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

Mouse dispatched mutants fail to distribute hedgehog proteins and are defective in hedgehog signaling

Hedgehog (Hh) signaling plays a major role in multiple aspects of embryonic development, which involves both short- and long-range signaling from localized Hh sources. One unusual aspect of Hh signaling is the autoproteolytic processing of Hh followed by lipid modification. As a consequence, the N-terminal fragment of Hh becomes membrane anchored on the cell surface of Hh-producing cells. A key issue in Hh signaling is to understand the molecular mechanisms by which lipid-modified Hh protein is transported from its sites of synthesis and subsequently moves through the morphogenetic field. The dispatched gene, which encodes a putative multipass membrane protein, was initially identified in Drosophila and is required in Hh-producing cells, where it facilitates the transport of cholesterol-modified Hh. We report the identification of the mouse dispatched (Disp) gene and a phenotypic analysis of Disp mutant mice. Disp-null mice phenocopy mice deficient in the smoothened gene, an essential component for Hh reception, suggesting that Disp is essential for Hh signaling. This conclusion was further supported by a detailed molecular analysis of Disp knockout mice, which exhibit defects characteristic of loss of Hh signaling. We also provide evidence that Disp is not required for Hh protein synthesis or processing, but rather for the movement of Hh protein from its sites of synthesis in mice. Taken together, our results reveal a conserved mechanism of Hh protein movement in Hh-producing cells that is essential for proper Hh signaling.

Morphogen gradients, positional information, and Xenopus: interplay of theory and experiment

The idea of morphogen gradients has long been an important one in developmental biology. Studies with amphibians and with Xenopus in particular have made significant contributions to demonstrating the existence, identity, and mechanisms of action of morphogens. Mesoderm induction and patterning by activin, nodals, bone morphogenetic proteins, and fibroblast growth factors have been analyzed thoroughly and reveal recurrent and combinatorial roles for these protein growth factor morphogens and their antagonists. The dynamics of nodal-type signaling and the intersection of VegT and beta-catenin intracellular gradients reveal detailed steps in early long-range patterning. Interpretation of gradients requires sophisticated mechanisms for sharpening thresholds, and the activin-Xbra-Gsc system provides an example of this. The understanding of growth factor signal transduction has elucidated growth factor morphogen action and provided tools for dissecting their direct long-range action and distribution. The physical mechanisms of morphogen gradient establishment are the focus of new interest at both the experimental and theoretical level. General themes and emerging trends in morphogen gradient studies are discussed.

Shh signaling within the dental epithelium is necessary for cell proliferation, growth and polarization

Sonic hedgehog (Shh), a member of the mammalian Hedgehog (Hh) family, plays a key role during embryogenesis and organogenesis. Tooth development, odontogenesis, is governed by sequential and reciprocal epithelial-mesenchymal interactions. Genetic removal of Shh activity from the dental epithelium, the sole source of Shh during tooth development, alters tooth growth and cytological organization within both the dental epithelium and mesenchyme of the tooth. In this model it is not clear which aspects of the phenotype are the result of the direct action of Shh on a target tissue and which are indirect effects due to deficiencies in reciprocal signalings between the epithelial and mesenchymal components. To distinguish between these two alternatives and extend our understanding of Shh's actions in odontogenesis, we have used the Cre-loxP system to remove Smoothened (Smo) activity in the dental epithelium. Smo, a seven-pass membrane protein is essential for the transduction of all Hh signals. Hence, removal of Smo activity from the dental epithelium should block Shh signaling within dental epithelial derivatives while preserving normal mesenchymal signaling. Here we show that Shh-dependent interactions occur within the dental epithelium itself. The dental mesenchyme develops normally up until birth. In contrast, dental epithelial derivatives show altered proliferation, growth, differentiation and polarization. Our approach uncovers roles for Shh in controlling epithelial cell size, organelle development and polarization. Furthermore, we provide evidence that Shh signaling between ameloblasts and the overlying stratum intermedium may involve subcellular localization of Patched 2 and Gli1 mRNAs, both of which are targets of Shh signaling in these cells.

A direct requirement for Hedgehog signaling for normal specification of all ventral progenitor domains in the presumptive mammalian spinal cord

The hedgehog signaling pathway organizes the developing ventral neural tube by establishing distinct neural progenitor fates along the dorsoventral axis. Smoothened (Smo) is essential for all Hedgehog (Hh) signaling, and genetic inactivation of Smo cells autonomously blocks the ability of cells to transduce the Hh signal. Using a chimeric approach, we examined the behavior of Smo null mutant neural progenitor cells in the developing vertebrate spinal cord, and we show that direct Hh signaling is essential for the specification of all ventral progenitor populations. Further, Hh signaling extends into the dorsal half of the spinal cord including the intermediate Dbx expression domain. Surprisingly, in the absence of Sonic hedgehog (Shh), we observe the presence of a Smo-dependent Hh signaling activity operating in the ventral half of the spinal cord that most likely reflects Indian hedgehog (Ihh) signaling originating from the underlying gut endoderm. Comparative studies of Shh, Smo, and Gli3 single and compound mutants reveal that Hh signaling acts in part to specify neural cell identity by counteracting the repressive action of Gli3 on p0, p1, p2, and pMN formation. However, whereas these cell identities are restored in Gli3/Smo compound mutants, correct stratification of the rescued ventral cell types is lost. Thus, Hh signaling is essential for organizing ventral cell pattern, possibly through the control of differential cell affinities.

Dorsal-ventral patterning of the spinal cord requires Gli3 transcriptional repressor activity

Sonic hedgehog (Shh) plays a critical role in organizing cell pattern in the developing spinal cord. Gli proteins are thought to mediate Shh signaling, but their role in directing neural tube patterning remains unclear. Here we identify a role for Gli3 transcriptional repressor activity in patterning the intermediate region of the spinal cord that complements the requirement for Gli2 in ventral regions. Moreover, blocking all Gli responses results in a complete dorsalization of ventral spinal cord, indicating that in addition to the specific roles of Gli2 and Gli3 in the neural tube, there is functional redundancy between Gli proteins. Finally, analysis of Shh/Gli3 compound mutant mice substantiates the idea that ventral patterning may involve a mechanism independent, or parallel, to graded Shh signaling. However, even in the absence of graded Shh signaling, Gli3 is required for the dorsal-ventral patterning of the intermediate neural tube. Together these data raise the possibility that Gli proteins act as common mediators integrating Shh signals, and other sources of positional information, to control patterning throughout the ventral neural tube.

Regulation of Tbx3 expression by anteroposterior signalling in vertebrate limb development

Tbx3, a T-box gene family member related to the Drosophila gene optomotor blind (omb) and encoding a transcription factor, is expressed in anterior and posterior stripes in developing chick limb buds. Tbx3 haploinsufficiency has been linked with the human condition ulnar-mammary syndrome, in which predominantly posterior defects occur in the upper limb. Omb is expressed in Drosophila wing development in response to a signalling cascade involving Hedgehog and Dpp. Homologous vertebrate signals Sonic hedgehog (Shh) and bone morphogenetic protein 2 (Bmp2) are associated in chick limbs with signalling of the polarising region which controls anteroposterior pattern. Here we carried out tissue transplantations, grafted beads soaked in Shh, Bmps, and Noggin in chick limb buds, and analysed Tbx3 expression. We also investigated Tbx3 expression in limb buds of chicken and mouse mutants and retinoid-deficient quail in which anteroposterior patterning is abnormal. We show that Tbx3 expression in anterior and posterior stripes is regulated differently. Posterior Tbx3 expression is stable and depends on the signalling cascade centred on the polarising region involving Shh and Bmps, while anterior Tbx3 expression is labile and depends on the balance between positive Bmp signals, produced anteriorly, and negative Shh signals, produced posteriorly. Our results are consistent with the idea that posterior Tbx3 expression is involved in specifying digit pattern and thus provides an explanation for the posterior defects in human patients. Anterior Tbx3 expression appears to be related to the width of limb bud, which determines digit number.

Structural basis for activation of fibroblast growth factor signaling by sucrose octasulfate

Sucrose octasulfate (SOS) is believed to stimulate fibroblast growth factor (FGF) signaling by binding and stabilizing FGFs. In this report, we show that SOS induces FGF-dependent dimerization of FGF receptors (FGFRs). The crystal structure of the dimeric FGF2-FGFR1-SOS complex at 2.6-A resolution reveals a symmetric assemblage of two 1:1:1 FGF2-FGFR1-SOS ternary complexes. Within each ternary complex SOS binds to FGF and FGFR and thereby increases FGF-FGFR affinity. SOS also interacts with the adjoining FGFR and thereby promotes protein-protein interactions that stabilize dimerization. This structural finding is supported by the inability of selectively desulfated SOS molecules to promote receptor dimerization. Thus, we propose that SOS potentiates FGF signaling by imitating the dual role of heparin in increasing FGF-FGFR affinity and promoting receptor dimerization. Hence, the dimeric FGF-FGFR-SOS structure substantiates the recently proposed "two-end" model, by which heparin induces FGF-FGFR dimerization. Moreover, the FGF-FGFR-SOS structure provides an attractive template for the development of easily synthesized SOS-related heparin agonists and antagonists that may hold therapeutic potential.

Megalin functions as an endocytic sonic hedgehog receptor

Embryos deficient in the morphogen Sonic hedgehog (Shh) or the endocytic receptor megalin exhibit common neurodevelopmental abnormalities. Therefore, we have investigated the possibility that a functional relationship exists between the two proteins. During embryonic development, megalin was found to be expressed along the apical surfaces of neuroepithelial cells and was coexpressed with Shh in the ventral floor plate of the neural tube. Using enzyme-linked immunosorbent assay, homologous ligand displacement, and surface plasmon resonance techniques, it was found that the amino-terminal fragment of Shh (N-Shh) bound to megalin with high affinity. Megalin-expressing cells internalized N-Shh through a mechanism that was inhibited by antagonists of megalin, viz. anti-receptor-associated protein and anti-megalin antibodies. Heparin also inhibited N-Shh endocytosis, implicating proteoglycans in the internalization process, as has been described for other megalin ligands. Use of chloroquine to inhibit lysosomal proteinase activity showed that N-Shh endocytosed via megalin was not efficiently targeted to the lysosomes for degradation. The ability of megalin-internalized N-Shh to bypass lysosomes may relate to the finding that the interaction between N-Shh and megalin was resistant to dissociation with low pH. Together, these findings show that megalin is an efficient endocytic receptor for N-Shh. Furthermore, they implicate megalin as a new regulatory component of the Shh signaling pathway.

Sonic hedgehog signaling from the urethral epithelium controls external genital development

External genital development begins with formation of paired genital swellings, which develop into the genital tubercle. Proximodistal outgrowth and axial patterning of the genital tubercle are coordinated to give rise to the penis or clitoris. The genital tubercle consists of lateral plate mesoderm, surface ectoderm, and endodermal urethral epithelium derived from the urogenital sinus. We have investigated the molecular control of external genital development in the mouse embryo. Previous work has shown that the genital tubercle has polarizing activity, but the precise location of this activity within the tubercle is unknown. We reasoned that if the tubercle itself is patterned by a specialized signaling region, then polarizing activity may be restricted to a subset of cells. Transplantation of urethral epithelium, but not genital mesenchyme, to chick limbs results in mirror-image duplication of the digits. Moreover, when grafted to chick limbs, the urethral plate orchestrates morphogenetic movements normally associated with external genital development. Signaling activity is therefore restricted to urethral plate cells. Before and during normal genital tubercle outgrowth, urethral plate epithelium expresses Sonic hedgehog (Shh). In mice with a targeted deletion of Shh, external genitalia are absent. Genital swellings are initiated, but outgrowth is not maintained. In the absence of Shh signaling, Fgf8, Bmp2, Bmp4, Fgf10, and Wnt5a are downregulated, and apoptosis is enhanced in the genitalia. These results identify the urethral epithelium as a signaling center of the genital tubercle, and demonstrate that Shh from the urethral epithelium is required for outgrowth, patterning, and cell survival in the developing external genitalia.

A gene network model accounting for development and evolution of mammalian teeth

Generation of morphological diversity remains a challenge for evolutionary biologists because it is unclear how an ultimately finite number of genes involved in initial pattern formation integrates with morphogenesis. Ideally, models used to search for the simplest developmental principles on how genes produce form should account for both developmental process and evolutionary change. Here we present a model reproducing the morphology of mammalian teeth by integrating experimental data on gene interactions and growth into a morphodynamic mechanism in which developing morphology has a causal role in patterning. The model predicts the course of tooth-shape development in different mammalian species and also reproduces key transitions in evolution. Furthermore, we reproduce the known expression patterns of several genes involved in tooth development and their dynamics over developmental time. Large morphological effects frequently can be achieved by small changes, according to this model, and similar morphologies can be produced by different changes. This finding may be consistent with why predicting the morphological outcomes of molecular experiments is challenging. Nevertheless, models incorporating morphology and gene activity show promise for linking genotypes to phenotypes.

Sonic Hedgehog as a mediator of long-range signaling

The ability of Hedgehog (Hh) proteins to exert their biological effects is regulated by a series of post-translational processes. These processes include an intramolecular cleavage, covalent addition of cholesterol and/or palmitate, and conversion into a multimeric freely diffusible form. The processing of Hh proteins affects their trafficking, potency, and ability to signal over many cell diameters. Accordingly, the loss of gene products required for these processes abrogates the Hh proteins' abilities to exert their effects, which can be long range, short range, or both. We review here recent evidence demonstrating that Hh proteins are directly responsible for their long-range biological effects. Additionally, we integrate both genetic and biochemical data to delineate a model illustrating how the unusual biochemistry of Hh family members may allow them to act as morphogens, signaling over both short and long distances.

Sonic hedgehog in the nervous system: functions, modifications and mechanisms

Signaling by Sonic hedgehog (Shh) controls important developmental processes, including dorsoventral neural tube patterning, neural stem cell proliferation, and neuronal and glial cell survival. Shh signaling involves lipid modifications to Shh itself, as well as changes in protein subcellular localization. Recent advances have revealed the importance of palmitoylation and acylation of Shh on its potency and migration capacity. Subsequent trafficking and organelle sorting in the Shh signaling pathway have been observed; these observations offer a new dimension to our understanding of downstream signal transduction events.

Growth, hedgehog and the price of GAS

Embryonic development in a given species is orchestrated by genes regulating growth and differentiation in a stereotyped and conserved manner, resulting in embryos of consistent size and shape. Several signaling pathways, including that of Sonic Hedgehog (SHH), have been implicated in these processes. Recent experiments with Gas1 indicate that it may act as a growth-inducing gene, challenging its previous function as a gene specifically involved in growth arrest. Moreover, GAS1, a GPI-linked membrane protein, can bind SHH, suggesting an interacting link between growth and patterning through SHH and GAS1.

Hedgehog-Gli signalling and the growth of the brain

The development of the vertebrate brain involves the creation of many cell types in precise locations and at precise times, followed by the formation of functional connections. To generate its cells in the correct numbers, the brain has to produce many precursors during a limited period. How this is achieved remains unclear, although several cytokines have been implicated in the proliferation of neural precursors. Understanding this process will provide profound insights, not only into the formation of the mammalian brain during ontogeny, but also into brain evolution. Here we review the role of the Sonic hedgehog-Gli pathway in brain development. Specifically, we discuss the role of this pathway in the cerebellar and cerebral cortices, and address the implications of these findings for morphological plasticity. We also highlight future directions of research that could help to clarify the mechanisms and consequences of Sonic hedgehog signalling in the brain.

Genetic manipulation of hedgehog signaling in the endochondral skeleton reveals a direct role in the regulation of chondrocyte proliferation

Indian hedgehog (Ihh), one of the three mammalian hedgehog (Hh) proteins, coordinates proliferation and differentiation of chondrocytes during endochondral bone development. Smoothened (Smo) is a transmembrane protein that transduces all Hh signals. In order to discern the direct versus indirect roles of Ihh in cartilage development, we have used the Cre-loxP approach to remove Smo activity specifically in chondrocytes. Animals generated by this means develop shorter long bones when compared to wild-type littermates. In contrast to Ihh mutants (Ihhn/Ihhn), chondrocyte differentiation proceeds normally. However, like Ihhn/Ihhn mice, proliferation of chondrocytes is reduced by about 50%, supporting a direct role for Ihh in the regulation of chondrocyte proliferation. Moreover, by overexpressing either Ihh or a constitutively active Smo allele (Smo*) specifically in the cartilage using the bigenic UAS-Gal4 system, we demonstrate that activation of the Ihh signaling pathway is sufficient to promote chondrocyte proliferation. Finally, expression of cyclin D1 is markedly downregulated when either Ihh or Smo activity is removed from chondrocytes, indicating that Ihh regulates chondrocyte proliferation at least in part by modulating the transcription of cyclin D1. Taken together, the present study establishes Ihh as a key mitogen in the endochondral skeleton.

"Fingering" the vertebrate limb

A detailed and precise picture is being pieced together about how the pattern of digits develops in vertebrate limbs. What is particularly exciting is that it will soon be possible to trace the process all the way from establishment of a signalling centre in a small bud of undifferentiated cells right through to final limb anatomy. The development of the vertebrate limb is a traditional model in which to explore mechanisms involved in pattern formation, and there is accelerating knowledge about the genes involved. One reason why the limb is holding its place in the post-genomic age is that it is rich in pre-genomic embryology. Here, we will focus on recent findings about the aspect of vertebrate limb development concerned with digit pattern across the anteroposterior axis of the limb. This process is controlled by a signalling region in the early limb bud known as the polarizing region. Interactions between polarizing region cells and other cells in the limb bud ensure that a thumb develops at one edge of the hand (anterior) and a little finger at the other (posterior).

BMP and Ihh/PTHrP signaling interact to coordinate chondrocyte proliferation and differentiation

During endochondral ossification, two secreted signals, Indian hedgehog (Ihh) and parathyroid hormone-related protein (PTHrP), have been shown to form a negative feedback loop regulating the onset of hypertrophic differentiation of chondrocytes. Bone morphogenetic proteins (BMPs), another family of secreted factors regulating bone formation, have been implicated as potential interactors of the Ihh/PTHrP feedback loop. To analyze the relationship between the two signaling pathways, we used an organ culture system for limb explants of mouse and chick embryos. We manipulated chondrocyte differentiation by supplementing these cultures either with BMP2, PTHrP and Sonic hedgehog as activators or with Noggin and cyclopamine as inhibitors of the BMP and Ihh/PTHrP signaling systems. Overexpression of Ihh in the cartilage elements of transgenic mice results in an upregulation of PTHrP expression and a delayed onset of hypertrophic differentiation. Noggin treatment of limbs from these mice did not antagonize the effects of Ihh overexpression. Conversely, the promotion of chondrocyte maturation induced by cyclopamine, which blocks Ihh signaling, could not be rescued with BMP2. Thus BMP signaling does not act as a secondary signal of Ihh to induce PTHrP expression or to delay the onset of hypertrophic differentiation. Similar results were obtained using cultures of chick limbs.

We further investigated the role of BMP signaling in regulating proliferation and hypertrophic differentiation of chondrocytes and identified three functions of BMP signaling in this process. First we found that maintaining a normal proliferation rate requires BMP and Ihh signaling acting in parallel. We further identified a role for BMP signaling in modulating the expression of Ihh. Finally, the application of Noggin to mouse limb explants resulted in advanced differentiation of terminally hypertrophic cells, implicating BMP signaling in delaying the process of hypertrophic differentiation itself. This role of BMP signaling is independent of the Ihh/PTHrP pathway.

Enamel knots as signaling centers linking tooth morphogenesis and odontoblast differentiation

Odontoblasts differentiate from the cells of the dental papilla, and it has been well-established that their differentiation in developing teeth is induced by the dental epithelium. In experimental studies, no other mesenchymal cells have been shown to have the capacity to differentiate into odontoblasts, indicating that the dental papilla cells have been committed to odontoblast cell lineage during earlier developmental stages. We propose that the advancing differentiation within the odontoblast cell lineage is regulated by sequential epithelial signals. The first epithelial signals from the early oral ectoderm induce the odontogenic potential in the cranial neural crest cells. The next step in the determination of the odontogenic cell lineage is the development of the dental papilla from odontogenic mesenchyme. The formation of the dental papilla starts at the onset of the transition from the bud to the cap stage of tooth morphogenesis, and this is regulated by epithelial signals from the primary enamel knot. The primary enamel knot is a signaling center which forms at the tip of the epithelial tooth bud. It becomes fully developed and morphologically discernible in the cap-stage dental epithelium and expresses at least ten different signaling molecules belonging to the BMP, FGF, Hh, and Wnt families. In molar teeth, secondary enamel knots appear in the enamel epithelium at the sites of the future cusps. They also express several signaling molecules, and their formation precedes the folding and growth of the epithelium. The differentiation of odontoblasts always starts from the tips of the cusps, and therefore, it is conceivable that some of the signals expressed in the enamel knots may act as inducers of odontoblast differentiation. The functions of the different signals in enamel knots are not precisely known. We have shown that FGFs stimulate the proliferation of mesenchymal as well as epithelial cells, and they may also regulate the growth of the cusps. We have proposed that the enamel knot signals also have important roles, together with mesenchymal signals, in regulating the patterning of the cusps and hence the shape of the tooth crown. We suggest that the enamel knots are central regulators of tooth development, since they link cell differentiation to morphogenesis.