dopaminergic growth factor GDNF as a positive control. The neurotoxin 6-hydroxydopamine was used to lesion the nigrostriatal dopaminergic innervation; RAd-ShhN and RAd-Gli-1 protected dopaminergic neuronal cell bodies in the substantia nigra, but not axonal terminals in the striatum, from 6-OHDA-induced cell death, while RAd-Nurr-1 was ineffective in protecting either cell bodies or axons. RAd-GDNF was able to protect both the dopaminergic cell bodies and the striatal axon terminals. Our results establish for the first time, to the best of our knowledge, that gene transfer of ShhN and one of its target transcription factors can selectively protect dopaminergic nigrostriatal neuronal cell bodies from a specific neurotoxic insult. Selective protection of nigrostriatal dopaminergic cell bodies by the differentiation factor ShhN and the transcription factor Gli-1 was achieved in a neurotoxic model that eliminates more than 70% of the nigral neurons under consideration. Differentiation and transcription factors can thus be used for the treatment of neurodegeneration by gene therapy.

Dose dependency of Disp1 and genetic interaction between Disp1 and other hedgehog signaling components in the mouse

Genetic analyses in Drosophila have demonstrated that a transmembrane protein Dispatched (Disp) is required for the release of lipid-modified Hedgehog (Hh) protein from Hh secreting cells. Analysis of Disp1 null mutant embryos has demonstrated that Disp1 plays a key role in hedgehog signaling in the early mouse embryo. Here we have used a hypomorphic allele in Disp1(Disp1Δ2), to extend our knowledge of Disp1 function in Hh-mediated patterning of the mammalian embryo. Through genetic combinations with null alleles of patched 1 (Ptch1),sonic hedgehog (Shh) and Indian hedgehog (Ihh), we demonstrate that Disp1 genetically interacts with Hh signaling components. As Disp1 activity is decreased we see a progressive increase in the severity of hedgehog-dependent phenotypes, which is further enhanced by reducing hedgehog ligand levels. Analysis of neural tube patterning demonstrates a progressive loss of ventral cell identities that most likely reflects decreased Shh signaling as Disp1 levels are attenuated. Conversely,increasing available Shh ligand by decreasing Ptch1 dosage leads to the restoration of ventral cell types in Disp1Δ2/Δ2 mutants. Together, these studies suggest that Disp1 actively regulates the levels of hedgehog ligand that are available to the hedgehog target field. Further, they provide additional support for the dose-dependent action of Shh signaling in patterning the embryo. Finally, in-vitro studies on Disp1 null mutant fibroblasts indicate that Disp1 is not essential for membrane targeting or release of lipid-modified Shh ligand.

Hedgehog-interacting protein is highly expressed in endothelial cells but down-regulated during angiogenesis and in several human tumors

BACKGROUND

The Hedgehog (Hh) signaling pathway regulates a variety of developmental processes, including vasculogenesis, and can also induce the expression of pro-angiogenic factors in fibroblasts postnatally. Misregulation of the Hh pathway has been implicated in a variety of different types of cancer, including pancreatic and small-cell lung cancer. Recently a putative antagonist of the pathway, Hedgehog-interacting protein (HIP), was identified as a Hh binding protein that is also a target of Hh signaling. We sought to clarify possible roles for HIP in angiogenesis and cancer.

METHODS

Inhibition of Hh signaling by HIP was assayed by measuring the induction of Ptc-1 mRNA in TM3 cells treated with conditioned medium containing Sonic hedgehog (Shh). Angiogenesis was assayed in vitro by EC tube formation on Matrigel. Expression of HIP mRNA was assayed in cells and tissues by Q-RT-PCR and Western blot. HIP expression in human tumors or mouse xenograft tumors compared to normal tissues was assayed by Q-RT-PCR or hybridization of RNA probes to a cancer profiling array.

RESULTS

We show that Hedgehog-interacting protein (HIP) is abundantly expressed in vascular endothelial cells (EC) but at low or undetectable levels in other cell types. Expression of HIP in mouse epithelial cells attenuated their response to Shh, demonstrating that HIP can antagonize Hh signaling when expressed in the responding cell, and supporting the hypothesis that HIP blocks Hh signaling in EC. HIP expression was significantly reduced in tissues undergoing angiogenesis, including PC3 human prostate cancer and A549 human lung cancer xenograft tumors, as well as in EC undergoing tube formation on Matrigel. HIP expression was also decreased in several human tumors of the liver, lung, stomach, colon and rectum when compared to the corresponding normal tissue.

CONCLUSION

These results suggest that reduced expression of HIP, a naturally occurring Hh pathway antagonist, in tumor neo-vasculature may contribute to increased Hh signaling within the tumor and possibly promote angiogenesis.

Synergistic and antagonistic roles of the Sonic hedgehog N- and C-terminal lipids

The Shh protein contains both N-terminal and C-terminal lipids. The functional redundancy of these lipid moieties is presently unclear. Here, we compare the relative roles of the N- and C-terminal lipids in early rat striatal neuronal differentiation, membrane association and multimerization, and ventralizing activity in the zebrafish forebrain. We show that these lipid act synergistically in cell tethering and the formation of a large (L) multimer (669 kDa). However, the C-terminal lipid antagonizes the rat striatal neuronal differentiation-inducing activity of the N-terminal lipid. In addition, multimerization is required but not sufficient for the differentiation-inducing activity. Based on the presence of different N- and C-lipid-containing Shh proteins in the rat embryo, and on their different activities, we propose that both N- and C-terminal lipids are required for the formation of multimers involved in long-range signaling, and that the C-terminal lipid may function in long-range signaling by reducing Shh activity until it reaches its long-range target. Comparative analysis of the ventralizing activities of different N- and C-terminal lipid-containing Shh proteins in the zebrafish forebrain shows that the presence of at least one lipid is required for signaling activity, suggesting that lipid modification of Shh is a conserved requirement for signaling in the forebrain of rodents and zebrafish.

Functional analysis of novel sonic hedgehog gene mutations identified in basal cell carcinomas from xeroderma pigmentosum patients

Altered sonic hedgehog (SHH) signaling is crucial in the development of basal cell carcinomas (BCC), the most common human cancer. Mutations in SHH signal transducers, PATCHED and SMOOTHENED, have already been identified, but SHH mutations are extremely rare; only 1 was detected in 74 sporadic BCCs. We present data showing unique SHH mutations in BCCs from repair-deficient, skin cancer-prone xeroderma pigmentosum (XP) patients, which are characterized by high levels of UV-specific mutations in key genes involved in skin carcinogenesis, including PATCHED and SMOOTHENED. Thus, 6 UV-specific SHH mutations were detected in 5 of 33 XP BCCs. These missense SHH alterations are not activating mutations for its postulated proto-oncogene function, as the mutant SHH proteins do not show transforming activity and induce differentiation or stimulate proliferation to the same level as the wild-type protein. Structural modeling studies of the 4 proteins altered at the surface residues, G57S, G64K, D147N, and R155C, show that they do not effect the protein conformation. Interestingly, they are all located on one face of the compact SHH protein suggesting that they may have altered affinity for different partners, which may be important in altering other functions. Additional functional analysis of the SHH mutations found in vivo in XP BCCs will help shed light on the role of SHH in skin carcinogenesis. In conclusion, we report for the first time, significant levels of SHH mutations found only in XP BCCs and none in squamous cell carcinomas, indicating their importance in the specific development of BCCs.

Functional characterization of sonic hedgehog mutations associated with holoprosencephaly

Mutations of the developmental gene Sonic hedgehog (SHH) and alterations of SHH signaling have been associated with holoprosencephaly (HPE), a rare disorder characterized by a large spectrum of brain and craniofacial anomalies. Based on the crystal structure of mouse N-terminal and Drosophila C-terminal hedgehog proteins, we have developed three-dimensional models of the corresponding human proteins (SHH-N, SHH-C) that have allowed us to identify within these two domains crucial regions associated with HPE missense mutations. We have further characterized the functional consequences linked to 11 of these mutations. In transfected HEK293 cells, the production of the active SHH-N fragment was dramatically impaired for eight mutants (W117R, W117G, H140P, T150R, C183F, L271P, I354T, A383T). The supernatants from these cell cultures showed no significant SHH-signaling activity in a reporter cell-based assay. Two mutants (G31R, D222N) were associated with a lower production of SHH-N and signaling activity. Finally, one mutant harboring the A226T mutation displays an activity comparable with the wild-type protein. This work demonstrates that most of the HPE-associated SHH mutations analyzed have a deleterious effect on the availability of SHH-N and its biological activity. However, because of the lack of correlation between genotype and phenotype for SHH-associated mutations, our study suggests that other factors intervene in the development of the spectrum of HPE anomalies.

Hedgehog interacting protein in the mature brain: membrane-associated and soluble forms

Hedgehog interacting protein (Hip) is considered as a membrane protein implicated in sequestering the hedgehog (hh) morphogens during embryonic development. Here, we demonstrate that Hip transcription also occurs in cells scattered in discrete brain areas of adult rodents and we identify the presence of membrane-associated and soluble forms of Hip in the mature brain. Moreover, we show that soluble forms of Hip, present in the conditioned medium of HEK293 cells overexpressing Hip, inhibit Sonic hedgehog (Shh)-induced differentiation of C3H10T1/2 cells, a well-characterised response associated with Shh signalling. After transfection in HEK293 cells, Hip partitions with the raft component ganglioside GM1 during density gradient centrifugation. Analysis of tagged Hip constructs reveals that the putative transmembrane domain of Hip is not cleaved suggesting that other mechanisms are implicated in the release of its soluble forms. Taken together, these data are consistent with the involvement of both membrane-associated and soluble Hip in the regulation of Shh signalling in adult neural tissues.

The cholesterol membrane anchor of the Hedgehog protein confers stable membrane association to lipid-modified proteins

The Hedgehog proteins are potent organizers of animal development. They carry a cholesterol ester at the C terminus of their signaling domain. The membrane anchoring mediated by this lipophilic modification was studied by means of an approach integrating cell biology, biochemistry, biophysics, and organic chemistry techniques. Sterol-modified and fluorescent-labeled Hedgehog-derived peptides and proteins were synthesized and investigated in biophysical and cell-biological assays. These experiments revealed that cholesterol alone anchors proteins to membranes with significant strength and half-times for spontaneous desorption of several hours. Its membrane anchoring ability is comparable to dual lipidation motifs such as double geranylgeranylation or S-palmitoylation plus S-farnesylation found in other lipidated proteins. The experiments also demonstrate that membrane binding changes dramatically if short lipidated peptides are equipped with a large protein. These data suggest that for Hedgehog release and subsequent signaling an interaction partner such as the Dispatched protein is necessary. In addition to these findings the described approach allows one to correlate biophysical data obtained with model peptides with data determined with fully functional proteins and to combine results from in vitro and in vivo experiments. It should be generally applicable to other membrane anchors and proteins.

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

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

Palmitoylation is required for the production of a soluble multimeric Hedgehog protein complex and long-range signaling in vertebrates

Hedgehog (Hh) signaling plays a major role in multiple aspects of embryonic development. A key issue in Hh signaling is to elucidate the molecular mechanism by which a Hh protein morphogen gradient is formed despite its membrane association. In this study, we used a combination of genetic, cellular, and biochemical approaches to address the role of lipid modifications in long-range vertebrate Hh signaling. Our molecular analysis of knockout mice deficient in Skn, the murine homolog of the Drosophila ski gene, which catalyzes Hh palmitoylation, and gene-targeted mice producing a nonpalmitoylated form of Shh indicates that Hh palmitoylation is essential for its activity as well as the generation of a protein gradient in the developing embryos. Furthermore, our biochemical data show that Hh lipid modifications are required for producing a soluble multimeric protein complex, which constitutes the major active component for Hh signaling. These results suggest that soluble Hh multimeric complex travels in the morphogenetic field to activate Hh signaling in distant Hh-responsive cells.

Hedgehog: an unusual signal transducer

Hedgehog proteins are of pivotal importance for development and maintenance of tissue patterns in adult organisms. Despite the role of Hedgehogs in differentiation and tumorigenesis, signal transduction of Hedgehog remains a relatively uncharted area of signalling biochemistry. For proper Hedgehog distribution into tissues, two highly unusual covalent modifications are necessary, palmitoylation of a secreted protein and the attachment of a cholesterol group, making Hedgehog the only established sterolated protein in nature. Hedgehog exerts its function via two membrane-bound receptors, Patched and Smoothened; presumably, Patched transports a cholesterol derivate out of cells which inhibits Smoothened. Binding of Hedgehog to Patched impedes this proposed pump function and thus Inhibition of Smoothened, which leads to expression of genetic Hedgehog targets via relief of transcriptional repression. These atypical features make Hedgehog physiology unique in biology and may explain why this field has attracted such significant attention.

Next stop, the twilight zone: hedgehog network regulation of mammary gland development

The hedgehog signal transduction network is a critical mediator of cell-cell communication during embryonic development. Evidence also suggests that properly regulated hedgehog network function is required in some adult organs for stem cell maintenance or renewal. Mutation, or misexpression, of network genes is implicated in the development of several different types of cancer, particularly that of skin, brain, lung, and pancreas. Recent studies in the mouse mammary gland have demonstrated roles for hedgehog network genes at virtually every phase of mammary gland development where it regulates such diverse processes as embryonic mammary gland induction, establishment of ductal histoarchitecture, and functional differentiation in lactation. Further, studies suggest a role for misregulated network function in the progression of breast cancer.