Dual roles for patched in sequestering and transducing Hedgehog

Promoter structure of human sonic hedgehog gene

Sonic hedgehog (Shh) is a secreted signal transducer responsible not only for patterning of the anterior-posterior axis during early limb and neuronal development, but also for generating cell-type diversity at later developmental stages. To elucidate the mechanism regulating human Shh gene expression, we cloned the 5'-flanking region of the human Shh gene and characterized it by transient transfection studies. Two transcription start sites were identified by primer extension analysis. Two TATA-boxes, a CCAAT-box and a palindrome-like structure constituted the basic promoter structure. Furthermore, two continuous E-boxes and a putative homeodomain containing an ATTA-box were located around 350-450 bp upstream of the upper TATA-box. Consensus binding sites of the RA, estrogen, D3 and glucocorticoid/progesterone receptors were not found within the cloned sequence. Short-term treatment with TPA increased luciferase activity up to 2.1-fold; on the other hand, treatment with dibutyryl-cyclic AMP decreased it to 0.8-fold. Retinoic acid (RA), vitamin D3, dexamethazone (DEX) and estradiol (E2) had no effect on the luciferase activity. Since the zebrafish Shh promoter contains two closely spaced axial (HNF3beta) binding sequences on its basic promoter, the palindrome-like structure located in the corresponding site of the human Shh promoter may be a crucial binding domain regulating human Shh gene expression.

Dpp receptor levels contribute to shaping the Dpp morphogen gradient in the Drosophila wing imaginal disc

Axis formation in the Drosophila wing depends on the localized expression of the secreted signaling molecule Decapentaplegic (Dpp). Dpp acts directly at a distance to specify discrete spatial domains, suggesting that it functions as a morphogen. Expression levels of the Dpp receptor thick veins (tkv) are not uniform along the anterior-posterior axis of the wing imaginal disc. Receptor levels are low where Dpp induces its targets Spalt and Omb in the wing pouch. Receptor levels increase in cells farther from the source of Dpp in the lateral regions of the disc. We present evidence that Dpp signaling negatively regulates tkv expression and that the level of receptor influences the effective range of the Dpp gradient. High levels of tkv sensitize cells to low levels of Dpp and also appear to limit the movement of Dpp outside the wing pouch. Thus receptor levels help to shape the Dpp gradient.

Repression of hedgehog signaling and BMP4 expression in growth plate cartilage by fibroblast growth factor receptor 3

Fibroblast growth factor receptor 3 (FGFR3) is a key regulator of skeletal growth and activating mutations in Fgfr3 cause achondroplasia, the most common genetic form of dwarfism in humans. Little is known about the mechanism by which FGFR3 inhibits bone growth and how FGFR3 signaling interacts with other signaling pathways that regulate endochondral ossification. To understand these mechanisms, we targeted the expression of an activated FGFR3 to growth plate cartilage in mice using regulatory elements from the collagen II gene. As with humans carrying the achondroplasia mutation, the resulting transgenic mice are dwarfed, with axial, appendicular and craniofacial skeletal hypoplasia. We found that FGFR3 inhibited endochondral bone growth by markedly inhibiting chondrocyte proliferation and by slowing chondrocyte differentiation. Significantly, FGFR3 downregulated the Indian hedgehog (Ihh) signaling pathway and Bmp4 expression in both growth plate chondrocytes and in the perichondrium. Conversely, Bmp4 expression is upregulated in the perichondrium of Fgfr3−/− mice. These data support a model in which Fgfr3 is an upstream negative regulator of the hedgehog (Hh) signaling pathway. Additionally, Fgfr3 may coordinate the growth and differentiation of chondrocytes with the growth and differentiation of osteoprogenitor cells by simultaneously modulating Bmp4 and patched expression in both growth plate cartilage and in the perichondrium.

In vivo evidence that Patched and Smoothened constitute distinct binding and transducing components of a Hedgehog receptor complex

During Drosophila development, cells belonging to the posterior compartment of each segment organize growth and patterning by secreting Hedgehog (Hh), a protein which induces a thin strip of adjacent cells in the anterior compartment to express the morphogens Decapentaplegic (Dpp) and Wingless (Wg). Hedgehog is bound and transduced by a receptor complex that includes Smoothened (Smo), a member of the Frizzled (Fz) family of seven-pass transmembrane receptors, as well as the multiple-pass transmembrane protein Patched (Ptc). Ptc is required for the binding of Hh to the complex as well as for the Hh-dependent activation of Smo within the complex. Here, we identify a likely null allele of the smo gene and use it to determine whether Hh is bound by Ptc alone, or by Smo in concert with Ptc. We find that cells devoid of Smo can sequester Hh, but that their ability to do so depends, as in wild-type cells, on the expression of high levels of Ptc protein. These results suggest that Ptc normally binds Hh without any help from Smo and hence favor a mechanism of signal transduction in which Hh binds specifically to Ptc and induces a conformational change leading to the release of latent Smo activity.

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

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

Functional interactions of genes mediating convergent extension, knypek and trilobite, during the partitioning of the eye primordium in zebrafish

Vertebrate eye development in the anterior region of the neural plate involves a series of inductive interactions dependent on the underlying prechordal plate and signals from the midline of the neural plate, including Hedgehog. The mechanisms controlling the spatiotemporal expression pattern of hedgehog genes are currently not understood. Cyclopia is observed in trilobite (tri) and knypek (kny) mutants with affected convergent extension of the embryonic axis during gastrulation. Here, we demonstrate that tri mutants show a high frequency of partial or complete cyclopia, kny mutants exhibit cyclopia infrequently, while knym119 trim209 double-mutant embryos have dramatically reduced convergent extension and are completely cyclopic. We analyzed the relationships between the convergent extension defect, the expression of hedgehog and prechordal plate genes, and the formation of cyclopia in knym119 and trim209 mutants. Our results correlate the cyclopia phenotype with the abnormal location of hh-expressing cells with respect to the optic primordium. We show that cyclopia in these mutants is not due to an incompetence of tri and kny cells to respond to Hedgehog signaling. Rather, it is a consequence of exceeding a critical distance (>40-50 micrometer) between hedgehog-expressing cells and the prospective eye field. We hypothesize that at this distance, midline cells are not in an appropriate position to physically separate the eye field and that HH and other signals do not reach the appropriate target cells. Furthermore, tri and kny have overlapping functions in establishing proper alignment of the anterior neural plate and midline cells expressing shh and twhh genes when the partitioning of the eye primordium takes place.

Characterization of two patched receptors for the vertebrate hedgehog protein family

The multitransmembrane protein Patched (PTCH) is the receptor for Sonic Hedgehog (Shh), a secreted molecule implicated in the formation of embryonic structures and in tumorigenesis. Current models suggest that binding of Shh to PTCH prevents the normal inhibition of the seven-transmembrane-protein Smoothened (SMO) by PTCH. According to this model, the inhibition of SMO signaling is relieved after mutational inactivation of PTCH in the basal cell nevus syndrome. Recently, PTCH2, a molecule with sequence homology to PTCH, has been identified. To characterize both PTCH molecules with respect to the various Hedgehog proteins, we have isolated the human PTCH2 gene. Biochemical analysis of PTCH and PTCH2 shows that they both bind to all hedgehog family members with similar affinity and that they can form a complex with SMO. However, the expression patterns of PTCH and PTCH2 do not fully overlap. While PTCH is expressed throughout the mouse embryo, PTCH2 is found at high levels in the skin and in spermatocytes. Because Desert Hedgehog (Dhh) is expressed specifically in the testis and is required for germ cell development, it is likely that PTCH2 mediates its activity in vivo. Chromosomal localization of PTCH2 places it on chromosome 1p33-34, a region deleted in some germ cell tumors, raising the possibility that PTCH2 may be a tumor suppressor in Dhh target cells.

Modulation of Hedgehog target gene expression by the Fused serine-threonine kinase in wing imaginal discs

The Fused (Fu) serine-threonine kinase and the Suppressor of fused (Su(fu)) product are part of the Hedgehog (Hh) signaling pathway both in embryos and in imaginal discs. In wing imaginal discs, the Hh signal induces Cubitus interruptus (Ci) accumulation and activates patched (ptc) and decapentaplegic (dpp) expression along the anterior/posterior (A/P) boundary. In this paper, we have examined the role of the Fu and Su(fu) proteins in the regulation of Hh target gene expression in wing imaginal discs, by using different classes of fu alleles and an amorphic Su(fu) mutation. We show that, at the A/P boundary, Fu kinase activity is involved in the maintenance of high ptc expression and in the induction of late anterior engrailed (en) expression. These combined effects can account for the modulation of Ci accumulation and for the precise localization of the Dpp morphogen stripe. In contrast, in more anterior cells which do not receive Hh signal, we show that Fu plays a role independent of its kinase function in the regulation of Ci accumulation. In these cells, Fu may be involved in the stabilization of a large protein complex which is probably responsible for the regulation of Ci cleavage and/or targeting to nucleus. We propose that the Fused function is necessary for the activation of full-length Ci and counteracts the negative Su(fu) effect on the pathway, leading to en, ptc and dpp expression.

Boundaries in the Drosophila wing imaginal disc organize vein-specific genetic programs

Previous studies have suggested that vein primordia in Drosophila form at boundaries along the A/P axis between discrete sectors of the larval wing imaginal disc. Genes involved in initiating vein development during the third larval instar are expressed either in narrow stripes corresponding to vein primordia or in broader ‘provein’ domains consisting of cells competent to become veins. In addition, genes specifying the alternative intervein cell fate are expressed in complementary intervein regions. The regulatory relationships between genes expressed in narrow vein primordia, in broad provein stripes and in interveins remains unknown, however. In this manuscript, we provide additional evidence for veins forming in narrow stripes at borders of A/P sectors. These experiments further suggest that narrow vein primordia produce secondary short-range signal(s), which activate expression of provein genes in a broad pattern in neighboring cells. We also show that crossregulatory interactions among genes expressed in veins, proveins and interveins contribute to establishing the vein-versus-intervein pattern, and that control of gene expression in vein and intervein regions must be considered on a stripe-by-stripe basis. Finally, we present evidence for a second set of vein-inducing boundaries lying between veins, which we refer to as paravein boundaries. We propose that veins develop at both vein and paravein boundaries in more ‘primitive’ insects, which have up to twice the number of veins present in Drosophila. We present a model in which different A/P boundaries organize vein-specific genetic programs to govern the development of individual veins.

Drosophila ciD encodes a hybrid Pangolin/Cubitus interruptus protein that diverts the Wingless into the Hedgehog signaling pathway

The Hedgehog (Hh) and Wingless (Wg) signaling pathways play important roles in animal development. The activities of the two pathways depend on each other during Drosophila embryogenesis. In the embryonic segment, Wg is required in anterior cells to sustain Hh secretion in adjacent posterior cells. Hh input in turn is necessary for anterior cells to maintain wg expression. The Hh and Wg pathways are mediated by the transcription factors Cubitus interruptus (Ci) and Pangolin/TCF (Pan), respectively. Coincidentally, pan and ci are adjacent genes on the fourth chromosome in a head-to-head orientation. Our genetic and in situ hybridization data indicate that ciD is a mutation affecting both ci and pan. Molecular analysis revealed that the ciD allele is caused by an inversion event that swapped the promoter regions and the first exons of the two genes. The ci gene in ciD is controlled by the ubiquitous pan promoter and encodes a hybrid Ci protein that carries the N-terminal region of Pan. This domain has previously been shown to bind to the b-catenin homolog Armadillo (Arm), raising the possibility that Wg input, in addition to Hh input, modulates the activity of the hybrid CiD protein. Indeed, we found that Wg signaling induces the expression of the Hh target gene patched (ptc) in ciD animals. We provide evidence that this effect depends on the ability of the CiD protein to bind Arm. Genetic and molecular data indicate that wild-type Pan and CiD compete for binding to Arm, leading to a compromised transduction of the Wg signal in heterozygous ciD/+ animals and to a dramatic enhancement of the gain-of-function activity of CiD in homozygous mutants. Thus, the Hh and the Wg pathways are affected by the ciD mutation, and the CiD fusion protein integrates the activities of both.

Neuropilin-1 extracellular domains mediate semaphorin D/III-induced growth cone collapse

Somatosensory axon outgrowth is repulsed when soluble semaphorin D (semD) binds to growth cone neuropilin-1 (Npn-1). Here, semD ligand binding studies of Npn-1 mutants demonstrate that the sema domain binds to the amino-terminal quarter, or complement-binding (CUB) domain, of Npn-1. By herpes simplex virus- (HSV-) mediated expression of Npn-1 mutants in chick retinal ganglion cells, we show that semD-induced growth cone collapse requires two segments of the ectodomain of Npn-1, the CUB domain and the juxtamembrane portion, or MAM (meprin, A5, mu) domain. In contrast, the transmembrane segment and cytoplasmic tail of Npn-1 are not required for biologic activity. These data imply that the CUB and MAM ectodomains of Npn-1 interact with another transmembrane growth cone protein that in turn transduces a semD signal into axon repulsion.

Synergistic signaling by two BMP ligands through the SAX and TKV receptors controls wing growth and patterning in Drosophila

In Drosophila wing discs, a morphogen gradient of DPP has been proposed to determine the transcriptional response thresholds of the downstream genes sal and omb. We present evidence that the concentration of the type I receptor TKV must be low to allow long-range DPP diffusion. Low TKV receptor concentrations result, however, in low signaling activity. To enhance signaling at low DPP concentrations, we find that a second ligand, GBB, augments DPP/TKV activity. GBB signals primarily through the type I receptor SAX, which synergistically enhances TKV signaling and is required for proper OMB expression. We show that OMB expression in wing discs requires synergistic signaling by multiple ligands and receptors to overcome the limitations imposed on DPP morphogen function by receptor concentration levels.

Signals transmitted along retinal axons in Drosophila: Hedgehog signal reception and the cell circuitry of lamina cartridge assembly

The arrival of retinal axons in the brain of Drosophila triggers the assembly of glial and neuronal precursors into a ‘neurocrystalline’ array of lamina synaptic ‘cartridges’. Hedgehog, a secreted protein, is an inductive signal delivered by retinal axons for the initial steps of lamina differentiation. In the development of many tissues, Hedgehog acts in a signal relay cascade via the induction of secondary secreted factors. Here we show that lamina neuronal precursors respond directly to Hedgehog signal reception by entering S-phase, a step that is controlled by the Hedgehog-dependent transcriptional regulator Cubitus interruptus. The terminal differentiation of neuronal precursors and the migration and differentiation of glia appear to be controlled by other retinal axon-mediated signals. Thus retinal axons impose a program of developmental events on their postsynaptic field utilizing distinct signals for different precursor populations.

Evidence for regulation of cartilage differentiation by the homeobox gene Hoxc-8

Homeobox genes of the Hox class are required for proper patterning of skeletal elements, but how they regulate the differentiation of specific tissues is unclear. We show here that overexpression of a Hoxc-8 transgene causes cartilage defects whose severity depends on transgene dosage. The abnormal cartilage is characterized by an accumulation of proliferating chondrocytes and reduced maturation. Since Hoxc-8 is normally expressed in chondrocytes, these results suggest that Hoxc-8 continues to regulate skeletal development well beyond pattern formation in a tissue-specific manner, presumably by controlling the progression of cells along the chondrocyte differentiation pathway. The comparison to Hoxd-4 and Isl-1 indicates that this role in chondrogenesis is specific to proteins of the Hox class. Their capacity for regulation of cartilage differentiation suggests that Hox genes could also be involved in human chondrodysplasias or other cartilage disorders.

New players and puzzles in the Hedgehog signaling pathway

Members of the Hedgehog (Hh) family of signaling proteins control cell fates and proliferation during animal development in part by regulating the transcription of specific genes. Depending on the tissue, Hh can act over long or short distances, to signal directly or by inducing secondary signals. Recent discoveries include new components of the pathway as well as novel regulatory mechanisms involving cholesterol, proteolysis, and the cytoskeleton. The role of Hh in carcinogenesis is underscored by the identification of mutations in several pathway components in skin and brain tumors.

Tout-velu is a Drosophila homologue of the putative tumour suppressor EXT-1 and is needed for Hh diffusion

Hedgehog (Hh) proteins act through both short-range and long-range signalling to pattern tissues during invertebrate and vertebrate development. The mechanisms allowing Hedgehog to diffuse over a long distance and to exert its long-range effects are not understood. Here we identify a new Drosophila gene, named tout-velu, that is required for diffusion of Hedgehog. Characterization of tout-velu shows that it encodes an integral membrane protein that belongs to the EXT gene family. Members of this family are involved in the human multiple exostoses syndrome, which affects bone morphogenesis. Our results, together with the previous characterization of the role of Indian Hedgehog in bone morphogenesis, lead us to propose that the multiple exostoses syndrome is associated with abnormal diffusion of Hedgehog proteins. These results show the existence of a new conserved mechanism required for diffusion of Hedgehog.

Transducing Hedgehog: the story so far

The secreted proteins of the Hedgehog family have been implicated in many different processes in vertebrate development including cartilage differentiation, myotome and sclerotome specification, hair follicle development, limb morphogenesis and the specification of different neuronal cell types. In addition, the aberrant activation of the Hedgehog pathway has been identified as the likely cause of a number of tumours in humans including basal cell carcinomas (BCCs) and primitive neurectodermal tumours (PNETs). Elucidating the mechanisms by which Hedgehog signals are transduced will thus have widespread implications for our understanding of both normal development and disease.

Wingless repression of Drosophila frizzled 2 expression shapes the Wingless morphogen gradient in the wing

In Drosophila wing imaginal discs, the Wingless (Wg) protein acts as a morphogen, emanating from the dorsal/ventral (D/V) boundary of the disc to directly define cell identities along the D/V axis at short and long range. Here, we show that high levels of a Wg receptor, Drosophila frizzled 2 (Dfz2), stabilize Wg, allowing it to reach cells far from its site of synthesis. Wg signaling represses Dfz2 expression, creating a gradient of decreasing Wg stability moving toward the D/V boundary. This repression of Dfz2 is crucial for the normal shape of Wg morphogen gradient as well as the response of cells to the Wg signal. In contrast to other ligand-receptor relationships where the receptor limits diffusion of the ligand, Dfz2 broadens the range of Wg action by protecting it from degradation.

The interpretation of position in a morphogen gradient as revealed by occupancy of activin receptors

Xenopus blastula cells activate different mesodermal genes as a concentration-dependent response to activin, which behaves like a morphogen. To understand how cells recognize morphogen concentration, we have bound naturally labeled activin to cells and related this to choice of gene activation. We find that the increasing occupancy of a single receptor type can cause cells to switch gene expression. Cells sense ligand concentration by the absolute number of occupied receptors per cell (100 and 300 molecules of bound activin induce Xbra and Xgsc, respectively, i.e., 2% and 6% of the total receptors) and not by a ratio of occupied to unoccupied receptors. The long duration of occupancy explains a previously described ratchet effect. Our results suggest a new concept of morphogen gradient formation and interpretation that is particularly well suited to the needs of early development.

Systematic gain-of-function genetics in Drosophila

A modular misexpression system was used to carry out systematic gain-of-function genetic screens in Drosophila. The system is based on inducible expression of genes tagged by insertion of a P-element vector carrying a GAL4-regulated promoter oriented to transcribe flanking genomic sequences. To identify genes involved in eye and wing development, the 2300 independent lines were screened for dominant phenotypes. Among many novel genes, the screen identified known genes, including hedgehog and decapentaplegic, implicated in these processes. A genetic interaction screen for suppressors of a cell migration defect in a hypomorphic slow border cells mutant identified known genes with likely roles in tyrosine kinase signaling and control of actin cytoskeleton, among many novel genes. These studies demonstrate the ability of the modular misexpression system to identify developmentally important genes and suggest that it will be generally useful for genetic interaction screens.

Cholesterol metabolism and embryogenesis

Although cholesterol has long been known to be an essential component of cell membranes in vertebrate organisms, recent studies have suggested that cholesterol plays a crucial role in specific processes during embryonic development, including the covalent modification of Hedgehog proteins. Here we review the overlapping developmental phenotypes associated with pharmacologically or genetically induced defects in cholesterol biosynthesis, embryonic cholesterol transport and Hedgehog proteins. Shared aspects of these phenotypes suggest that common mechanisms underlie impaired central nervous system development associated with cholesterol deficiency.

Encrypted morphogens of skeletogenesis: biological errors and pharmacologic potentials

Bone morphogenetic proteins (BMPs) are members of a class of ancient, highly conserved signalling molecules that play major roles in embryonic axis determination, organ development, tissue repair, and regeneration throughout the animal kingdom. The bone morphogenetic proteins are potent developmental morphogens that act in a concentration-dependent manner to specify cell fates in developing and regenerating systems. Complementary DNAs have been cloned for approximately twenty BMPs, and recombinant proteins have been produced for many of these genes. Transgenic and naturally occurring animal models demonstrate a wide variety of potential functions for BMP genes during development and tissue regeneration, and a wide range of pharmacologic effects are predicted from knock-out or over-expression of the BMP genes. Fibrodysplasia ossificans progressiva (FOP), a rare and devastating genetic disease of ectopic osteogenesis in humans, is associated with over-expression of at least one of the BMPs. The BMPs, their transmembrane receptors, their intracellular signal transducers, and their secreted antagonists hold great promise as pharmacologic agents in modulating a vast array of developmental and regenerative pathways in human diseases.

Control of somite patterning by Sonic hedgehog and its downstream signal response genes

In the avian embryo, previous work has demonstrated that the notochord provides inductive signals to activate myoD and pax1 regulatory genes, which are expressed in the dorsal and ventral somite cells that give rise to myotomal and sclerotomal lineages. Here, we present bead implantation and antisense inhibition experiments that show that Sonic hedgehog is both a sufficient and essential notochord signal molecule for myoD and pax1 activation in somites. Furthermore, we show that genes of the Sonic hedgehog signal response pathway, specifically patched, the Sonic hedgehog receptor, and gli and gli2/4, zinc-finger transcription factors, are activated in coordination with somite formation, establishing that Sonic hedgehog response genes play a regulatory role in coordinating the response of somites to the constitutive notochord Sonic hedgehog signal. Furthermore, the expression of patched, gli and gli2/4 is differentially patterned in the somite, providing mechanisms for differentially transducing the Sonic hedgehog signal to the myotomal and sclerotomal lineages. Finally, we show that the activation of gli2/4 is controlled by the process of somite formation and signals from the surface ectoderm, whereas upregulation of patched and activation of gli is controlled by the process of somite formation and a Sonic hedgehog signal. The Sonic hedgehog signal response genes, therefore, have important functions in regulating the initiation of the Sonic hedgehog response in newly forming somites and in regulating the patterned expression of myoD and pax1 in the myotomal and sclerotomal lineages following somite formation.

The patched gene in development and cancer

The cloning of vertebrate homologues of the Drosophila segment polarity gene patched has led to confirmation of a role for the multipass transmembrane protein which it encodes as a receptor for secreted signalling proteins of the Hedgehog family. In addition, human patched has been identified as a tumour suppressor gene implicated in basal cell carcinomas and medullablastomas.

Sonic hedgehog is not required for polarising activity in the Doublefoot mutant mouse limb bud

The mouse mutant Doublefoot (Dbf) shows preaxial polydactyly of all four limbs. We have analysed limb development in this mutant with respect to morphogenesis, gene expression patterns and ectopic polarising activity. The results reveal a gain-of-function mutation at a locus that mediates pattern formation in the developing limb. Shh expression is identical with that of wild-type embryos, i.e. there is no ectopic expression. However, mesenchyme from the anterior aspects of Dbf/+ mutant limb buds, when transplanted to the anterior side of chick wing buds, induces duplication of the distal skeletal elements. Mid-distal mesenchymal transplants from early, but not later, Dbf/+ limb buds are also able to induce duplication. This demonstration of polarising activity in the absence of Shh expression identifies the gene at the Dbf locus as a new genetic component of the Shh signalling pathway, which (at least in its mutated form) is able to activate signal transduction independently of Shh. The mutant gene product is sufficient to fulfil the signalling properties of Shh including upregulation of the direct Shh target genes Ptc and Gli, and induction of the downstream target genes Bmp2, Fgf4 and Hoxd13. The expression domains of all these genes extend from their normal posterior domains into the anterior part of the limb bud without being focused on a discrete ectopic site. These observations dissociate polarising activity from Shh gene expression in the Dbf/+ limb bud. We suggest that the product of the normal Dbf gene is a key active constituent of the polarising region, possibly acting in the extracellular compartment.

Regulation of the Hedgehog and Wingless signalling pathways by the F-box/WD40-repeat protein Slimb

Members of the Hedgehog (Hh) and Wnt/Wingless (Wg) families of secreted proteins control many aspects of growth and patterning during animal development. Hh signal transduction leads to increased stability of a transcription factor, Cubitus interruptus (Ci), whereas Wg signal transduction causes increased stability of Armadillo (Arm/beta-catenin), a possible co-factor for the transcriptional regulator Lef1/TCF. Here we describe a new gene, slimb (for supernumerary limbs), which negatively regulates both of these signal transduction pathways. Loss of function of slimb results in a cell-autonomous accumulation of high levels of both Ci and Arm, and the ectopic expression of both Hh- and Wg- responsive genes. The slimb gene encodes a conserved F-box/WD40-repeat protein related to Cdc4p, a protein in budding yeast that targets cell-cycle regulators for degradation by the ubiquitin/proteasome pathway. We propose that Slimb protein normally targets Ci and Arm for processing or degradation by the ubiquitin/proteasome pathway, and that Hh and Wg regulate gene expression at least in part by inducing changes in Ci and Arm, which protect them from Slimb-mediated proteolysis.

A Hedgehog activity gradient contributes to AP axial patterning of the Drosophila wing

The secreted protein Hedgehog (Hh) transmits a signal from posterior to anterior cells that is essential for limb development in insects and vertebrates. In Drosophila, Hh has been thought to act primarily to induce localized expression of Decapentaplegic and Wingless which in turn relay patterning cues at long range. We report here that Hh plays an additional role in patterning the wing. By replacing endogenous Hh activity with that of a membrane-tethered form of Hh, we show that Hh acts directly to pattern the central region of the wing, in addition to its role as an inducer of Dpp. Comparing the biological activities of secreted and membrane-tethered Hh provides evidence that Hh forms a local concentration gradient and functions as a concentration-dependent morphogen in the fly wing.

Smoothened-mediated Hedgehog signalling is required for the maintenance of the anterior-posterior lineage restriction in the developing wing of Drosophila

It is thought that the posterior expression of the ‘selector’ genes engrailed and invected control the subdivision of the growing wing imaginal disc of Drosophila into anterior and posterior lineage compartments. At present, the cellular mechanisms by which separate lineage compartments are maintained are not known. Most models have assumed that the presence or absence of selector gene expression autonomously drives the expression of compartment-specific adhesion or recognition molecules that inhibit intermixing between compartments. However, our present understanding of Hedgehog signalling from posterior to anterior cells raises some interesting alternative models based on a cell's response to signalling. We show here that anterior cells that lack smoothened, and thus the ability to receive the Hedgehog signal, no longer obey a lineage restriction in the normal position of the anterior-posterior boundary. Rather these clones extend into anatomically posterior territory, without any changes in engrailed/invected gene expression. We have also examined clones lacking both en and inv; these too show complex behaviors near the normal site of the compartment boundary, and do not always cross entirely into anatomically anterior territory. Our results suggest that compartmentalization is a complex process involving intercompartmental signalling; models based on changes in affinity or growth will be discussed.

Inhibition of Caenorhabditis elegans vulval induction by gap-1 and by let-23 receptor tyrosine kinase

During induction of the Caenorhabditis elegans hermaphrodite vulva, a signal from the anchor cell activates the LET-23 epidermal growth factor receptor (EGFR)/LET-60 Ras/MPK-1 MAP kinase signaling pathway in the vulval precursor cells. We have characterized two mechanisms that limit the extent of vulval induction. First, we found that gap-1 may directly inhibit the LET-60 Ras signaling pathway. We identified the gap-1 gene in a genetic screen for inhibitors of vulval induction. gap-1 is predicted to encode a protein similar to GTPase-activating proteins that likely functions to inhibit the signaling activity of LET-60 Ras. A loss-of-function mutation in gap-1 suppresses the vulvaless phenotype of mutations in the let-60 ras signaling pathway, but a gap-1 single mutant does not exhibit excess vulval induction. Second, we found that let-23 EGFR prevents vulval induction in a cell-nonautonomous manner, in addition to its cell-autonomous role in activating the let-60 ras/mpk-1 signaling pathway. Using genetic mosaic analysis, we show that let-23 activity in the vulval precursor cell closest to the anchor cell (P6.p) prevents induction of vulval precursor cells further away from the anchor cell (P3.p, P4.p, and P8.p). This result suggests that LET-23 in proximal vulval precursor cells might bind and sequester the inductive signal LIN-3 EGF, thereby preventing diffusion of the inductive signal to distal vulval precursor cells.

Control of compartmental affinity boundaries by hedgehog

In Drosophila, each segmental primordium is subdivided into two cell populations, the anterior (A) and posterior (P) compartments by the selective activity of the transcription factor Engrailed (En) in P cells. Under En control, P cells secrete, but cannot respond to, the signalling protein Hedgehog (Hh). In contrast, and by default, A cells are programmed to respond to Hh by expressing other signalling molecules, such as Decapentaplegic (Dpp) and Wingless (Wg), which organize growth and patterning in both compartments. Cells of the A and P compartments do not intermix, apparently as a consequence of their having distinct cell affinities that cause them to maximize contact with cells of the same compartment, while minimizing contact with cells from the other compartment. This failure to mix has previously been ascribed to an autonomous and direct role for En in specifying a P, as opposed to an A, cell affinity. However, an alternative hypothesis is that Hh secreted by P cells induces A cells to acquire a distinct cell affinity, ensuring that a stable 'affinity boundary' forms wherever P and A cells meet. Here we show that the affinity boundary that segregates A and P cells into adjacent but immiscible cell populations is to a large extent a consequence of local Hh signalling, rather than a reflection of an intrinsic affinity difference between A and P cells.

Signalling networks regulating dental development

There has been rapid progress recently in the identification of signalling pathways regulating tooth development. It has become apparent that signalling networks involved in Drosophila development and development of mammalian organs such as the limb are also used in tooth development. Teeth are epithelial appendages formed in the oral region of vertebrates and their early developmental anatomy resembles that of other appendages, such as hairs and glands. The neural crest origin of tooth mesenchyme has been confirmed and recent evidence suggests that specific combinations of homeobox genes expressed in the neural crest cells may regulate the types of teeth and their patterning. Signalling molecules in the Shh, FGF, BMP and Wnt families appear to regulate the early steps of tooth morphogenesis and some transcription factors associated with these pathways have been shown to be necessary for tooth development. Several of the conserved signals are also transiently expressed in the enamel knots in the dental epithelium. The enamel knots are associated with the characteristic epithelial folding morphogenesis which is responsible for the development of tooth shape and it is currently believed that the enamel knots function as signalling centres regulating tooth shape development. The developing tooth has proven to be an excellent model in studies of the molecular basis of patterning and morphogenesis of organs and it can be expected that continuing studies will rapidly increase the understanding of these mechanisms.

Pax genes and organogenesis

Pax genes are a family of developmental control genes that encode nuclear transcription factors. They are characterized by the presence of the paired domain, a conserved amino acid motif with DNA-binding activity. Originally, paired-box-containing genes were detected in Drosophila melanogaster, where they exert multiple functions during embryogenesis. In vertebrates, Pax genes are also involved in embryogenesis. Mutations in four out of nine characterized Pax genes have been associated with either congenital human diseases such as Waardenburg syndrome (PAX3), Aniridia (PAX6), Peter's anomaly (PAX6), renal coloboma syndrome (PAX2) or spontaneous mouse mutants (undulated (Pax1), Splotch (Pax3), Small eye (Pax6), Pax2(1)Neu), which all show defects in development. Recently, analysis of spontaneous and transgenic mouse mutants has revealed that vertebrate pax genes are key regulators during organogenesis of kidney, eye, ear, nose, limb muscles, vertebral column and brain. Like their Drosophila counterparts, vertebrate Pax genes are involved in pattern formation during embryogenesis, possibly by determining the time and place of organ initiation or morphogenesis. For most tissues, however, the nature of the primary developmental action of Pax transcription factors remains to be elucidated. One predominant theme is signal transduction during tissue interactions, which may lead to a position-specific regulation of cell proliferation.

The homeobox gene Distal-less induces ventral appendage development in Drosophila

This study investigates the role of the homeobox gene Distal-less (Dll) in the development of the legs, antennae, and wings of Drosophila. Lack of Dll function causes a change in the identity of ventral appendage cells (legs and antennae) that often results in the loss of the appendage. Ectopic Dll expression in the proximal region of ventral appendages induces nonautonomous duplication of legs and antennae by the activation of wingless and decapentaplegic. Ectopic Dll expression in dorsal appendages produces transformation into corresponding ventral appendages; wings and halteres develop ectopic legs and the head-eye region develops ectopic antennae. In the wing, the exogenous Dll product induces this transformation by activating the endogenous Dll gene and repressing the wing determinant gene vestigial. It is proposed that Dll induces the development of ventral appendages and also participates in a genetic address that specifies the identity of ventral appendages and discriminates the dorsal versus the ventral appendages in the adult. However, unlike other homeotic genes, Dll expression and function is not defined by a cell lineage border. Dll also performs a secondary and late function required for the normal patterning of the wing.

Dual pathways for induction of wingless expression by protein kinase A and Hedgehog in Drosophila embryos

The secreted Drosophila Hedgehog (Hh) protein induces transcription of specific genes by an unknown mechanism that requires the serpentine transmembrane protein Smoothened (Smo) and the transcription factor Cubitus interruptus (Ci). Protein kinase A (PKA) has been implicated in the mechanism of Hh signal transduction because it acts to repress Hh target genes in imaginal disc cells that express Ci. Changes in Ci protein levels, detected by an antibody that recognizes an epitope in the carboxy-terminal half of Ci, have been suggested to mediate the positive effects of Hh and the negative effects of PKA on Hh target gene expression in imaginal discs. Here we show that PKA inhibition, like Hh, leads to increased "carboxy-terminal" Ci staining and Hh target gene expression in embryos. In addition, we find that Hh and Smo can stimulate target gene expression at constant Ci levels and that increased PKA activity can induce ectopic Hh target gene expression in a manner that requires Smo and Ci activities but does not involve changes in Ci protein concentration. This suggests a branching pathway of Hh signal transduction downstream of Smo and that PKA exerts opposite effects on the two branches. Finally we show that Hh signaling in embryos does not depend on cAMP-dependent regulation of PKA activity.

CNS midline to mesoderm signaling in Drosophila

The dorsal median cells are unique mesodermal cells that reside on the surface of the ventral nerve cord in the Drosophila embryo. The Buttonless homeodomain protein is specifically expressed in these cells and is required for their differentiation. We have determined that proper buttonless gene expression and dorsal median cell differentiation requires signals from underlying CNS midline cells. Thus, dorsal median cells fail to form in single-minded mutants and do not persist in slit mutants. Through analysis of rhomboid mutants and targeted rhomboid expression, we also show that the EGF signaling pathway regulates the number of both the dorsal median cells, as well as a set of mesodermal cells that arise next to the midline and express the single-minded gene. Finally, wingless-patched double mutants exhibit defects in the restriction of dorsal median cells to segment boundaries and alterations in CNS midline cell fates. Taken together, these data define a novel neuroectoderm to mesoderm signaling pathway and suggest that unique mesodermal cell types are specified by a combination of midline and segmental cues.

Hedgehog is an indirect regulator of morphogenetic furrow progression in the Drosophila eye disc

Pattern formation in the eye imaginal disc of Drosophila occurs in a wave that moves from posterior to anterior. The anterior edge of this wave is marked by a contracted band of cells known as the morphogenetic furrow, behind which photoreceptors differentiate. The movement of the furrow is dependent upon the secretion of the signalling protein Hedgehog (Hh) by more posterior cells, and it has been suggested that Hh acts as an inductive signal to induce cells to enter a furrow fate and begin differentiation. To further define the role of Hh in this process, we have analysed clones of cells lacking the function of the smoothened (smo) gene, which is required for transduction of the Hh signal and allows the investigation of the autonomous requirement for hh signalling. These experiments demonstrate that the function of hh in furrow progression is indirect. Cells that cannot receive/transduce the Hh signal are still capable of entering a furrow fate and differentiating normally. However, hh is required to promote furrow progression and regulate its rate of movement across the disc, since the furrow is significantly delayed in smo clones.

Altered neural cell fates and medulloblastoma in mouse patched mutants

The PATCHED (PTC) gene encodes a Sonic hedgehog (Shh) receptor and a tumor suppressor protein that is defective in basal cell nevus syndrome (BCNS). Functions of PTC were investigated by inactivating the mouse gene. Mice homozygous for the ptc mutation died during embryogenesis and were found to have open and overgrown neural tubes. Two Shh target genes, ptc itself and Gli, were derepressed in the ectoderm and mesoderm but not in the endoderm. Shh targets that are, under normal conditions, transcribed ventrally were aberrantly expressed in dorsal and lateral neural tube cells. Thus Ptc appears to be essential for repression of genes that are locally activated by Shh. Mice heterozygous for the ptc mutation were larger than normal, and a subset of them developed hindlimb defects or cerebellar medulloblastomas, abnormalities also seen in BCNS patients.

Developmental genes and cancer: role of patched in basal cell carcinoma of the skin

Many genes originally identified because of their role in embryonic development are also important in postnatal control of cell growth and differentiation. Mutations in some of these genes have been shown to cause cancer. Basal cell carcinoma (BCC) of the skin is the most common cancer in humans. More than 750000 new cases are diagnosed annually, and the incidence is rising. BCCs are slow-growing, locally invasive tumors that rarely metastasize but can result in extensive morbidity through local recurrence and tissue destruction. Epidemiologic studies suggest that sunlight (particularly UVB radiation) is a strong risk factor for BCC formation, although other factors are also involved. The nevoid basal cell carcinoma syndrome (NBCCS), a rare genetic disorder, is characterized by predisposition to BCCs and other tumors as well as to a wide range of developmental defects. NBCCS maps to chromosome 9q22.3, and loss of heterozygosity at this site in both sporadic and hereditary BCCs suggests that it functions as a tumor suppressor. The gene for NBCCS was recently cloned and is the human homologue of the Drosophila gene "patched." Genetic studies in Drosophila show that patched is part of the hedgehog signaling pathway, which is important in determining embryonic patterning and cell fate in multiple structures of the developing embryo. Human patched is mutated in both hereditary and sporadic BCCs, and inactivation of this gene is probably a necessary, if not sufficient, step for BCC formation. Delineation of the biochemical pathway in which patched functions may lead to rational medical therapy for BCCs and possibly for other tumors associated with NBCCS.

A role for Indian hedgehog in extraembryonic endoderm differentiation in F9 cells and the early mouse embryo

Hedgehog genes in Drosophila and vertebrates control patterning of a number of different structures during embryogenesis. They code for secreted signaling proteins that are cleaved into an active aminopeptide and a carboxypeptide. The aminopeptide can mediate local and long range events and can act as a morphogen, inducing differentiation of distinct cell types in a concentration-dependent manner. We demonstrate here that the expression of Indian hedgehog mRNA and protein is upregulated dramatically as F9 cells differentiate in response to retinoic acid, into either parietal endoderm or embryoid bodies, containing an outer visceral endoderm layer. The ES cell line D3 forms embryoid bodies in suspension culture without addition of retinoic acid and also upregulates Indian hedgehog expression. RT-PCR analysis of blastocyst outgrowth cultures demonstrates that whereas little or no Indian hedgehog message is present in blastocysts, significant levels appear upon subsequent days of culture, coincident with the emergence of parietal endoderm cells. In situ hybridization analysis for Indian hedgehog mRNA expression demonstrates the presence of elevated levels of message in the outer visceral endoderm cells relative to the core cells in mature embryoid bodies and in the visceral endoderm of Day 6.5 embryos. Whole-mount in situ hybridization analysis of Day 7.5 and 8.5 embryos indicates that Indian hedgehog expression is highest in the visceral yolk sac at this stage. F9 cell lines expressing a full length Indian hedgehog cDNA express a number of characteristics of differentiated cells, in the absence of retinoic acid. Taken together, these data suggest that Indian hedgehog is involved in mediating differentiation of extraembryonic endoderm during early mouse embryogenesis.

Gli1 is a target of Sonic hedgehog that induces ventral neural tube development

The vertebrate zinc finger genes of the Gli family are homologs of the Drosophila gene cubitus interruptus. In frog embryos, Gli1 is expressed transiently in the prospective floor plate during gastrulation and in cells lateral to the midline during late gastrula and neurula stages. In contrast, Gli2 and Gli3 are absent from the neural plate midline with Gli2 expressed widely and Gli3 in a graded fashion with highest levels in lateral regions. In mouse embryos, the three Gli genes show a similar pattern of expression in the neural tube but are coexpressed throughout the early neural plate. Because Gli1 is the only Gli gene expressed in prospective floor plate cells of frog embryos, we have investigated a possible involvement of this gene in ventral neural tube development. Here we show that Shh signaling activates Gli1 transcription and that widespread expression of endogenous frog or human glioma Gli1, but not Gli3, in developing frog embryos results in the ectopic differentiation of floor plate cells and ventral neurons within the neural tube. Floor-plate-inducing ability is retained when cytoplasmic Gli1 proteins are forced into the nucleus or are fused to the VP16 transactivating domain. Thus, our results identify Gli1 as a midline target of Shh and suggest that it mediates the induction of floor plate cells and ventral neurons by Shh acting as a transcriptional regulator.

Sonic hedgehog participates in craniofacial morphogenesis and is down-regulated by teratogenic doses of retinoic acid

The face is one of the most intricately patterned structures in human and yet little is known of the mechanisms by which the tissues are instructed to grow, fuse, and differentiate. We undertook a study to determine if the craniofacial primordia used the same molecular cues that mediate growth and patterning in other embryonic tissues such as the neural tube and the limb. Here we provide evidence for the presence of organizer-like tissues in the craniofacial primordia. These candidate organizers express the polarizing signal sonic hedghog (shh) and its putative receptor, patched, as well as fibroblast growth factor 8 and bone morphogeneic protein 2. Shh-expressing epithelial grafts functioned as organizing tissues in a limb bud assay system, where they evoked duplications of the digit pattern. High doses of retinoic acid, which are known to truncate the growth of the frontonasal and maxillary processes and thus produce bilateral clefting of the lip and palate, inhibited the expression of shh and patched but not fgf8, in the craniofacial primordia, and abolished polarizing activity of these tissues. From these studies we conclude that the embryonic face contains signaling centers in the epithelium that participate in craniofacial growth and patterning. In addition, we discuss a novel mechanism whereby retinoids can exert a teratogenic effect on craniofacial morphogenesis independent of its effects on Hox gene expression or neural crest cell migration.

Induction of basal cell carcinoma features in transgenic human skin expressing Sonic Hedgehog

Hedgehog (HH) signaling proteins mediate inductive events during animal development. Mutation of the only known HH receptor gene, Patched (PTC), has recently been implicated in inherited and sporadic forms of the most common human cancer, basal cell carcinoma (BCC). In Drosophila, HH acts by inactivating PTC function, raising the possibility that overexpression of Sonic Hedgehog (SHH) in human epidermis might have a tumorigenic effect equivalent to loss of PTC function. We used retroviral transduction of normal human keratinocytes to constitutively express SHH. SHH-expressing cells demonstrated increased expression of both the known HH target, BMP-2B, as well as bcl-2, a protein prominently expressed by keratinocytes in BCCs. These keratinocytes were then used to regenerate human skin transgenic for long terminal repeat-driven SHH (LTR-SHH) on immune-deficient mice. LTR-SHH human skin consistently displays the abnormal specific histologic features seen in BCCs, including downgrowth of epithelial buds into the dermis, basal cell palisading and separation of epidermis from the underlying dermis. In addition, LTR-SHH skin displays the gene expression abnormalities previously described for human BCCs, including decreased BP180/BPAG2 and laminin 5 adhesion proteins and expression of basal epidermal keratins. These data indicate that expression of SHH in human skin recapitulates features of human BCC in vivo, suggest that activation of this conserved signaling pathway contributes to the development of epithelial neoplasia and describe a new transgenic human tissue model of neoplasia.

Proteolysis that is inhibited by hedgehog targets Cubitus interruptus protein to the nucleus and converts it to a repressor

Cell-cell communication at anterior/posterior compartment borders in Drosophila involves Hedgehog (Hh), a protein secreted by posterior cells, and Cubitus interruptus (Ci), a protein in the Hh response pathway in anterior cells. Although Ci is thought to have roles as a transcription factor repressing hh expression and activating target genes, it localizes in the cytoplasm of anterior cells. We report here the identification of a domain that tethers Ci in the cytoplasm and show that in some anterior cells, Ci is cleaved to generate a form that lacks the tethering domain. This form translocates to the nucleus where it represses hh and other target genes. Hh inhibits proteolysis of Ci, and we suggest that this inhibition leads to the observed patterns of expression of key target genes at the compartment border.

Cloning of a mouse smoothened cDNA and expression patterns of hedgehog signalling molecules during chondrogenesis and cartilage differentiation in clonal mouse EC cells, ATDC5

Hedgehog (hh) family proteins appear to use the conserved targets in their signalling pathway including Patched (Ptc), Smoothened (Smo), and Gli. Although Indian hedgehog (Ihh) plays an important role in endochondral bone formation, the involvement of hh signalling molecules in skeletogenesis is unknown. We cloned a mouse (m) Smo cDNA and studied the expression patterns of Ihh, Ptc, Smo, and Gli mRNAs in mouse chondrogenic EC cells, ATDC5. The deduced amino acid sequence of mSmo consisted of 793 amino acids and was 98 and 93% homologous to the rat (r) Smo and human (h) Smo, respectively. In ATDC5 cells, the expression of Ihh mRNA paralleled that of type X collagen mRNA. Smo, Ptc, and Gli mRNAs were constitutively expressed throughout chondrogenesis and the subsequent cartilage differentiation processes except for the transient decrease in Ptc mRNA at the cellular condensation stage. Our data suggest that hh signalling molecules may be involved in chondrogenesis and cartilage differentiation in ATDC5 cells.

Hedgehog organises the pattern and polarity of epidermal cells in the Drosophila abdomen

The abdomen of adult Drosophila, like that of other insects, is formed by a continuous epithelium spanning several segments. Each segment is subdivided into an anterior (A) and posterior (P) compartment, distinguished by activity of the selector gene engrailed (en) in P but not A compartment cells. Here we provide evidence that Hedgehog (Hh), a protein secreted by P compartment cells, spreads into each A compartment across the anterior and the posterior boundaries to form opposing concentration gradients that organize cell pattern and polarity. We find that anteriorly and posteriorly situated cells within the A compartment respond in distinct ways to Hh: they express different combinations of genes and form different cell types. They also form polarised structures that, in the anterior part, point down the Hh gradient and, in the posterior part, point up the gradient - therefore all structures point posteriorly. Finally, we show that ectopic Hh can induce cells in the middle of each A compartment to activate en. Where this happens, A compartment cells are transformed into an ectopic P compartment and reorganise pattern and polarity both within and around the transformed tissue. Many of these results are unexpected and lead us to reassess the role of gradients and compartments in patterning insect segments.

Hedgehog acts by distinct gradient and signal relay mechanisms to organise cell type and cell polarity in the Drosophila abdomen

The epidermis of the adult Drosophila abdomen is formed by a chain of anterior (A) and posterior (P) compartments, each segment comprising one A and one P compartment. In the accompanying paper (Struhl et al., 1997), we provide evidence that Hedgehog protein (Hh), being secreted from P compartment cells, organises the pattern and polarity of A compartment cells. Here we test whether Hh acts directly or by a signal relay mechanism. We use mutations in Protein Kinase A (PKA) or smoothened (smo) to activate or to block Hh signal transduction in clones of A compartment cells. For cell type, a scalar property, both manipulations cause strictly autonomous transformations: the cells affected are exactly those and only those that are mutant. Hence, we infer that Hh acts directly on A compartment cells to specify the various types of cuticular structures that they differentiate. By contrast, these same manipulations cause non-autonomous effects on cell polarity, a vectorial property. Consequently, we surmise that Hh influences cell polarity indirectly, possibly by inducing other signalling factors. Finally, we present evidence that Hh does not polarise abdominal cells by utilising either Decapentaplegic (Dpp) or Wingless (Wg), the two morphogens through which Hh acts during limb development. We conclude that, in the abdomen, cell type and cell polarity reflect distinct outputs of Hh signalling and propose that these outputs are controlled by separable gradient and signal relay mechanisms.