The whereabouts of a morphogen: direct evidence for short- and graded long-range activity of hedgehog signaling peptides

Constitutively active PTH/PTHrP receptor in odontoblasts alters odontoblast and ameloblast function and maturation

Parathyroid hormone (PTH)-related protein (PTH-rP) is an important autocrine/paracrine attenuator of programmed cell differentiation whose expression is restricted to the epithelial layer in tooth development. The PTH/PTHrP receptor (PPR) mRNA in contrast is detected in the dental papilla, suggesting that PTHrP and the PPR may modulate epithelial-mesenchymal interactions. To explore the possible interactions, we studied the previously described transgenic mice in which a constitutively active PPR is targeted to osteoblastic cells. These transgenic mice have a vivid postnatal bone and tooth phenotype, with normal tooth eruption but abnormal, widened crowns. Transgene mRNA expression was first detected at birth in the dental papilla and, at 1 week postnatally, in odontoblasts. There was no transgene expression in ameloblasts or in other epithelial structures. Prenatally, transgenic molars and incisors revealed no remarkable change. By the age of 1 week, the dental papilla was widened, with disorganization of the odontoblastic layer and decreased dentin matrix. In addition, the number of cusps was abnormally increased, the ameloblastic layer disorganized, and enamel matrix decreased. Odontoblastic and, surprisingly, ameloblastic cytodifferentiation was impaired, as shown by in situ hybridization and electron microscopy. Interestingly, ameloblastic expression of Sonic Hedgehog, a major determinant of ameloblastic cytodifferentiation, was dramatically altered in the transgenic molars. These data suggest that odontoblastic activation of the PPR may play an important role in terminal odontoblastic and, indirectly, ameloblastic cytodifferentiation, and describe a useful model to study how this novel action of the PPR may modulate mesenchymal/epithelial interactions at later stages of tooth morphogenesis and development.

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

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

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

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

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

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

Indian hedgehog: Its roles and regulation in endochondral bone development

Normal endochondral bone development requires the coordination of chondrocyte proliferation and differentiation. Indian hedgehog (Ihh) is a morphogen produced by chondrocytes in the early stage of terminal differentiation and plays several key roles in this process. Ihh regulates growth of adjacent proliferative chondrocytes and can also regulate the rate of differentiation of chondrocytes indirectly through its stimulation of parathyroid hormone-related protein (PTHrP). In this review, we focus on recent studies that have identified new functions of Ihh and how Ihh itself is being regulated.

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

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

Indian hedgehog and syndecans-3 coregulate chondrocyte proliferation and function during chick limb skeletogenesis

Hedgehog proteins exert critical roles in embryogenesis and require heparan sulfate proteoglycans (HS-PGs) for action. Indian hedgehog (Ihh) is produced by prehypertrophic chondrocytes in developing long bones and regulates chondrocyte proliferation and other events, but it is not known whether it requires HS-PGs for function. Because the HS-PG syndecan-3 is preferentially expressed by proliferating chondrocytes, we tested whether it mediates Ihh action. Primary chick chondrocyte cultures were treated with recombinant Ihh (rIhh-N) in absence or presence of heparinase I or syndecan-3 neutralizing antibodies. While rIhh-N stimulated proliferation in control cultures, it failed to do so in heparinase- or antibody-treated cultures. In reciprocal gain-of-function studies, chondrocytes were made to overexpress syndecan-3 by an RCAS viral vector. Cells became more responsive to rIhh-N, but even this response was counteracted by heparinase or antibody treatment. To complement the in vitro data, RCAS viral particles were microinjected in day 4-5 chick wing buds and effects of syndecan-3 misexpression were monitored over time. Syndecan-3 misexpression led to widespread chondrocyte proliferation and, interestingly, broader expression and distribution of Ihh. In addition, the syndecan-3 misexpressing skeletal elements were short, remained cartilaginous, lacked osteogenesis, and exhibited a markedly reduced expression of collagen X and osteopontin, products characteristic of hypertrophic chondrocytes and bone cells. The data are the first to indicate that Ihh action in chondrocyte proliferation involves syndecan-3 and to identify a specific member of the syndecan family as mediator of hedgehog function.

Sonic hedgehog-patched Gli signaling in the developing rat prostate gland: lobe-specific suppression by neonatal estrogens reduces ductal growth and branching

While prostate gland development is dependent on androgens, other hormones including retinoids and estrogens can influence this process. Brief exposure to high-dose estrogen during the neonatal period in rats leads to permanent, lobe-specific aberrations in the prostate gland, a phenomenon referred to as developmental estrogenization. We have previously shown that this response is mediated through alterations in steroid receptor expression; however, further downstream mechanisms remain unclear. Herein, we examined Sonic hedgehog (Shh)-patched (ptc)-gli in the developing rat prostate gland, its role in branching morphogenesis, and the effects of neonatal estrogens on its expression and localization to determine whether a disturbance in this signaling pathway is involved in mediating the estrogenized phenotype. Shh was expressed in epithelial cells at the distal tips of elongating ducts in discreet, heterogeneous foci, while ptc and gli1-3 were expressed in the adjacent mesenchymal cells in the developing gland. The addition of Shh protein to cultured neonatal prostates reduced ductal growth and branching, decreased Fgf10 transcript, and increased Bmp4 expression in the adjacent mesenchyme. Shh-induced growth suppression was reversed by exogenous Fgf10, but not noggin, indicating that Fgf10 suppression is the proximate cause of the growth inhibition. A model is proposed to show how highly localized Shh expression along with regulation of downstream morphogens participates in dichotomous branching during prostate morphogenesis. Neonatal exposure to high-dose estradiol suppressed Shh, ptc, gli1, and gli3 expressions and concomitantly blocked ductal branching in the dorsal and lateral prostate lobes specifically. In contrast, ventral lobe branching and Shh-ptc-gli expression were minimally affected by estrogen exposure. Organ culture studies with lateral prostates confirmed that estradiol suppressed Shh-ptc-gli expression directly at the prostatic level. Taken together, the present findings indicate that lobe-specific decreases in Shh-ptc-gli expression are involved in mediating estradiol-induced suppression of dorsal and lateral lobe ductal growth and branching during prostate morphogenesis.

Sox5 and Sox6 are needed to develop and maintain source, columnar, and hypertrophic chondrocytes in the cartilage growth plate

Sox5 and Sox6 encode Sry-related transcription factors that redundantly promote early chondroblast differentiation. Using mouse embryos with three or four null alleles of Sox5 and Sox6, we show that they are also essential and redundant in major steps of growth plate chondrocyte differentiation. Sox5 and Sox6 promote the development of a highly proliferating pool of chondroblasts between the epiphyses and metaphyses of future long bones. This pool is the likely cellular source of growth plates. Sox5 and Sox6 permit formation of growth plate columnar zones by keeping chondroblasts proliferating and by delaying chondrocyte prehypertrophy. They allow induction of chondrocyte hypertrophy and permit formation of prehypertrophic and hypertrophic zones by delaying chondrocyte terminal differentiation induced by ossification fronts. They act, at least in part, by down-regulating Ihh signaling, Fgfr3, and Runx2 and by up-regulating Bmp6. In conclusion, Sox5 and Sox6 are needed for the establishment of multilayered growth plates, and thereby for proper and timely development of endochondral bones.

Organogenesis of the exocrine gland

Morphogenesis of exocrine glands is a complex stepwise process of epithelial ingrowth, ductal elongation, ductal branching, and alveolar or acinar differentiation. Emerging from an increasing number of mouse gene knockout, dominant-negative, and antisense models is the identification of a remarkable collection of cell adhesion molecules, growth factors, and their receptors whose time-dependent contributions to glandular organogenesis are essential. Many have cryptically overlapping and interdependent but noncompensatory roles. Discoidin domain receptor 1 tyrosine kinase (DDR1) and the ErbB1 receptor of amphiregulin are, for example, required for ductal branching and elongation. Each is in turn dependent on the Wnt family of morphogenic factors for autophosphorylation or transactivation, respectively. Here we review the current cast of exocrine glandular morphogens, as a foundation for a global or systems biology appreciation of the interweaving signaling pathways that underlie mammalian glandular morphogenesis.

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

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

Dynamic Changes in the Response of Cells to Positive Hedgehog Signaling during Mouse Limb Patterning

In the vertebrate limb, the posteriorly located zone of polarizing activity (ZPA) regulates digit identity through the morphogen Sonic Hedgehog (Shh). By genetically marking Shh-responding cells in mice, we have addressed whether the cumulative influence of positive Shh signaling over time and space reflects a linear gradient of Shh responsiveness and whether Shh could play additional roles in limb patterning. Our results show that all posterior limb mesenchyme cells, as well as the ectoderm, respond to Shh from the ZPA and become the bone, muscle, and skin of the posterior limb. Further, the readout of Shh activator function integrated over time and space does not display a stable and linear gradient along the A-P axis, as in a classical morphogen view. Finally, by fate mapping Shh-responding cells in Gli2 and Gli3 mutant limbs, we demonstrate that a specific level of positive Hh signaling is not required to specify digit identities.

Disruption of Fgf10/Fgfr2b-coordinated epithelial-mesenchymal interactions causes cleft palate

Classical research has suggested that early palate formation develops via epithelial-mesenchymal interactions, and in this study we reveal which signals control this process. Using Fgf10-/-, FGF receptor 2b-/- (Fgfr2b-/-), and Sonic hedgehog (Shh) mutant mice, which all exhibit cleft palate, we show that Shh is a downstream target of Fgf10/Fgfr2b signaling. Our results demonstrate that mesenchymal Fgf10 regulates the epithelial expression of Shh, which in turn signals back to the mesenchyme. This was confirmed by demonstrating that cell proliferation is decreased not only in the palatal epithelium but also in the mesenchyme of Fgfr2b-/- mice. These results reveal a new role for Fgf signaling in mammalian palate development. We show that coordinated epithelial-mesenchymal interactions are essential during the initial stages of palate development and require an Fgf-Shh signaling network.

Beta1-integrins are critical for cerebellar granule cell precursor proliferation

We have previously shown that mice with a CNS restricted knock-out of the integrin beta1 subunit gene (Itgb1-CNSko mice) have defects in the formation of lamina and folia in the cerebral and cerebellar cortices that are caused by disruption of the cortical marginal zones. Cortical structures in postnatal and adult Itgb1-CNSko animals are also reduced in size, but the mechanism that causes the size defect has remained unclear. We now demonstrate that proliferation of granule cell precursors (GCPs) is severely affected in the developing cerebellum of Itgb1-CNSko mice. In the absence of beta1 expression, GCPs lose contact with laminin in the meningeal basement membrane, cease proliferating, and differentiate prematurely. In vitro studies provide evidence that beta1 integrins act at least in part cell autonomously in GCPs to regulate their proliferation. Previous studies have shown that sonic hedgehog (Shh)-induced GCP proliferation is potentiated by the integrin ligand laminin. We show that Shh directly binds to laminin and that laminin-Shh induced cell proliferation is dependent on beta1 integrin expression in GCPs. Taken together, these data are consistent with a model in which beta1 integrin expression in GCPs is required to recruit a laminin-Shh complex to the surface of GCPs and to subsequently modulate the activity of signaling pathways that regulate proliferation.

Restriction of sonic hedgehog signalling during early tooth development

The signalling peptide encoded by the sonic hedgehog gene is restricted to localised thickenings of oral epithelium, which mark the first morphological evidence of tooth development, and is known to play a crucial role during the initiation of odontogenesis. We show that at these stages in the murine mandibular arch in the absence of epithelium, the Shh targets Ptc1and Gli1 are upregulated in diastema mesenchyme, an edentulous region between the sites of molar and incisor tooth formation. This ectopic expression is not associated with Shh transcription but with Shh protein, undetectable in the presence of epithelium. These findings suggest that, in diastema mesenchyme, restriction of Shh activity is dependent upon the overlying epithelium. This inhibitory activity was demonstrated by the ability of transplanted diastema epithelium to downregulate Ptc1 in tooth explants, and for isolated diastema mesenchyme to express Ptc1. A candidate inhibitor in diastema mesenchyme is the glycosylphosphatidylinositol-linked membrane glycoprotein Gas1. Gas1is normally expressed throughout mandibular arch mesenchyme; however, in the absence of epithelium this expression was downregulated specifically in the diastema where ectopic Shh protein was identified. Although Shh signalling has no effect upon Gas1 expression in mandibular arch mesenchyme,overexpression of Gas1 results in downregulation of ectopic Ptc1. Therefore, control of the position of tooth initiation in the mandibular arch involves a combination of Shh signalling at sites where teeth are required and antagonism in regions destined to remain edentulous.

Ext1-Dependent Heparan Sulfate Regulates the Range of Ihh Signaling during Endochondral Ossification

Exostosin1 (Ext1) belongs to a family of glycosyltransferases necessary for the synthesis of the heparan sulfate (HS) chains of proteoglycans, which regulate signaling of several growth factors. Loss of tout velu (ttv), the homolog of Ext1 in Drosophila, inhibits Hedgehog movement. In contrast, we show that reduced HS synthesis in mice carrying a hypomorphic mutation in Ext1 results in an elevated range of Indian hedgehog (Ihh) signaling during embryonic chondrocyte differentiation. Our data suggest a dual function for HS: First, HS is necessary to bind Hedgehog in the extracellular space. Second, HS negatively regulates the range of Hedgehog signaling in a concentration-dependent manner. Additionally, our data indicate that Ihh acts as a long-range morphogen, directly activating the expression of parathyroid hormone-like hormone. Finally, we propose that the development of exostoses in the human Hereditary Multiple Exostoses syndrome can be attributed to activation of Ihh signaling.

Sonic hedgehog signaling is required for expansion of granule neuron precursors and patterning of the mouse cerebellum

The signals that promote regional growth and development of the brain are not well understood. Sonic hedgehog (Shh) is produced by Purkinje cells of the cerebellum and is a potent inducer of granule cell proliferation. Here, we demonstrate that Shh protein is present in the murine cerebellum during late stages of embryogenesis and is associated with Purkinje cell bodies and their processes. To better determine the role of Shh during cerebellar development, we genetically removed Shh activity specifically from Purkinje cells and the cerebellar anlage of the mouse embryo. We show that Shh is required for expansion of the granule neuron precursor population, but not for the subsequent differentiation of these cells. In addition, the loss of Shh activity influences Purkinje cell development and the formation of folia in the cerebellum. A role for Shh in compartmentalization of the cerebellum is also suggested by the more severe rostral defects observed in the absence of Hedgehog signaling. Together, these findings provide additional evidence for Shh's key regulatory role in controlling growth of the cerebellar primordium.

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

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

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

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

All mouse ventral spinal cord patterning by hedgehog is Gli dependent and involves an activator function of Gli3

An important question is how the gradient of Hedgehog is interpreted by cells at the level of the Gli transcription factors. The full range of Gli activity and its dependence on Hh have not been determined, although the Gli2 activator and Gli3 repressor have been implicated. Using the spinal cord as a model system, we demonstrate that Gli3 can transduce Hedgehog signaling as an activator. All expression of the Hh target gene Gli1 is dependent on both Gli2 and Gli3. Unlike Gli2, however, Gli3 requires endogenous Gli1 for induction of floor plate and V3 interneurons. Strikingly, embryos lacking all Gli function develop motor neurons and three ventral interneuron subtypes, similar to embryos lacking Hh signaling and Gli3. Therefore, in the spinal cord all Hh signaling is Gli dependent. Furthermore, a combination of Gli2 and Gli3 is required to regulate motor neuron development and spatial patterning of ventral spinal cord progenitors.

A novel role of the hedgehog pathway in lens regeneration

Lens regeneration in the adult newt is a classic example of replacing a lost organ by the process of transdifferentiation. After lens removal, the pigmented epithelial cells of the dorsal iris proliferate and dedifferentiate to form a lens vesicle, which subsequently differentiates to form a new lens. In searching for factors that control this remarkable process, we investigated the expression and role of hedgehog pathway members. These molecules are known to affect retina and pigment epithelium morphogenesis and have been recently shown to be involved in repair processes. Here we show that Shh, Ihh, ptc-1, and ptc-2 are expressed during lens regeneration. The expression of Shh and Ihh is quite unique since these genes have never been detected in lens. Interestingly, both Shh and Ihh are only expressed in the regenerating and developing lens, but not in the intact lens. Interfering with the hedgehog pathway results in considerable inhibition of the process of lens regeneration, including decreased cell proliferation as well as interference with lens fiber differentiation in the regenerating lens vesicle. Down-regulation of ptc-1 was also observed when inhibiting the pathway. These results provide the first evidence of a novel role for the hedgehog pathway in specific regulation of the regenerating lens.

Shh expression is required for embryonic hair follicle but not mammary gland development

The embryonic mammary gland and hair follicle are both derived from the ventral ectoderm, and their development depends on a number of common fundamental developmental pathways. While the Hedgehog (Hh) signaling pathway is required for hair follicle morphogenesis, the role of this pathway during embryonic mammary gland development remains undetermined. We demonstrate here that, unlike the hair follicle, both Shh and Ihh are expressed in the developing embryonic mouse mammary rudiment as early as E12.5. In Shh(-/-) embryos, hair follicle development becomes arrested at an early stage, while the mammary rudiment, which continues to express Ihh, develops in a manner indistinguishable from that of wild-type littermates. The five pairs of mammary buds in Shh(-/-) female embryos exhibit normal branching morphogenesis at E16.5, forming a rudimentary ductal structure identical to wild-type embryonic mammary glands. We further demonstrate that loss of Hh signaling causes altered cyclin D1 expression in the embryonic dermal mesenchyme. Specifically, cyclin D1 is expressed at E14.5 principally in the condensed mesenchymal cells of the presumptive hair follicles and in both mesenchymal and epithelial cells of the mammary rudiments in wild-type and Shh-deficient embryos. By E18.5, robust cyclin D1 expression is maintained in mammary rudiments of both wild-type and Shh-deficient embryos. In hair follicles of wild-type embryos by E18.5, cyclin D1 expression switches to follicular epithelial cells. In contrast, strong cyclin D1 expression is observed principally in the mesenchymal cells of arrested hair follicles in Shh(-/-) embryos at E18.5. These data reveal that, despite the common embryonic origin of hair follicles and mammary glands, distinct patterns of Hh-family expression occur in these two tissues. Furthermore, these data suggest that cyclin D1 expression in the embryonic hair follicle is mediated by both Hh-independent and Hh-dependent mechanisms.

Heparan sulfate proteoglycans: Coordinators of multiple signaling pathways during chondrogenesis

Heparan sulfate proteoglycans are abundantly expressed in the pericellular matrix of both developing and mature cartilage. Increasing evidence indicates that the action of numerous chondroregulatory molecules depends on these proteoglycans. This review summarizes the current understanding of the interactions of heparan sulfate chains of cartilage proteoglycans with both soluble and nonsoluble ligands during the process of chondrogenesis. In addition, the consequences of mutating genes encoding heparan sulfate biosynthetic enzymes or heparan sulfate proteoglycan core proteins on cartilage development are discussed.

Role ofSonic hedgehog in patterning of tracheal-bronchial cartilage and the peripheral lung

Sonic hedgehog (Shh) was conditionally deleted in respiratory epithelial cells of the embryonic lung in vivo. Deletion of Shh before embryonic day (E) 13.5 resulted in respiratory failure at birth. While lobulation was not perturbed, the lungs were hypoplastic, with reduced branching of peripheral lung tubules, evident from E13.5. Smooth muscle and endothelial cells were absent or reduced, the latter in relationship to the loss of peripheral lung parenchyma. Tracheal-bronchial ring abnormalities occurred when Shh was deleted between E8.5 and E12.5. Deletion of Shh later in gestation (after E13.5) caused mild abrogation of peripheral branching morphogenesis but did not disrupt tracheal-bronchial development. Defects in branching morphogenesis and vascularization seen in Shh null mutant (Shh(-/-)) mice were substantially corrected when SHH was ectopically expressed in the respiratory epithelium; however, peripheral expression of SHH failed to correct cartilage abnormalities in the trachea and bronchi, indicating a spatial requirement for SHH expression near sites of cartilage formation. Expression of SHH by the respiratory epithelium plays an important role in the patterning of tracheal-bronchial mesenchyme required for formation of cartilage rings in conducting airways. SHH regulates branching morphogenesis and influences differentiation of the peripheral lung mesenchyme required for formation of bronchial and vascular smooth muscle.

Overexpression of Sonic Hedgehog suppresses embryonic hair follicle morphogenesis

The Sonic Hedgehog (Shh) signalling pathway plays a central role in the development of the skin and hair follicle and is a major determinant of skin tumorigenesis, most notably of basal cell carcinoma (BCC). Various mouse models involving either ablation or overexpression of key members of the Shh signalling pathway display a range of skin tumours. To further examine the role of Shh in skin development, we have overexpressed Shh in a subset of interfollicular basal cells from 12.5 dpc under the control of the human keratin 1 (HK1) promoter. The HK1-Shh transgenic mice display a range of skin anomalies, including highly pigmented inguinal lesions and regions of alopecia. The most striking hair follicle phenotype is a suppression in embryonic follicle development between 14.0 and 19.0 dpc, resulting in a complete absence of guard, awl, and auchene hair fibres. These data indicate that alternative signals are responsible for the development of different hair follicles and point to a major role of Shh signalling in the morphogenesis of guard, awl, and auchene hair fibres. Through a comparison with other mouse models, the characteristics of the HK1-Shh transgenic mice suggest that the precise timing and site of Shh expression are key in dictating the resultant skin and tumour phenotype.

Chondroitin sulfate proteoglycans are required for lung growth and morphogenesis in vitro

Proteoglycans (PGs) have been shown to play a key role in the development of many tissues. We have investigated the role of sulfated PGs in early rat lung development by treating cultured tissues with 30 mM sodium chlorate, a global inhibitor of PG sulfation. Chlorate treatment disrupted growth and branching of embryonic day 13 lung explants. Isolated lung epithelium (LgE) migrated toward and invaded lung mesenchyme (LgM), and chlorate irreversibly suppressed this response. Chlorate also inhibited migration of LgE toward beads soaked in FGF10. Chlorate severely decreased branching morphogenesis in tissue recombinants consisting of LgM plus either LgE or tracheal epithelium (TrE) and decreased expression of surfactant protein C gene (SP-C). Chlorate also reduced bone morphogenetic protein-4 expression in cultured tips and recombinants but had no effect on the expression of clara cell 10-kDa protein (CC10), sonic hedgehog (Shh), FGF10, and FGF receptor 2IIIb. Chlorate reduced the growth of LgE in mesenchyme-free culture but did not affect SP-C expression. In contrast, chlorate inhibited both rudiment growth and the induction of SP-C in mesenchyme-free cultured TrE. Treatment of lung tips and tissue recombinants with chondroitinase ABC abolished branching morphogenesis. Chondroitinase also suppressed growth of TrE in mesenchyme-free culture. Chondroitinase treatment, however, had no effect on the induction of SP-C expression in any of these cultures. These results demonstrate the overall importance of sulfated PGs to normal lung development and demonstrate a dynamic role for chondroitin sulfate PGs in embryonic lung growth and morphogenesis.

Beta1 integrins regulate chondrocyte rotation, G1 progression, and cytokinesis

Beta1 integrins are highly expressed on chondrocytes, where they mediate adhesion to cartilage matrix proteins. To assess the functions of beta1 integrin during skeletogenesis, we inactivated the beta1 integrin gene in chondrocytes. We show here that these mutant mice develop a chondrodysplasia of various severity. beta1-deficient chondrocytes had an abnormal shape and failed to arrange into columns in the growth plate. This is caused by a lack of motility, which is in turn caused by a loss of adhesion to collagen type II, reduced binding to and impaired spreading on fibronectin, and an abnormal F-actin organization. In addition, mutant chondrocytes show decreased proliferation caused by a defect in G1/S transition and cytokinesis. The G1/S defect is, at least partially, caused by overexpression of Fgfr3, nuclear translocation of Stat1/Stat5a, and up-regulation of the cell cycle inhibitors p16 and p21. Altogether these findings establish that beta1-integrin-dependent motility and proliferation of chondrocytes are mandatory events for endochondral bone formation to occur.

Sonic hedgehog regulates the position of the trigeminal ganglia

The trigeminal ganglia differentiate in part from specialized ectodermal structures in the embryonic head termed the trigeminal placodes. However, the signals which govern the migration of trigeminal precursors and the final morphology of the ganglia are poorly defined. Here, we show that notochord or floor plate tissue can induce the formation of ectopic sensory ganglia adjacent to the developing dorsal mesencephalon. Neurons within these ganglia coexpress the transcription factors Brn3a and Islet, which together characterize primary sensory neurons throughout the developing embryo. The ectopic ganglia originate from Pax3-expressing regions of the surface ectoderm that normally contribute to the ophthalmic trigeminal (op5), and can only be induced at developmental stages during which op5 precursors are present in the mesencephalic region. The migration of trigeminal precursors is also blocked by a local source of recombinant Shh, while in mouse embryos lacking Shh, these cells continue to migrate until they fuse into a single ganglion at the ventral midline. Together, these results suggest that Shh acts to arrest the migration of sensory precursors rather than to induce sensory neurons de novo. Consistent with this hypothesis, Shh induces the expression of the proteoglycan PG-M/versican in the cranial mesoderm, which has been previously implicated in the regulation of the movement of sensory neural precursors.

A dual role for Sonic hedgehog in regulating adhesion and differentiation of neuroepithelial cells

In vertebrates, the nervous system arises from a flat sheet of epithelial cells, the neural plate, that gradually transforms into a hollow neural tube. This process, called neurulation, involves sequential changes in cellular interactions that are precisely coordinated both spatially and temporally by the combined actions of morphogens. To gain further insight into the molecular events regulating cell adhesion during neurulation, we investigated whether the adhesive and migratory capacities of neuroepithelial cells might be modulated by Sonic hedgehog (Shh), a signaling molecule involved in the control of cell differentiation in the ventral neural tube. When deposited onto extracellular matrix components in vitro, neural plates explanted from avian embryos at early neurulation readily dispersed into monolayers of spread cells, thereby revealing their intrinsic ability to migrate. In the presence of Shh added in solution to the culture medium, the explants still exhibited the same propensity to disperse. In contrast, when Shh was immobilized to the substrate or produced by neuroepithelial cells themselves after transfection, neural plate explants failed to disperse and instead formed compact structures. Changes in the adhesive capacities of neuroepithelial cells caused by Shh could be accounted for by inactivation of surface beta1-integrins combined with an increase in N-cadherin-mediated cell adhesion. Furthermore, immobilized Shh promoted differentiation of neuroepithelial cells into motor neurons and floor plate cells with the same potency as soluble Shh. However, the effect of Shh on the neuroepithelial cell adhesion was discernible and apparently independent from its differentiation effect and was not mediated by the signaling cascade elicited by the Patched-Smoothened receptor and involving the Gli transcription factors. Thus, our experiments indicate that Shh is able to control sequentially adhesion and differentiation of neuroepithelial cells through different mechanisms, leading to a coordinated regulation of the various cell interactions essential for neural tube morphogenesis.

Heparan sulfates expressed in the distal lung are required for Fgf10 binding to the epithelium and for airway branching

Fibroblast growth factor (Fgf) 10 is a critical regulator of bud formation during lung morphogenesis. fgf10 is expressed in distal lung mesenchyme at sites of prospective budding from the earliest developmental stages and signals through its epithelial receptor Fgfr2b. Experiments in intact lung organ cultures demonstrate that Fgf10 is a chemotactic factor for distal, but not for proximal, epithelium. This differential response suggests the involvement of an additional mechanism regulating Fgf10-Fgfr2b interactions, because Fgfr2b is uniformly expressed throughout the respiratory tract. Here we use an immunohistochemistry-based binding assay to show that O-sulfated heparan sulfates (HS) are critical for Fgf10 binding to the distal epithelium. We show that altering endogenous gradients of HS sulfation with sodium chlorate or over-O-sulfated synthetic heparin in lung organ cultures dramatically decreases Fgf10 binding. Moreover, we show that under these conditions epithelial binding is not improved by providing exogenous FGF10. Our data suggest that, not only ligand availability, but also the presence of specific patterns of HS modification in the distal lung epithelium are critical determinants of Fgf10 binding to the epithelium and signaling.

The emergent design of the neural tube: prepattern, SHH morphogen and GLI code

The Sonic hedgehog (Shh) pathway plays an important role in the development of many tissues and organs. The secreted ligand Shh has been shown to act as a mitogen, morphogen and survival factor in different contexts whereas the three Gli transcription factors act as Shh mediators in a context-dependent combinatorial fashion. The common wisdom has been that Gli protein function is subject to Shh signaling. One can ask how Gli proteins act and what the nature of Shh signaling during CNS dorsal-ventral patterning is. Is it possible that Hedgehog signals are only one of several ways to regulate Gli activity? Moreover, in light of the partial rescue of the neural tube phenotype of Shh or Smoothened mutant embryos in Shh(-/-);Gli3(-/-), Smoothened(-/-);Gli3(-/-), and Shh(-/-);Rab23(-/-) double null embryos, one can consider the roles that the Shh-Gli pathway may have taken to orchestrate congruent prepattern and growth, and the importance of creating the correct number of precursors in patterning mechanisms.

Sonic hedgehog is required for progenitor cell maintenance in telencephalic stem cell niches

To directly test the requirement for hedgehog signaling in the telencephalon from early neurogenesis, we examined conditional null alleles of both the Sonic hedgehog and Smoothened genes. While the removal of Shh signaling in these animals resulted in only minor patterning abnormalities, the number of neural progenitors in both the postnatal subventricular zone and hippocampus was dramatically reduced. In the subventricular zone, this was partially attributable to a marked increase in programmed cell death. Consistent with Hedgehog signaling being required for the maintenance of stem cell niches in the adult brain, progenitors from the subventricular zone of floxed Smo animals formed significantly fewer neurospheres. The loss of hedgehog signaling also resulted in abnormalities in the dentate gyrus and olfactory bulb. Furthermore, stimulation of the hedgehog pathway in the mature brain resulted in elevated proliferation in telencephalic progenitors. These results suggest that hedgehog signaling is required to maintain progenitor cells in the postnatal telencephalon.

Short- and long-range effects of Sonic hedgehog in limb development

The secreted protein Sonic hedgehog (Shh) and its transmembrane receptor Patched (Ptc) control a major signal transduction pathway in early vertebrate limb development. Ligand-free Ptc interacts with the transmembrane protein Smoothened (Smo) and blocks expression of Smo-controlled genes including ptc. Ligand-bound Ptc removes the block and leads to further expression of ptc, which in turn restricts the range of Shh transport. Currently it is not certain that Shh functions as a morphogen on the 300-microm scale of early chick limb development, because it has been difficult to determine how far different forms of Shh are transported. We develop a model to study the effects of two forms of Shh used experimentally and propose a mechanism for Shh signal transduction based on a two-state model for the Ptc-Smo interaction. Recent bead- and tissue-implant experiments can be explained by using this model without postulating different diffusivities for the two forms of Shh; a difference in other parameters such as the rate of release of Shh from the bead or transplant can explain the results equally well. The model also predicts that lower concentrations of Shh in a bead will produce a response similar to that after a tissue transplant. Our results provide an explanation for the counterintuitive experimental results and show that the same signal transduction mechanism can explain both short- and long-range Shh signaling. We conclude that Shh can function as a long-range morphogen.

Genetic analysis of the roles of Hh, FGF8, and nodal signaling during catecholaminergic system development in the zebrafish brain

CNS catecholaminergic neurons can be distinguished by their neurotransmitters as dopaminergic or noradrenergic and form in distinct regions at characteristic embryonic stages. This raises the question of whether all catecholaminergic neurons of one transmitter type are specified by the same set of factors. Therefore, we performed genetic analyses to define signaling requirements for the specification of distinct clusters of catecholaminergic neurons in zebrafish. In mutants affecting midbrain- hindbrain boundary (MHB) organizer formation, the earliest ventral diencephalic dopaminergic neurons appear normal. However, after 2 d of development, we observed fewer cells than in wild types, which suggests that the MHB provides proliferation or survival factors rather than specifying ventral diencephalic dopaminergic clusters. In hedgehog (Hh) pathway mutants, the formation of catecholaminergic neurons is affected only in the pretectal cluster. Surprisingly, neither fibroblast growth factor 8 (FGF8) alone nor in combination with Hh signaling is required for specification of early developing dopaminergic neurons. We analyzed the formation of prosomeric territories in the forebrain of Hh and Nodal pathway mutants to determine whether the absence of specific dopaminergic clusters may be caused by early patterning defects ablating corresponding parts of the CNS. In Nodal pathway mutants, ventral diencephalic and pretectal catecholaminergic neurons fail to develop, whereas both anatomical structures form at least in part. This suggests that Nodal signaling is required for catecholaminergic neuron specification. In summary, our results do not support the previously suggested dominant roles for sonic hedgehog and Fgf8 in specification of the first catecholaminergic neurons, but instead indicate a novel role for Nodal signaling in this process.

HIP/RPL29 down-regulation accompanies terminal chondrocyte differentiation

HIP is a heparin/heparan sulfate (Hp/HS) binding protein identical to ribosomal protein L29 that displays diverse biological functions. There is strong evidence that abnormal expression and quantitative deficiencies of essential molecules such as extracellular matrix (ECM) proteins, transcription factors, and ribosomal proteins can seriously impair embryonic development. As observed for HS-bearing molecules, high levels of HIP/RPL29 are found in proliferating chondrocytic precursors and chondrocytes of developing growth plate. Here, we demonstrate both in vitro and in developing mouse embryos that HIP/RPL29 is down-regulated in terminally differentiated chondrocytes corresponding to the late hypertrophic zone of the growth plate. Because cartilage serves as a template for endochondral bone formation, we hypothesize that the presence of HIP/RPL29 during early chondrogenesis is essential for normal skeletal growth and patterning. In particular, we believe that HIP/RPL29 expression is required to maintain proliferation of chondrocytes and avoid skeletal shortening. Increasing evidence suggests that multifunctional ribosomal proteins of eukaryotic cells are important regulators of cell growth and differentiation, not simply structural parts of translational machinery. To investigate the role of HIP/RPL29 normal expression during cartilage formation, we designed a ribozyme-mediated knock-down approach to partially down-regulate HIP/RPL29 expression in the multipotent mouse embryonic skin fibroblast cell line C3H/10T (1/2). This technology permitted us to avoid the insufficient expression associated with more severe consequences, such as lethality, and provided advantages similar to those obtained with mutations generating hypomorphic phenotypes. Our results show that partial reduction of HIP/RPL29 levels accelerates differentiation of C3H/10T(1/2) into cartilage-like cells. In conclusion, our data indicate that HIP/RPL29 constitutes an important novel regulator of chondrocytic growth and differentiation.

Differential requirement for Gli2 and Gli3 in ventral neural cell fate specification

Sonic hedgehog (Shh) directs the development of ventral cell fates, including floor plate and V3 interneurons, in the mouse neural tube. Here, we show that the transcription factors Gli2 and Gli3, mediators of Shh signaling, are required for the development of the ventral cell fates but make distinct contributions to controlling cell fates at different locations along the rostral-caudal axis. Mutants lacking Patched1 (Ptc1), the putative receptor of Shh, were used to analyze Gli functions. Ptc1(-/-) mutants develop floor plate, motor neuron, and V3 interneuron progenitors in lateral and dorsal regions, suggesting that the normal role of Ptc1 is to suppress ventral cell development in dorsal neural tube. The Ptc1(-/-) phenotype is rescued, with restoration of dorsal cell types, by the lack of Gli2, but only in the caudal neural tube. In triple mutants of Gli2, Gli3, and Ptc1, dorsal and lateral cell fates are restored in the entire neural tube. These observations suggest that Gli2 is essential for ventral specification in the caudal neural tube, and that in more rostral regions, only Gli3 can promote development of ventral cells if Gli2 is absent. Thus, Shh signaling is mediated by overlapping but distinct functions of Gli2 and Gli3, and their relative contributions vary along the rostral-caudal axis.

Developmental roles and clinical significance of hedgehog signaling

Cell signaling plays a key role in the development of all multicellular organisms. Numerous studies have established the importance of Hedgehog signaling in a wide variety of regulatory functions during the development of vertebrate and invertebrate organisms. Several reviews have discussed the signaling components in this pathway, their various interactions, and some of the general principles that govern Hedgehog signaling mechanisms. This review focuses on the developing systems themselves, providing a comprehensive survey of the role of Hedgehog signaling in each of these. We also discuss the increasing significance of Hedgehog signaling in the clinical setting.

Heparan sulfate-FGF10 interactions during lung morphogenesis

Signaling by fibroblast growth factor 10 (FGF10) through FGFR2b is essential for lung development. Heparan sulfates (HS) are major modulators of growth factor binding and signaling present on cell surfaces and extracellular matrices of all tissues. Although recent studies provide evidence that HS are required for FGF-directed tracheal morphogenesis in Drosophila, little is known about the HS role in FGF10-mediated bud formation in the vertebrate lung. Here, we mapped HS expression in the early lung and we investigated how HS interactions with FGF10-FGFR2b influence lung morphogenesis. Our data show that a specific set of HS low in O-sulfates is dynamically expressed in the lung mesenchyme at the sites of prospective budding near Fgf10-expressing areas. In turn, highly sulfated HS are present in basement membranes of branching epithelial tubules. We show that disrupting endogenous gradients of HS or altering HS sulfation in embryonic lung culture systems prevents FGF10 from inducing local responses and markedly alters lung pattern formation and gene expression. Experiments with selectively sulfated heparins indicate that O-sulfated groups in HS are critical for FGF10 signaling activation in the epithelium during lung bud formation, and that the effect of FGF10 in pattern is in part determined by regional distribution of O-sulfated HS. Moreover, we describe expression of a HS 6-O-sulfotransferase preferentially at the tips of branching tubules. Our data suggest that the ability of FGF10 to induce local budding is critically influenced by developmentally regulated regional patterns of HS sulfation.

Developmental skeletal anomalies

A genetic and molecular revolution is taking place in medicine today. Led by the Human Genome Project, genetic information and concepts are changing the way diseases are defined, diagnoses are made, and treatment strategies are developed. The profound implications of actually understanding the molecular abnormalities of many clinical problems are affecting virtually all medical and surgical disciplines. The ability to apply knowledge gleaned from the laboratory is our best hope for developing strategies to modify the pathologic effects of genes (by drug therapy), repair genes (gene therapy), and restore lost or affected tissues (tissue engineering). Instead of an empiric trial-and-error approach to therapy, it may become feasible to tailor treatment to the specific molecular malfunction. In this review we have chosen to emphasize a few selected musculoskeletal disorders, including skeletal dysplasias, spinal deformities, developmental dislocation of the hip, and idiopathic clubfoot. The logical extension of our understanding of the molecular players in many of these disorders is to establish precisely what the products of the affected genes do during skeletal development, and how mutations disturb these functions to produce the characteristic phenotype. Despite the many hypotheses generated from the work in human genetics, and the knowledge that has been gained from animal models, there remains a relatively poor understanding of how these genes interfere with skeletal development. Unraveling these mysteries and defining them in molecular and cellular terms will be the challenges for the near future.

Hedgehog signalling is required for correct anteroposterior patterning of the zebrafish otic vesicle

Currently, few factors have been identified that provide the inductive signals necessary to transform the simple otic placode into the complex asymmetric structure of the adult vertebrate inner ear. We provide evidence that Hedgehog signalling from ventral midline structures acts directly on the zebrafish otic vesicle to induce posterior otic identity. We demonstrate that two strong Hedgehog pathway mutants, chameleon (con(tf18b)) and slow muscle omitted (smu(b641)) exhibit a striking partial mirror image duplication of anterior otic structures, concomitant with a loss of posterior otic domains. These effects can be phenocopied by overexpression of patched1 mRNA to reduce Hedgehog signalling. Ectopic activation of the Hedgehog pathway, by injection of sonic hedgehog or dominant-negative protein kinase A RNA, has the reverse effect: ears lose anterior otic structures and show a mirror image duplication of posterior regions. By using double mutants and antisense morpholino analysis, we also show that both Sonic hedgehog and Tiggy-winkle hedgehog are involved in anteroposterior patterning of the zebrafish otic vesicle.

Disruption of sonic hedgehog signaling alters growth and patterning of lingual taste papillae

Taste buds on the anterior part of the tongue develop in conjunction with epithelial-mesenchymal specializations in the form of gustatory (taste) papillae. Sonic hedgehog (Shh) and Bone Morphogenetic Protein 4 (BMP4) are expressed in developing taste papillae, but the roles of these signaling molecules in specification of taste bud progenitors and in papillary morphogenesis are unclear. We show here that BMP4 is not expressed in the early tongue, but is precisely coexpressed with Shh in papillary placodes, which serve as a signaling center for both gustatory and papillary development. To elucidate the role of Shh, we used an in vitro model of mouse fungiform papillary development to determine the effects of two functional inhibitors of Shh signaling: anti-Shh (5E1) antibody and cyclopamine. Cultured E11.5 tongue explants express Shh and BMP4(LacZ) in a pattern similar to that of intact embryos, localizing to developing papillary placodes after 2 days in culture. Tongues cultured with 5E1 antibody continue to express these genes in papillary patterns but develop more papillae that are larger and closer together than in controls. Tongues cultured with cyclopamine have a dose-dependent expansion of Shh and BMP4(LacZ) expression domains. Both antibody-treated and cyclopamine-treated tongue explants also are smaller than controls. Taken together, these results suggest that, although Shh is not involved in the initial specification of papillary placodes, Shh does play two key roles during pmcry development: (1) as a morphogen that directs cells toward a nonpapillary fate, and (2) as a mitogen, causing expansion of the interplacodal epithelium and underlying mesenchyme.

Specific congenital heart defects in RSH/Smith-Lemli-Opitz syndrome: postulated involvement of the sonic hedgehog pathway in syndromes with postaxial polydactyly or heterotaxia

BACKGROUND

RSH/Smith-Lemli-Opitz syndrome is an autosomal recessive syndrome due to an inborn error of cholesterol metabolism and is characterized by developmental delay, facial anomalies, hypospadias, congenital heart defect (CHD), postaxial polydactyly, and 2-3 toe syndactyly. CHD is found in half of the propositi, and a specific association with atrioventricular canal defect (AVCD) and anomalous pulmonary venous return has been demonstrated.

METHODS

We report on an additional patient with RSH/SLOS presenting with complete AVCD and anomalous pulmonary venous return, and discuss the possible relationship of the Sonic Hedgehog (SHH) pathway as causative factor of these CHDs and those in heterotaxia patients with postaxial polydactyly syndromes.

RESULTS

Anatomic similarities between heterotaxia and CHDs of several syndromes with postaxial polydactyly have been noted previously, considering the frequent association of AVCD with common atrium in these conditions. It is known that both CHDs of heterotaxia and postaxial polydactyly can be related to abnormalities of the SHH pathway. Cholesterol has a critical role in the formation of normally active hedgehog proteins. It could be hypothesized that specific types of CHDs in RSH/SLOS can be caused by modifications of the SHH protein related to the defect of cholesterol biosynthesis.

CONCLUSIONS

The specific association of AVCD and anomalous pulmonary venous return in patients with RSH/SLOS and the finding of AVCD +/- common atrium in several syndromes with polydactyly leads to the hypothesis that heterotaxia due to SHH anomalies could be involved in a large spectrum of conditions. Perturbations in different components of the SHH pathway could lead to several developmental errors presenting with partially overlapping clinical manifestations.

Cyclopamine and jervine in embryonic rat tongue cultures demonstrate a role for Shh signaling in taste papilla development and patterning: fungiform papillae double in number and form in novel locations in dorsal lingual epithelium

From time of embryonic emergence, the gustatory papilla types on the mammalian tongue have stereotypic anterior and posterior tongue locations. Furthermore, on anterior tongue, the fungiform papillae are patterned in rows. Among the many molecules that have potential roles in regulating papilla location and pattern, Sonic hedgehog (Shh) has been localized within early tongue and developing papillae. We used an embryonic, tongue organ culture system that retains temporal, spatial, and molecular characteristics of in vivo taste papilla morphogenesis and patterning to study the role of Shh in taste papilla development. Tongues from gestational day 14 rat embryos, when papillae are just beginning to emerge on dorsal tongue, were maintained in organ culture for 2 days. The steroidal alkaloids, cyclopamine and jervine, that specifically disrupt the Shh signaling pathway, or a Shh-blocking antibody were added to the standard culture medium. Controls included tongues cultured in the standard medium alone, and with addition of solanidine, an alkaloid that resembles cyclopamine structurally but that does not disrupt Shh signaling. In cultures with cyclopamine, jervine, or blocking antibody, fungiform papilla numbers doubled on the dorsal tongue with a distribution that essentially eliminated inter-papilla regions, compared with tongues in standard medium or solanidine. In addition, fungiform papillae developed on posterior oral tongue, just in front of and beside the single circumvallate papilla, regions where fungiform papillae do not typically develop. The Shh protein was in all fungiform papillae in embryonic tongues, and tongue cultures with standard medium or cyclopamine, and was conspicuously localized in the basement membrane region of the papillae. Ptc protein had a similar distribution to Shh, although the immunoproduct was more diffuse. Fungiform papillae did not develop on pharyngeal or ventral tongue in cyclopamine and jervine cultures, or in the tongue midline furrow, nor was development of the single circumvallate papilla altered. The results demonstrate a prominent role for Shh in fungiform papilla induction and patterning and indicate differences in morphogenetic control of fungiform and circumvallate papilla development and numbers. Furthermore, a previously unknown, broad competence of dorsal lingual epithelium to form fungiform papillae on both anterior and posterior oral tongue is revealed.

Developmental mechanisms underlying polydactyly in the mouse mutant Doublefoot

The pre-axial polydactylous mouse mutant Doublefoot has 6-9 digits per limb but lacks anteroposterior polarity (there is no biphalangeal digit 1). It differs from other polydactylous mutants in showing normal Shh expression, but polarizing activity (shown by mouse-chick grafting experiments) and hedgehog signalling activity (shown by expression of Ptc1) are present throughout the distal mesenchyme. The Dbf mutation has not yet been identified. Here we review current understanding of this mutant, and briefly report new results indicating (1) that limb bud expansion is concomitant with ectopic lhh expression and with extension of the posterior high cell proliferation rate into the anterior region, and (2) that the Dbf mutation is epistatic to Shh in the limb.

Hedgehog signaling in pancreas development

Hedgehog proteins are secreted molecules that bind to their cell surface receptors to elicit concentration dependent responses essential for numerous tissue patterning and cell differentiation events during embryogenesis. However, during early stages of pancreas organogenesis, hedgehog signaling has been shown to inhibit tissue morphogenesis and cell differentiation. By contrast, recent cell culture studies indicate that an active hedgehog pathway might be required for maintenance of adult endocrine cell functions. This review describes our current understanding of the requirement of hedgehog signaling during pancreas morphogenesis and cell differentiation and discusses how individual hedgehog genes might act at various stages to ensure proper pancreas development and organ function.

Tooth and jaw: molecular mechanisms of patterning in the first branchial arch

The mammalian jaw apparatus is ultimately derived from the first branchial arch derivatives, the maxillary and mandibular processes, and composed of a highly specialised group of structures. Principle amongst these are the skeletal components of the mandible and maxilla and the teeth of the mature dentition. Integral to the development of these structures are signalling interactions between the stomodeal ectoderm and underlying neural crest-derived ectomesenchymal cells that populate this region. Recent evidence suggests that in the early mouse embryo, regionally restricted expression of homeobox-containing genes, such as members of the Dlx, Lhx and Gsc classes, are responsible for generating early polarity in the first branchial arch and establishing the molecular foundations for patterning of the skeletal elements. Teeth also develop on the first branchial arch and are derived from both ectoderm and the underlying ectomesenchyme. Reciprocal signalling interactions between these cell populations also control the odontogenic developmental programme, from early patterning of the future dental axis to the initiation of tooth development at specific sites within the ectoderm. In particular, members of the Fibroblast growth factor (Fgf), Bmp, Hedgehog and Wnt families of signalling molecules induce regionally restricted expression of downstream target genes in the odontogenic ectomesenchyme. Finally, the processes of morphogenesis and cellular differentiation ultimately generate a tooth of specific class. Many of the same genetic interactions that are involved in early tooth development mediate these effects through the activity of localised signalling centres within the developing tooth germ.

Chick limbs with mouse teeth: an effective in vivo culture system for tooth germ development and analysis

Mouse tooth germ development is currently studied by three main approaches: in wild-type and mutant mouse lines, after transplantation of tooth germs to ectopic sites, and in organ culture. The in vivo approaches are the most physiological but do not provide accessibility to tooth germs for further experimental manipulation. Organ cultures, although readily accessible, do not sustain full tooth germ development and are appropriate for short-term analysis. Thus, we sought to establish a new approach that would combine experimental accessibility with sustained development. We implanted fragments of embryonic day 12 mouse embryo first branchial arch containing early bud stage tooth germs into the lateral mesenchyme of day 4-5 chick embryo wing buds in ovo. Eggs were reincubated, and implanted tissues were examined by histochemistry and in situ hybridization over time. The tooth germs underwent seemingly normal growth, differentiation, and morphogenesis. They reached the cap, bell, and crown stages in approximately 3, 6, and 10 days, respectively, mimicking in a striking manner native temporal patterns. To examine mechanisms regulating tooth germ development, we first implanted tooth germ fragments, microinjected them with neutralizing antibodies to the key signaling molecule Sonic hedgehog (Shh), and examined them over time. Tooth germ development was markedly delayed, as revealed by poor morphogenesis and lack of mature ameloblasts and odontoblasts displaying characteristic traits such as an elongated cell shape, nuclear relocalization, and amelogenin gene expression. These phenotypic changes began to be reversed upon further incubation. The data show that the limb bud represents an effective, experimentally accessible as well as economical system for growth and analysis of developing tooth germs. The inhibitory effects of Shh neutralizing antibody treatment are discussed in relation to roles of this signaling pathway proposed by this and other groups previously.

The role of growth factors in tooth development

Growth factors and other paracrine signal molecules regulate communication between cells in all developing organs. During tooth morphogenesis, molecules in several conserved signal families mediate interactions both between and within the epithelial and mesenchymal tissue layers. The same molecules are used repeatedly during advancing development, and several growth factors are coexpressed in epithelial signaling centers. The enamel knots are signaling centers that regulate the patterning of teeth and are associated with foldings of the epithelial sheet. Different signaling pathways form networks and are integrated at many levels. Many targets of the growth factors have been identified, and mutations in several genes within the signaling networks cause defective tooth formation in both humans and mice.