The tumour-suppressor gene patched encodes a candidate receptor for Sonic hedgehog

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.

Involvement of the sonic hedgehog, patched 1 and bmp2 genes in patterning of the zebrafish dermal fin rays

The signaling molecule encoded by Sonic hedgehog (shh) participates in the patterning of several embryonic structures including limbs. During early fin development in zebrafish, a subset of cells in the posterior margin of pectoral fin buds express shh. We have shown that regulation of shh in pectoral fin buds is consistent with a role in mediating the activity of a structure analogous to the zone of polarizing activity (ZPA) (Akimenko and Ekker (1995) Dev. Biol. 170, 243–247). During growth of the bony rays of both paired and unpaired fins, and during fin regeneration, there does not seem to be a region equivalent to the ZPA and one would predict that shh would play a different role, if any, during these processes specific to fish fins. We have examined the expression of shh in the developing fins of 4-week old larvae and in regenerating fins of adults. A subset of cells in the basal layer of the epidermis in close proximity to the newly formed dermal bone structures of the fin rays, the lepidotrichia, express shh, and ptc1 which is thought to encode the receptor of the SHH signal. The expression domain of ptc1 is broader than that of shh and adjacent blastemal cells releasing the dermal bone matrix also express ptc1. Further observations indicate that the bmp2 gene, in addition to being expressed in the same cells of the basal layer of the epidermis as shh, is also expressed in a subset of the ptc1-expressing cells of the blastema. Amputations of caudal fins immediately after the first branching point of the lepidotrichia, and global administration of all-trans-retinoic acid, two procedures known to cause fusion of adjacent rays, result in a transient decrease in the expression of shh, ptc1 and bmp2. The effects of retinoic acid on shh expression occur within minutes after the onset of treatment suggesting direct regulation of shh by retinoic acid. These observations suggest a role for shh, ptc1 and bmp2 in patterning of the dermoskeleton of developing and regenerating teleost fins.

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.

The parathyroid hormone/parathyroid hormone-related peptide receptor coordinates endochondral bone development by directly controlling chondrocyte differentiation

During vertebrate limb development, growth plate chondrocytes undergo temporally and spatially coordinated differentiation that is necessary for proper morphogenesis. Parathyroid hormone-related peptide (PTHrP), its receptor, the PTH/PTHrP receptor, and Indian hedgehog are implicated in the regulation of chondrocyte differentiation, but the specific cellular targets of these molecules and specific cellular interactions involved have not been defined. Here we generated chimeric mice containing both wild-type and PTH/PTHrP receptor (-/-) cells, and analyzed cell-cell interactions in the growth plate in vivo. Abnormal differentiation of mutant cells shows that PTHrP directly signals to the PTH/PTHrP receptor on proliferating chondrocytes to slow their differentiation. The presence of ectopically differentiated mutant chondrocytes activates the Indian hedgehog/PTHrP axis and slows differentiation of wild-type chondrocytes. Moreover, abnormal chondrocyte differentiation affects mineralization of cartilaginous matrix in a non-cell autonomous fashion; matrix mineralization requires a critical mass of adjacent ectopic hypertrophic chondrocytes. Further, ectopic hypertrophic chondrocytes are associated with ectopic bone collars in adjacent perichondrium. Thus, the PTH/PTHrP receptor directly controls the pace and synchrony of chondrocyte differentiation and thereby coordinates development of the growth plate and adjacent bone.

Common molecular pathways in skeletal morphogenesis and repair

The formation of bone is a continual process in vertebrate development, initiated during fetal development and persisting in adulthood in the form of remodeling and repair. The remarkable capacity of skeletal tissues to regenerate has led to the hypothesis that the molecular signaling pathways regulating skeletogenesis are shared during fetal development and adult wound healing. A number of key regulatory pathways that are required for endochondral ossification during fetal development are described, and their reintroduction in fracture repair demonstrated. Secreted proteins such as Sonic and Indian hedgehog exert their effect on pattern formation and chondrogenesis in the appendicular skeleton, partly through regulation of molecules such as bone morphogenic proteins (Bmps) and parathyroid hormone-related peptide (PTHrP). Once chondrocytes have matured and hypertrophied, they undergo apoptosis and are replaced by bone; the transcription factor Cbfal plays a critical role in this process of chondrocyte differentiation and ossification. Analyses of the expression patterns of these genes during fracture healing strongly suggest that they play equivalent roles in adult wound repair. Knowledge acquired through the study of fetal skeletogenesis will undoubtedly contribute to an understanding of fracture repair, and subsequently guide the development of biologically based therapeutic interventions.

Analysis of epithelial-mesenchymal interactions in the initial morphogenesis of the mammalian tooth

Epithelial-mesenchymal interactions govern the development of epidermal organs such as teeth. During the early stages of tooth development, a local ectodermal thickening which expresses several signaling molecules appears. It is believed that these in turn signal to the underlying mesenchyme triggering mesenchymal condensation and tooth development. For example, epithelially expressed Bmp4 induces Msx1 and Lef1 as well as itself in the underlying mesenchyme. In this paper we have investigated the role of four epithelial signaling molecules, Bmp2, Shh, Wnt10a, and Wnt10b, in the early inductive cascades that govern tooth development. We show that all four genes are specifically expressed in the epithelium between E11.0 and E12.0 when tooth morphogenesis is first apparent. Although Shh, Bmp2, and Wnt10b have similar, if not identical, expression patterns, each signal has a distinct molecular action on the jaw mesenchyme. Whereas Shh and Wnt10b can induce general Hedgehog and Wnt targets, Ptc and Gli for Shh and Lef1 for Wnt10b, only Bmp2 is able to induce tooth-specific expression of Msx1. Thus, there are distinct targets for all three pathways. Interestingly, both Bmp and Wnt signaling activate Lef1, making it a candidate for integrating the two distinct signaling pathways.

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.

Functional promiscuity of gene regulation by serpentine receptors in Dictyostelium discoideum

Serpentine receptors such as smoothened and frizzled play important roles in cell fate determination during animal development. In Dictyostelium discoideum, four serpentine cyclic AMP (cAMP) receptors (cARs) regulate expression of multiple classes of developmental genes. To understand their function, it is essential to know whether each cAR is coupled to a specific gene regulatory pathway or whether specificity results from the different developmental regulation of individual cARs. To distinguish between these possibilities, we measured gene induction in car1 car3 double mutant cell lines that express equal levels of either cAR1, cAR2, or cAR3 under a constitutive promoter. We found that all cARs efficiently mediate both aggregative gene induction by cAMP pulses and induction of postaggregative and prespore genes by persistent cAMP stimulation. Two exceptions to this functional promiscuity were observed. (i) Only cAR1 can mediate adenosine inhibition of cAMP-induced prespore gene expression, a phenomenon that was found earlier in wild-type cells. cAR1's mediation of adenosine inhibition suggests that cAR1 normally mediates prespore gene induction. (ii) Only cAR2 allows entry into the prestalk pathway. Prestalk gene expression is induced by differentiation-inducing factor (DIF) but only after cells have been prestimulated with cAMP. We found that DIF-induced prestalk gene expression is 10 times higher in constitutive cAR2 expressors than in constitutive cAR1 or cAR3 expressors (which still have endogenous cAR2), suggesting that cAR2 mediates induction of DIF competence. Since in wild-type slugs cAR2 is expressed only in anterior cells, this could explain the so far puzzling observations that prestalk cells differentiate at the anterior region but that DIF levels are actually higher at the posterior region. After the initial induction of DIF competence, cAMP becomes a repressor of prestalk gene expression. This function can again be mediated by cAR1, cAR2, and cAR3.

Ethanol impairs migration of the prechordal plate in the zebrafish embryo

Exposure of vertebrate embryos to ethanol causes cyclopia, but little is known about the underlying mechanisms of this effect. Here we show that cyclopia can be induced in the zebrafish by a short ethanol treatment during early gastrula stages and is accompanied by loss of gene expression characteristic of the ventral aspects of the fore- and midbrain. Interestingly, defects in the expression of ventral brain markers are linked to impaired migration of the prechordal plate mesoderm indicating that the correct position of the prechordal plate mesoderm under the anterior neural plate in the zebrafish embryo is required for specification of the anterior neural midline. Ethanol-induced cyclopia does not, however, impair the induction of anterior neuroectodermal structures in general. Finally, as genes like goosecoid and islet-1 are expressed in prechordal plate cells in a temporal pattern similar to control embryos despite the ectopic position of expressing cells, it appears that regulation of prechordal plate-specific gene expression is largely independent of the final position of the prechordal plate.

The teratogenic Veratrum alkaloid cyclopamine inhibits sonic hedgehog signal transduction

The steroidal alkaloid cyclopamine produces cyclopia and holoprosencephaly when administered to gastrulation-stage amniote embryos. Cyclopamine-induced malformations in chick embryos are associated with interruption of Sonic hedgehog (Shh)-mediated dorsoventral patterning of the neural tube and somites. Cell types normally induced in the ventral neural tube by Shh are either absent or appear aberrantly at the ventral midline after cyclopamine treatment, while dorsal cell types normally repressed by Shh appear ventrally. Somites in cyclopamine-treated embryos show Pax7 expression throughout, indicating failure of sclerotome induction. Cyclopamine at concentrations of 20–100 nM blocks the response of neural plate explants to recombinant Shh-N in a dose-dependent manner. Similar concentrations have no effect on the post-translational modification of Shh by cholesterol in transfected COS-1 cells. Comparison of the effects of cyclopamine to those of the holoprosencephaly-inducing cholesterol synthesis inhibitor AY-9944 shows that cyclopamine does not induce malformations by interfering with cholesterol metabolism. Although AY-9944 does not interrupt Shh signaling in ovo, it blocks the response to Shh-N in explants cultured without an exogenous cholesterol source. As predicted by current models of the regulation of cholesterol metabolism, the response to Shh-N in AY-9944-treated explants is restored by providing exogenous cholesterol. However, exogenous cholesterol does not restore Shh signaling in cyclopamine-treated explants. These findings suggest that cyclopamine-induced teratogenesis is due to a more direct antagonism of Shh signal transduction.

Stage-specific effects of sonic hedgehog expression in the epidermis

Sonic hedgehog (Shh) is expressed in the ectoderm of the forming hair follicle and feather bud during normal development. However, inappropriate activation of the Shh signal transduction cascade in human epidermis can cause basal cell carcinoma. Here we show that during normal development of avian skin, Shh is first expressed only after the responsiveness to this protein has been suppressed in most of the surrounding ectodermal cells. Forced expression of Shh in avian skin prior to this time causes a disorganized ectodermal proliferation. However, as skin begins to differentiate, the forced expression of Shh causes feather bud formation. Subsequently, expression of Shh in interfollicular epidermis has little or no morphological effect. Restricted responsiveness to Shh in developing skin has functional consequences for morphogenesis and may have important implications for cutaneous pathologies as well.

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.

Evidence that preaxial polydactyly in the Doublefoot mutant is due to ectopic Indian Hedgehog signaling

Patterning of the vertebrate limb along the anterior-posterior axis is controlled by the zone of polarizing activity (ZPA) located at the posterior limb margin. One of the vertebrate Hh family members, Shh, has been shown to be able to mediate the function of the ZPA. Several naturally occurring mouse mutations with the phenotype of preaxial polydactyly exhibit ectopic Shh expression at the anterior limb margin. In this study, we report the molecular characterization of a spontaneous mouse mutation, Doublefoot (Dbf). Dbf is a dominant mutation which maps to chromosome 1. Heterozygous and homozygous embryos display a severe polydactyly with 6 to 8 digits on each limb. We show here that Shh is expressed normally in Dbf mutants. In contrast, a second Hh family member, Indian hedgehog (Ihh) which maps close to Dbf, is ectopically expressed in the distal limb bud. Ectopic Ihh expression in the distal and anterior limb bud results in the ectopic activation of several genes associated with anterior-posterior and proximal-distal patterning (Fgf4, Hoxd13, Bmp2). In addition, specific components in the Hedgehog pathway are either ectopically activated (Ptc, Ptc-2, Gli1) or repressed (Gli2). We propose that misexpression of Ihh, and not a novel Smoothened ligand as recently suggested (Hayes et al., 1998), is responsible for the Dbf phenotype. We consider that Ihh has a similar activity to Shh when expressed in the early Shh-responsive limb bud. To determine whether Dbf maps to the Ihh locus, which is also on chromosome 1, we performed an interspecific backcross. These results demonstrate that Dbf and Ihh are genetically separated by approximately 1.3 centimorgans, suggesting that Dbf mutation may cause an exceptionally long-range disruption of Ihh regulation. Although this leads to ectopic activation of Ihh, normal expression of Ihh in the cartilaginous elements is retained.

The hereditary renal cell carcinoma 3;8 translocation fuses FHIT to a patched-related gene, TRC8

The 3;8 chromosomal translocation, t(3;8)(p14.2;q24.1), was described in a family with classical features of hereditary renal cell carcinoma. Previous studies demonstrated that the 3p14.2 breakpoint interrupts the fragile histidine triad gene (FHIT) in its 5' noncoding region. However, evidence that FHIT is causally related to renal or other malignancies is controversial. We now show that the 8q24.1 breakpoint region encodes a 664-aa multiple membrane spanning protein, TRC8, with similarity to the hereditary basal cell carcinoma/segment polarity gene, patched. This similarity involves two regions of patched, the putative sterol-sensing domain and the second extracellular loop that participates in the binding of sonic hedgehog. In the 3;8 translocation, TRC8 is fused to FHIT and is disrupted within the sterol-sensing domain. In contrast, the FHIT coding region is maintained and expressed. In a series of sporadic renal carcinomas, an acquired TRC8 mutation was identified. By analogy to patched, TRC8 might function as a signaling receptor and other pathway members, to be defined, are mutation candidates in malignant diseases involving the kidney and thyroid.

Holoprosencephaly: a paradigm for the complex genetics of brain development

Holoprosencephaly (HPE) is the most common major developmental defect of the forebrain in humans. Clinical expression is variable, ranging from a small brain with a single cerebral ventricle and cyclopia to clinically unaffected carriers in familial HPE. Significant aetiological heterogeneity exists in HPE and includes both genetic and environmental causes. Recently, defects in the cell signalling pathway involving the Sonic Hedgehog (SHH) gene, as well as defects in the cholesterol biosynthesis, have been shown to cause HPE in humans. These discoveries and current genetic approaches serve as a paradigm for studying normal and abnormal brain morphogenesis.

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.

The Shh signalling pathway in tooth development: defects in Gli2 and Gli3 mutants

The expression of genes involved in the Sonic Hedgehog signalling pathway, including Shh, Ptc, Smo, Gli1, Gli2 and Gli3, were found to be expressed in temporal and spatial patterns during early murine tooth development, suggestive of a role in early tooth germ initiation and subsequent epithelial-mesenchymal interactions. Of these Ptc, Smo, Gli1, Gli2 and Gli3 were expressed in epithelium and mesenchyme whereas Shh was only detected in epithelium. This suggests that Shh is involved in both lateral (epithelial-mesenchymal) and planar (epithelial-epithelial) signalling in early tooth development. Ectopic application of Shh protein to mandibular mesenchyme induced the expression of Ptc and Gli1. Addition of exogenous Shh protein directly into early tooth germs and adjacent to tooth germs, resulted in abnormal epithelial invagination, indicative of a role for Shh in epithelial cell proliferation. In order to assess the possible role of this pathway, tooth development in Gli2 and Gli3 mutant embryos was investigated. Gli2 mutants were found to have abnormal development of maxillary incisors, probably resulting from a mild holoprosencephaly, whereas Gli3 mutants had no major tooth abnormalities. Gli2/Gli3 double homozygous mutants did not develop any normal teeth and did not survive beyond embryonic day 14.5; however, Gli2(−/−); Gli3(+/−) did survive until birth and had small molars and mandibular incisors whereas maxillary incisor development was arrested as a rudimentary epithelial thickening. These results show an essential role for Shh signalling in tooth development that involves functional redundancy of downstream Gli genes.

Human developmental disorders and the Sonic hedgehog pathway

Sonic hedgehog (Shh) is a morphogen that is crucial for normal development of a variety of organ systems, including the brain and spinal cord, the eye, craniofacial structures, and the limbs. Mutations in the human SHH gene and genes that encode its downstream intracellular signaling pathway cause several clinical disorders. These include holoprosencephaly (HPE, the most common anomaly of the developing forebrain), nevoid basal cell carcinoma syndrome, sporadic tumors, including basal cell carcinomas, and three distinct congenital disorders: Greig syndrome Pallister-Hall syndrome, and isolated postaxial polydactyly. These conditions caused by abnormalities in the SHH pathway demonstrate the crucial role of SHH in complex developmental processes, and molecular analyses of these disorders provide insight into the normal function of the SHH pathway in human development.

Patching together the genetics of Gorlin syndrome

BACKGROUND

Gorlin syndrome is an autosomal dominant disorder characterized by developmental defects and susceptibility to cancer, especially to basal cell carcinomas. The genetic basis of this disorder has recently been elucidated.

METHODS

In this article previous studies are reviewed in which loss of heterozygosity analysis of tumours and normal tissue pointed to a region on chromosome 9 as being involved in Gorlin syndrome. In this light, Knudson's two-hit model is discussed. The identification of the involvement of the patched gene in Gorlin syndrome is reviewed. New data on genotype-phenotype correlations in the syndrome are presented.

RESULTS

Loss-of-heterozygosity studies, together with standard family studies using linkage analysis, have proved useful in identifying the location of a gene with complex phenotypic expression.

CONCLUSION

The application of the two-hit model, as utilized in loss-of-heterozygosity studies, has been very useful in elucidating the genetic basis of Gorlin syndrome. There may be a correlation between certain aspects of the mutations in patched and the clinical presentation of the disorder in families.

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.

GFRalpha1 is an essential receptor component for GDNF in the developing nervous system and kidney

Glial cell line-derived neurotrophic factor (GDNF) is a distant member of the TGFbeta protein family that is essential for neuronal survival and renal morphogenesis. We show that mice who are deficient in the glycosyl-phosphatidyl inositol (GPI) -linked protein GFRalpha1 (GDNFRalpha) display deficits in the kidneys, the enteric nervous system, and spinal motor and sensory neurons that are strikingly similar to those of the GDNF- and Ret-deficient mice. GFRalpha1-deficient dopaminergic and nodose sensory ganglia neurons no longer respond to GDNF or to the structurally related protein neurturin (NTN) but can be rescued when exposed to GDNF or neurturin in the presence of soluble GFRalpha1. In contrast, GFRalpha1-deficient submandibular parasympathetic neurons retain normal response to these two factors. Taken together with the available genetic and biochemical data, these findings support the idea that GFRalpha1 and the transmembrane tyrosine kinase Ret are both necessary receptor components for GDNF in the developing kidney and nervous system, and that GDNF and neurturin can mediate some of their activities through a second receptor.

Sonic hedgehog is required for survival of both myogenic and chondrogenic somitic lineages

In vertebrates, the medial moieties of the somites give rise to the vertebrae and epaxial muscles, which develop in close relationship with the axial organs, neural tube and notochord. The lateral moieties contribute to the ribs and to limb and body wall muscles (hypaxial muscles) after a phase of lateral and ventral migration. Surgical ablation of the neural tube and notochord in the chick embryo during segmentation and early differentiation of the somites (day 2 of incubation) does not affect primary development of the hypaxial muscles, but leads to a complete absence of epaxial muscles, vertebrae and ribs, due to cell death in the somites. Here we demonstrate that cell death, which occurs within 24 hours of excision of the axial organs, affects both myogenic and chondrogenic cell lineages defined, respectively, by the expression of MyoD and Pax-1 genes. In contrast, Pax-3 transcripts, normally present in cells giving rise to hypaxial muscles, are preserved in the excised embryos. Backgrafting either the ventral neural tube or the notochord allows survival of MyoD- and Pax-1-expressing cells. Similarly, Sonic hedgehog-producing cells grafted in place of axial organs also rescue MyoD- and Pax-1-expressing cells from death and allow epaxial muscles, ribs and vertebrae to undergo organogenesis. These results demonstrate that the ventral neural tube and the notochord promote the survival of both myogenic and chondrogenic cell lineages in the somites and that this action is mediated by Sonic hedgehog.

Identification of mutations in the human PATCHED gene in sporadic basal cell carcinomas and in patients with the basal cell nevus syndrome

Mutations in PATCHED (PTC), the human homolog of the Drosophila patched gene, have been identified in most exons of the gene in patients with the basal cell nevus syndrome and in sporadic basal cell carcinomas. We have screened the 23 PTC exons for mutations using single strand conformation polymorphism analysis of DNA from 86 basal cell nevus syndrome probands, 26 sporadic basal cell carcinomas, and seven basal cell nevus syndrome-associated basal cell carcinomas. This screen identified mutations located in eight exons in 13 of the basal cell nevus syndrome patients and in three of the tumors. The most common mutations were frameshifts resulting in premature chain termination. These results provide further evidence for the crucial role of PTC as a tumor suppressor in human keratinocytes.

Identification of a palmitic acid-modified form of human Sonic hedgehog

During hedgehog biosynthesis, autocatalytic processing produces a lipid-modified amino-terminal fragment (residues 24-197 in the human Sonic hedgehog sequence) that is responsible for all known hedgehog signaling activity and that is highly conserved evolutionarily. Published in vitro biochemical studies using Drosophila hedgehog identified the membrane anchor as a cholesterol, and localized the site of attachment to the COOH terminus of the fragment. We have expressed full-length human Sonic hedgehog in insect and in mammalian cells and determined by mass spectrometry that, in addition to cholesterol, the human hedgehog protein is palmitoylated. Peptide mapping and sequencing data indicate that the palmitoyl group is attached to the NH2 terminus of the protein on the alpha-amino group of Cys-24. Cell-free palmitoylation studies demonstrate that radioactive palmitic acid is readily incorporated into wild type Sonic hedgehog, but not into variant forms lacking the Cys-24 attachment site. The lipid-tethered forms of hedgehog showed about a 30-fold increase in potency over unmodified soluble hedgehog in a cell- based (C3H10T1/2 alkaline phosphatase induction) assay, suggesting that the lipid tether plays an important role in hedgehog function. The observation that an extracellular protein such as Shh is palmitoylated is highly unusual and further adds to the complex nature of this protein.

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.

Expression of wingless in the Drosophila embryo: a conserved cis-acting element lacking conserved Ci-binding sites is required for patched-mediated repression

Patterning of the Drosophila embryo depends on the accurate expression of wingless (wg), which encodes a secreted signal required for segmentation and many other processes. Early expression of wg is regulated by the nuclear proteins of the gap and pair-rule gene classes but, after gastrulation, wg transcription is also dependent on cell-cell communication. Signaling to the Wg-producing cells is mediated by the secreted protein, Hedgehog (Hh), and by Cubitus interruptus (Ci), a transcriptional effector of the Hh signal transduction pathway. The transmembrane protein Patched (Ptc) acts as a negative regulator of wg expression; ptc- embryos have ectopic wg expression. According to the current models, Ptc is a receptor for Hh. The default activity of Ptc is to inhibit Ci function; when Ptc binds Hh, this inhibition is released and Ci can control wg transcription. We have investigated cis-acting sequences that regulate wg during the time that wg expression depends on Hh signaling. We show that approximately 4.5 kb immediately upstream of the wg transcription unit can direct expression of the reporter gene lacZ in domains similar to the normal wg pattern in the embryonic ectoderm. Expression of this reporter construct expands in ptc mutants and responds to hh activity. Within this 4.5 kb, a 150 bp element, highly conserved between D. melanogaster and Drosophila virilis, is required to spatially restrict wg transcription. Activity of this element depends on ptc, but it contains no consensus Ci-binding sites. The discovery of an element that is likely to bind a transcriptional repressor was unexpected, since the prevailing model suggests that wg expression is principally controlled by Hh signaling acting through the Ci activator. We show that wg regulatory DNA can drive lacZ in a proper wg-like pattern without any conserved Ci-binding sites and suggest that Ci can not be the sole endpoint of the Hh pathway.

The transfer of left-right positional information during chick embryogenesis

The earliest known left-right asymmetric genes are expressed at Hensen's node during chick gastrulation. Gene expression following reorientation of the node shows asymmetry is instructed by adjacent tissue, hence left-right information originates outside the node. Subsequently, the node signals back to the lateral tissue, initiating a cascade leading to left-sided expression of nodal in the lateral plate mesoderm. Loss of nodal expression in the presence of blocking antibodies confirms that Sonic hedgehog is the key signal conveying left-right information from the node; however, manipulation of explant cultures suggests that the induction of nodal requires secondary signals produced in the paraxial mesoderm. These experiments establish the time of action of these signals to and from Hensen's node in establishing left-right asymmetry.

Related membrane domains in proteins of sterol sensing and cell signaling provide a glimpse of treasures still buried within the dynamic realm of intracellular metabolic regulation

Recent discoveries in the regulation of cholesterol metabolism have documented a two step proteolytic pathway that regulates nuclear targeting of the sterol regulatory element binding proteins. Sterol regulatory element binding protein cleavage activating protein is a newly identified protein that modulates the proteolytic maturation of the sterol regulatory element binding proteins. It contains a domain that is quite similar in sequence to the membrane spanning region of the rate controlling enzyme of cholesterol biosynthesis, 3-hydroxy-3-methylglutaryl coenzyme A reductase. The membrane domain of the reductase is involved in its post-translational regulation by cholesterol. The molecular defect in the intracellular cholesterol storage disease, Niemann-Pick type C, has also recently been identified. Surprisingly, the affected gene encodes a protein with similarity to the membrane domains that are conserved in 3-hydroxy-3-methylglutaryl reductase and sterol regulatory element binding protein cleavage activating protein. Furthermore, the cell surface receptor for the sterol modified hedgehog morphogen, Patched, also contains a membrane domain with significant similarity to this putative sterol monitoring domain. These recent developments suggest a common mechanism for sensing intracellular sterol levels and cell signaling, which is based on the function of related membrane domains that are contained in key regulatory proteins.

FGF-, BMP- and Shh-mediated signalling pathways in the regulation of cranial suture morphogenesis and calvarial bone development

The development of calvarial bones is tightly co-ordinated with the growth of the brain and needs harmonious interactions between different tissues within the calvarial sutures. Premature fusion of cranial sutures, known as craniosynostosis, presumably involves disturbance of these interactions. Mutations in the homeobox gene Msx2 as well as the FGF receptors cause human craniosynostosis syndromes. Our histological analysis of mouse calvarial development demonstrated morphological differences in the sagittal suture between embryonic and postnatal stages. In vitro culture of mouse calvaria showed that embryonic, but not postnatal, dura mater regulated suture patency. We next analysed by in situ hybridisation the expression of several genes, which are known to act in conserved signalling pathways, in the sagittal suture during embryonic (E15-E18) and postnatal stages (P1-P6). Msx1 and Msx2 were expressed in the sutural mesenchyme and the dura mater. FGFR2(BEK), as well as Bmp2 and Bmp4, were intensely expressed in the osteogenic fronts and Bmp4 also in the mesenchyme of the sagittal suture and in the dura mater. Fgf9 was expressed throughout the calvarial mesenchyme, the dura mater, the developing bones and the overlying skin, but Fgf4 was not detected in these tissues. Interestingly, Shh and Ptc started to be expressed in patched pattern along the osteogenic fronts at the end of embryonic development and, at this time, the expression of Bmp4 and sequentially those of Msx2 and Bmp2 were reduced, and they also acquired patched expression patterns. The expression of Msx2 in the dura mater disappeared after birth. <P> FGF and BMP signalling pathways were further examined in vitro, in E15 mouse calvarial explants. Interestingly, beads soaked in FGF4 accelerated sutural closure when placed on the osteogenic fronts, but had no such effect when placed on the mid-sutural mesenchyme. BMP4 beads caused an increase in tissue volume both when placed on the osteogenic fronts and on the mid-sutural area, but did not effect suture closure. BMP4 induced the expression of both Msx1 and Msx2 genes in sutural tissue, while FGF4 induced only Msx1. We suggest that the local application of FGF on the osteogenic fronts accelerating suture closure in vitro, mimics the pathogenesis of human craniosynostosis syndromes in which mutations in the FGF receptor genes apparently cause constitutive activation of the receptors. Taken together, our data suggest that conserved signalling pathways regulate tissue interactions during suture morphogenesis and intramembranous bone formation of the calvaria and that morphogenesis of mouse sagittal suture is controlled by different molecular mechanisms during the embryonic and postnatal stages. Signals from the dura mater may regulate the maintenance of sutural patency prenatally, whereas signals in the osteogenic fronts dominate after birth.

Regulation of growth region cartilage proliferation and differentiation by perichondrium

Endochondral bone formation in vertebrates requires precise coordination between proliferation and differentiation of the participating chondrocytes. We examined the role of perichondrium in this process using an organ culture system of chicken embryonic tibiotarsi. A monoclonal antibody against chicken collagen type X, specifically expressed by hypertrophic chondrocytes, was utilized to monitor the terminal differentiation of chondrocytes. Proliferation of chondrocytes was examined by a BrdU-labeling procedure. The absence of perichondrium is correlated with an extended zone of cartilage expressing collagen type X, suggesting that the perichondrium regulates chondrocyte hypertrophy in a negative manner. Removal of perichondrium, in addition, resulted in an extended zone of chondrocytes incorporating BrdU, indicating that the perichondrium also negatively regulates the proliferation of chondrocytes. Partial removal of perichondrium from one side of the tibiotarsus led to expansion of both the collagen type X-positive domain and the BrdU-positive zone at the site of removal but not where the perichondrium remained intact. This suggests that both types of regulation by the perichondrium are local effects. Furthermore, addition of bovine parathyroid hormone (PTH) to perichondrium-free cultures reversed the expansion of the collagen type X-positive domain but not that of the proliferative zone. This suggests that the regulation of differentiation is dependent upon the PTH/PTHrP receptor and that the regulation of proliferation is likely independent of it. Taken together, these results are consistent with a model where perichondrium regulates both the exit of chondrocytes from the cell cycle, and their subsequent differentiation.

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.

Holoprosencephaly: from Homer to Hedgehog

Holoprosencephaly (HPE), a common developmental defect affecting the forebrain and face, is etiologically heterogeneous and exhibits wide phenotypic variation. Graded degrees of severity of the brain malformation are also reflected in the highly variable craniofacial malformations associated with HPE. In addition, individuals with microforms of HPE, who usually have normal cognition and normal brain imaging, are at risk for having children with HPE. Some obligate carriers for HPE may not have any phenotypic abnormalities. Recurrent chromosomal rearrangements in individuals with HPE suggest loci containing genes important for brain development, and abnormalities in these genes may result in HPE. Recently, Sonic Hedgehog (SHH) was the first gene identified as causing HPE in humans. Proper function of SHH depends on cholesterol modification. Other candidate genes that may be involved in HPE include components of the SHH pathway, elements involved in cholesterol metabolism, and genes expressed in the developing forebrain.

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.

Patterning of the embryonic forebrain

The vertebrate forebrain is derived from the anterior neural plate, where anteroposterior, dorsoventral and local patterning mechanisms specify regional identify. The recent identification of genetic regulators of these processes has opened the way to elucidating how the major forebrain regions (i.e. cerebral cortex, basal ganglia, thalamus, and hypothalamus) are formed, and how molecular lesions in these processes cause human birth defects.

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 expression in developing chicken digestive organs is regulated by epithelial-mesenchymal interactions

Sonic hedgehog (Shh) gene encodes a secreted protein that acts as an important mediator of cell-cell interactions. A detailed analysis of Shh expression in the digestive organs of the chicken embryo was carried out. Shh expression in the endoderm begins at stage 7, when the formation of the foregut commences, and is found as narrow bands in the midgut. Shh expression around the anterior intestinal portal at stage 15 is restricted to the columnar endoderm lined by the thick splanchnic mesoderm, suggesting that the existence of thick splanchnic mesoderm might be necessary for Shh expression in the columnar endoderm. After the gut is closed, Shh expression is found universally in digestive epithelia, including the cecal epithelium. However, its expression ceases in the epithelium of the proventricular glands, the ductus choledochus and ductus pancreaticus that protrude from the main digestive duct. When the gizzard epithelium differentiated into glands under the influence of the proventricular mesenchyme, the glandular epithelium lost the ability to express Shh. These findings suggest that Shh expression in the epithelium may be regulated by surrounding mesenchyme throughout organogenesis of the digestive organs and is closely involved in epithelial-mesenchymal interactions in developing digestive organs.

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.

Sonic Hedgehog induces proliferation of committed skeletal muscle cells in the chick limb

Myogenic Regulatory Factors (MRFs) are a family of transcription factors whose expression in a cell reflects the commitment of this cell to a myogenic fate before any cytological sign of muscle differentiation is detectable. Myogenic cells in limb skeletal muscles originate from the lateral half of the somites. Cells that migrate away from the lateral part of the somites to the limb bud do not initially express any member of the MRF family. Expression of MRFs in the muscle precursor cells starts after the migration process is completed. The extracellular signals involved in activating the myogenic programme in muscle precursor cells in the limb in vivo are not known. We wished to investigate whether Sonic Hedgehog (SHH) expressed in the posterior part of the limb bud could be involved in differentiation of the muscle precursor cells in the limb. We found that retrovirally overexpressed SHH in the limb bud induced the extension of the expression domain of the Pax-3 gene, then that of the MyoD gene and finally that of the myosin protein. This led to an hypertrophy of the muscles in vivo. Addition of SHH to primary cultures of myoblasts resulted in an increase in the proportion of myoblasts that incorporate bromodeoxyuridine, resulting in an increase of myotube number. These data show that SHH is able to activate myogenesis in vivo and in vitro in already committed myoblasts and suggest that the stimulation of the myogenic programme by SHH involves activation of cell proliferation.

Expression profile of Gli family members and Shh in normal and mutant mouse limb development

Gli genes represent a small family, encoding zinc-finger proteins of the Krüppel-type. The family consists of Gli(1), Gli2, and Gli3, all of which are expressed in the developing mouse limb bud. To assess the role of the Gli family and Sonic hedgehog (Shh) in mouse limb development, we compared the expression domains of all three Gli genes and of Shh. Although each Gli gene has its own distinct expression pattern in limb buds, at 10.5-11.5 dpc all three genes were found not to be expressed in the posterior region, the presumptive Shh expression domain. This transient mutually exclusive expression suggested a potential interaction between Gli genes and Shh. To address this matter, we analysed the expression of Gli genes and Shh in two polydactyly mouse mutants, Extra toes (Xt) and Hemimelic-extra toes (Hx) which express Shh ectopically in the anterior region of the limb field. Since Xt mice lack Gli3 expression, the ectopic Shh expression is genetically linked to the absence of Gli3. In Hx mice we found a down-regulation of Gli3 in the anterior region of the limb bud. In both mutants Gli2 expression pattern was not altered, whereas Gli1 expression was anteriorly up-regulated adjacent to the ectopic Shh domain. These results strongly suggest a positive regulation of Gli1 by Shh and a negative interaction between Shh and Gli3.

Activating Smoothened mutations in sporadic basal-cell carcinoma

Basal-cell carcinomas (BCCs) are the commonest human cancer. Insight into their genesis came from identification of mutations in the PATCHED gene (PTCH) in patients with the basal-cell nevus syndrome, a hereditary disease characterized by multiple BCCs and by developmental abnormalities. The binding of Sonic hedgehog (SHH) to its receptor, PTCH, is thought to prevent normal inhibition by PTCH of Smoothened (SMO), a seven-span transmembrane protein. According to this model, the inhibition of SMO signalling is relieved following mutational inactivation of PTCH in basal-cell nevus syndrome. We report here the identification of activating somatic missense mutations in the SMO gene itself in sporadic BCCs from three patients. Mutant SMO, unlike wild type, can cooperate with adenovirus E1A to transform rat embryonic fibroblast cells in culture. Furthermore, skin abnormalities similar to BCCs developed in transgenic murine skin overexpressing mutant SMO. These findings support the role of SMO as a signalling component of the SHH-receptor complex and provide direct evidence that mutated SMO can function as an oncogene in BCCs.

Emergence of determined myotome precursor cells in the somite

Myotome and sclerotome precursor cells are derived, respectively, from cells in the dorsomedial and ventromedial regions of the somite. To assay changes in the specification of myotomal precursor cells during somite maturation, we implanted dorsomedial quadrant fragments, from staged quail somites, next to the notochords of host chick embryos, and superimposed two additional notochords on these implants. In this notochord signalling environment, dorsomedial quadrant cells that are developmentally plastic are expected to differentiate as cartilage, while cells determined to a myogenic fate are expected to differentiate as skeletal muscle. Large numbers of differentiated chondrocytes developed from dorsomedial quadrant grafts of all stages of paraxial mesoderm development tested, indicating that persistent chondrogenic potential in cells fated to form muscle and dermis can be elicited by notochord signals. Differentiated myocytes, however, appeared in two somite-stage-dependent phases. In the first phase, dorsomedial quadrants from segmental plate and early stage somites (II and IV) form small, disorganized clusters of individual myocytes. The frequency of first-phase myocluster formation increases as myogenic factor expression begins in the dorsomedial quadrant, indicating that myogenic determination assayed by this method is closely linked to the expression of myogenic factors in the dorsomedial quadrant. In the second phase, dorsomedial quadrants from somite stages XI-XIII consistently form morphologically organized muscle tissue containing large numbers of parallel-oriented, multinucleated myotubes. Mitotic labelling demonstrated that muscle precursors were determined to the muscle phenotype prior to withdrawal from the cell cycle. Thus, myogenic determination in cells of the dorsomedial quadrant is acquired at earlier stages of somite maturation than the ability to proliferate and form muscle tissue. These results are consistent with the hypothesis that successive lineages of myotome precursor cells with different mitotic and morphogenetic properties arise in the dorsomedial quadrant during somite maturation.