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

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.

Drosophila glypicans control the cell-to-cell movement of Hedgehog by a dynamin-independent process

The signalling molecule Hedgehog (Hh) functions as a morphogen to pattern a field of cells in animal development. Previous studies in Drosophila have demonstrated that Tout-velu (Ttv), a heparan sulphate polymerase, is required for Hh movement across receiving cells. However, the molecular mechanism of Ttv- mediated Hh movement is poorly defined. We show that Dally and Dally-like (Dly), two Drosophila glypican members of the heparan sulphate proteoglycan (HSPG) family, are the substrates of Ttv and are essential for Hh movement. We show that embryos lacking dly activity exhibit defects in Hh distribution and its subsequent signalling. However, both Dally and Dly are involved and are functionally redundant in Hh movement during wing development. We further demonstrate that Hh movement in its receiving cells is regulated by a cell-to-cell mechanism that is independent of dynamin-mediated endocytosis. We propose that glypicans transfer Hh along the cell membrane to pattern a field of cells.

Wnts and Hedgehogs: lipid-modified proteins and similarities in signaling mechanisms at the cell surface

This review compares the signaling mechanisms of the Wnt and the Hedgehog proteins. Although Wnts and Hedgehogs are unrelated proteins, they are both modified by lipids, possibly through the action of enzymes that are related to each other. At the surface of target cells, the reception of Wnt and Hedgehog signals is regulated by several molecules, some of which, in particular the Frizzled and Smoothened receptors, are related to each other. Several other aspects of Wnt and Hedgehog transport and signaling are discussed, as well as the possible origin of these pathways.

Identification of a novel left-right asymmetrically expressed gene in the mouse belonging to the BPI/PLUNC superfamily

In the process of left-right (L-R) axis formation in the mouse, the node plays a critical role as a structure where the initial breaking of L-R symmetry occurs. Here, we report on the gene LPlunc1, a member of BPI/PLUNC gene superfamily, which is asymmetrically expressed in the developing mouse node. LPlunc1 protein is secreted as a processed form of relative molecular mass 54K-60K and shares sequence features with the other members of BPI/PLUNC superfamily, including the N-terminal and C-terminal homology domains, each of which is considered to form a lipid binding pocket. LPlunc1 is transiently expressed in the crown cells of the node asymmetrically. This expression pattern of Lplunc1 highly overlaps with that of Nodal, a major player during the L-R formation. Interestingly, this asymmetric expression pattern is randomized in the iv mutant and reversed in the inv mutant, indicating that LPlunc1 is downstream of iv and inv. Our results suggest a link between lipid binding/transfer and the axis development.

Expression of hedgehog proteins in the human thymus

The Hedgehog (Hh) family of secreted proteins includes intercellular signaling molecules that specify cell fate and patterning during the development of many tissues. In this study we show that the different components of the Hh signaling pathway are expressed in human thymus. The three mammalian Hh proteins, Sonic (Shh), Indian (Ihh), and Desert (Dhh) hedgehog, are produced by thymic epithelial cells. Shh-expressing epithelial cells are restricted to the thymic subcapsula and medulla, whereas Ihh- and Dhh-producing epithelial cells are distributed throughout the thymus. The requisite Hh receptors, Patched 1(Ptc1) and Smoothened (Smo), and the Gli transcription factors are expressed by thymocytes and also by epithelial cells. Ptc1 is expressed in most thymocyte subsets, whereas Smo expression is mainly associated with immature thymocytes. The isoform of the Ptc receptor, Ptc2, is expressed only by intrathymic progenitor cells and epithelial cells. Other Hh-binding proteins with modulating functions, such as Hedgehog-interacting protein (Hip) and growth arrest-specific gene-1 (Gas-1), are also expressed in human thymus. Our study shows that the intrathymic expression pattern of the Hh signaling pathway components is complex and suggests that Hh proteins may regulate human thymocyte differentiation from the earliest developmental stages, as well as thymic epithelial cell function.

Replicated anterior zeugopod (raz): a polydactylous mouse mutant with lowered Shh signaling in the limb bud

A unique limb phenotype is described in a radiation-induced mutant mouse resulting from an inversion of a proximal segment of chromosome 5. The limb phenotype in the homozygous mutant presents with two anterior skeletal elements in the zeugopod but no posterior bone, hence the name replicated anterior zeugopod, raz. The zeugopod phenotype is accompanied by symmetrical central polydactyly of hand and foot. The chromosomal inversion includes the Shh gene and the regulatory locus, located ∼1 Mb away, within the Lmbr1 gene. In homozygous mutants, the expression of Shh mRNA and Shh protein is severely downregulated to about 20% of wild-type limb buds, but Shh expression appears normal throughout the remainder of the embryo. Correspondingly, Gli3 expression is upregulated and posteriorly expanded in the raz/raz limb bud. We propose that the double anterior zeugopod and symmetrical central polydactyly are due to an increased and uniform concentration of the Gli3 repressor form because of lowered Shh signaling.

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.

Sonic hedgehog and bone morphogenetic protein regulate interneuron development from dorsal telencephalic progenitors in vitro

Cortical progenitors are competent to produce interneurons, but do not generate large numbers of interneurons in vivo under normal circumstances. This could reflect the absence of an inductive signal in the environment of the dorsal telencephalon and/or the presence of an inhibitory signal. To determine whether either or both mechanisms regulate interneuron generation, progenitors in dorsomedial and dorsolateral wall explants of mouse telencephalon were marked with a retrovirus and cultured under several conditions. When cultured separately, progenitors in dorsomedial wall explants produced fewer GABAergic interneurons than progenitors in dorsolateral wall explants. When cocultured with ventral telencephalic cells, however, dorsomedial wall progenitors produced more GABAergic interneurons than in dorsomedial wall explants alone. The inductive effect of ventral telencephalon depended on sonic hedgehog (Shh) and could be mimicked by exogenous Shh. In contrast, exogenous bone morphogenetic protein 4 (BMP4) reduced the production of interneurons in dorsolateral wall explants and inhibited the induction by exogenous Shh. Moreover, inhibiting BMP signaling in dorsomedial wall progenitors with a dominant-negative BMP receptor Ib (dnBMPIb) virus increased their production of interneurons, even if Shh was blocked. Shh and dnBMPRIb increased proliferation and the generation of interneurons, but FGF2 did not induce interneurons, although it increased proliferation. This suggests that proliferation per se does not control the production of interneurons. Our findings suggest that the generation of interneurons by dorsal telencephalic progenitors is normally limited by excess levels of BMPs. Shh may promote the generation of interneurons by antagonizing BMP, but may not be required directly for the generation of interneurons.

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.

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.

A restrictive role for Hedgehog signalling during otic specification in Xenopus

Vertebrate inner ear development is initiated by the specification of the otic placode, an ectodermal structure induced by signals from neighboring tissue. Although several signaling molecules have been identified as candidate otic inducers, many details of the process of inner ear induction remain elusive. Here, we report that otic induction is responsive to the level of Hedgehog (Hh) signaling activity in Xenopus, making use of both gain- and loss-of-function approaches. Ectopic activation of Hedgehog signaling resulted in the development of ectopic vesicular structures expressing the otic marker genes XPax-2, Xdll-3, and Xwnt-3A, thus revealing otic identity. Induction of ectopic otic vesicles was also achieved by misexpression of two different inhibitors of Hh signaling: the putative Hh antagonist mHIP and XPtc1deltaLoop2, a dominant-negative form of the Hh receptor Patched. In addition, misexpression of XPtc1deltaLoop2 as well as treatment of Xenopus embryos with the specific Hh signaling antagonist cyclopamine resulted in the formation of enlarged otic vesicles. In summary, our observations suggest that a defined level of Hh signaling provides a restrictive environment for otic fate in Xenopus embryos.

Induction of neurogenin-1 expression by sonic hedgehog: Its role in development of trigeminal sensory neurons

We have examined the roles of signaling molecules in the mechanisms underlying the induction of neurogenin (ngn)-1 expression. ngn-1 is a basic helix-loop-helix (bHLH) transcription factor, which is essential for the specification of trigeminal sensory neurons. Semiquantitative reverse transcriptase-polymerase chain reaction using cranial explants in organ cultures showed that sonic hedgehog (Shh) promotes ngn-1 expression. This promoting activity was not observed in other signaling molecules examined. The promotion of ngn-1 expression by Shh, furthermore, was inhibited by cyclopamine, a specific inhibitor of Shh signaling. Shh did not affect the expression of ngn-2, a bHLH transcription factor that plays an important role in the specification of epibranchial placode-derived sensory neurons. The expression levels of ngn-1 and ngn-2 decreased after fibroblast growth factor-2 treatment. These results suggest that Shh induces ngn-1 expression specifically and that expression of ngn-1 and ngn-2 is regulated by different mechanisms. The induction of ngn-1 expression by Shh suggests that this signaling molecule participates in the specification of trigeminal sensory neurons. We therefore examined the effect of Shh on the development of these neurons. Immunostaining using anti-ngn-1 demonstrated that Shh promotes ngn-1 expression in trigeminal neural crest cells. Trigeminal neural crest cells are derived from the posterior mesencephalon and the most-anterior rhombencephalon, and they contain a subset of precursors of trigeminal sensory neurons. Moreover, a subpopulation of trigeminal neural crest cells expressed the Shh receptor Patched. The number of cells that express Brn3a, a POU-domain transcription factor that plays an important role in differentiation of sensory neurons, also increased with Shh treatment. Our data suggest that Shh signaling is involved in the specification of trigeminal sensory neurons through the induction of ngn-1 expression. Furthermore, Shh promotes the differentiation of neural crest cells into trigeminal sensory neurons.

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.

Regulatory principles of developmental signaling

Signaling between cells is a widely used mechanism by which cell fate and tissue patterning is determined in development. We review the mechanisms by which signaling between cells is regulated so that a cell receives the right amount of signal, at the right time, to achieve its intended developmental fate and position. In nearly all cases, we find that the supply of signal factor (ligand) is the limiting step in initiating a signaling process. Ligand supply is regulated by the transcription and localization of RNA, the spread of ligand from a source, and by inhibitors that operate at several different levels. We emphasize the different regulatory strategies that operate for threshold as opposed to concentration-dependent (morphogen) signaling. Threshold signaling is extensively regulated by feedback mechanisms. Morphogen signaling is regulated quantitatively by receptor loading and transduction flow.

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.

The interplay of genetic and environmental factors in craniofacial morphogenesis: holoprosencephaly and the role of cholesterol

Cyclopia, the paradigmatic "face [that] predicts the brain" in severe holoprosencephaly (HPE) (DeMyer et al., 1964), has been recognized since ancient times. Descriptive embryologists and pathologists have noted the continuum of defective separation of the forebrain and loss of central nervous system (CNS) midline structures for more than a century. It has been recognized more recently that inhibitors of cholesterol biosynthesis, whether consumed in native plants by range sheep, or experimentally applied to early embryos, could phenocopy the natural malformation, as could a variety of other teratogens (maternal diabetes, alcohol). Yet it has been less than a decade that the genomic knowledge base and powerful analytic methods have brought the sciences of descriptive, molecular, and genetic embryology within range of each other. In this review, we discuss the clinical presentations and pathogenesis of HPE. We will outline various genetic and teratogenic mechanisms leading to HPE. Lastly, we will attempt to examine the pivotal role of cholesterol and the Sonic Hedgehog (Shh) pathway in this disorder and in normal embryonic forebrain development.

Notochord anomalies in the adriamycin rat model: A morphologic and molecular basis for the VACTERL association

BACKGROUND/PURPOSE

The Adriamycin rat model (ARM) is a reliable model of the VACTERL association. The notochord is structurally abnormal in the region of the foregut, midgut, and hindgut in the ARM. The authors hypothesised that notochord anomalies allow ectopic expression of molecular signals in the developing embryo and thus lead to VACTERL malformations. This study was designed to investigate this hypothesis.

METHODS

Adriamycin (1.75 mg/kg) was administered intraperitoneally to pregnant rats on days 7, 8, and 9 of gestation. Control animals were given saline. Embryos were recovered on gestational days 10.5 to 14 at (1/2)-day intervals and at full term. The first group of embryos were embedded in resin, and sagittal sections stained with Toluidine blue were studied for morphologic abnormalities. The second group of embryos were examined using in situ hybridization for the expression of Sonic Hedgehog (Shh), a patterning gene implicated in the etiology of the VACTERL association.

RESULTS

Twenty-seven of the 28 (96.4%) full-term embryos showed VACTERL anomalies. Forty-five of the 50 (90%) experimental embryos (gestational days 10.5 to 14) showed notochord abnormalities. Abnormal ventral branches from the notochord toward the gut were a commonly observed abnormality. These were seen to impinge on the developing foregut, midgut, dorsal aorta, and kidney. In situ hybridization for Shh showed that these branches from the notochord expressed Shh in 66.6% of experimental embryos. This abnormal Shh expression was not seen in the control embryos.

CONCLUSIONS

Adriamycin diffusely induces altered notochord morphology in the rat embryo. The abnormal notochord morphology may allow ectopic expression of Sonic Hedgehog, and, thus, contribute to the malformations found in the VACTERL association.

Cholesterol modification of hedgehog is required for trafficking and movement, revealing an asymmetric cellular response to hedgehog

Hedgehog family members are secreted proteins involved in numerous patterning mechanisms. Different posttranslational modifications have been shown to modulate Hedgehog biological activity. We investigated the role of these modifications in regulating subcellular localization of Hedgehog in the Drosophila embryonic epithelium. We demonstrate that cholesterol modification of Hedgehog is responsible for its assembly in large punctate structures and apical sorting through the activity of the sterol-sensing domain-containing Dispatched protein. We further show that movement of these specialized structures through the cellular field is contingent upon the activity of proteoglycans synthesized by the heparan sulfate polymerase Tout-Velu. Finally, we show that the Hedgehog large punctate structures are necessary only for a subset of Hedgehog target genes across the parasegmental boundary, suggesting that presentation of Hedgehog from different membrane compartments is responsible for Hedgehog functional diversity in epithelial cells.

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.

Patterning the limb before and after SHH signalling

The vertebrate limb is one of the most relevant experimental models for analysing cell-cell signalling during patterning of embryonic fields and organogenesis. Recently, the combination of molecular and genetic studies with experimental manipulation of developing limb buds has significantly advanced our understanding of the complex molecular interactions co-ordinating limb bud outgrowth and patterning. Some of these studies have shown that there is a need to revise some of the textbook views of vertebrate limb development. In this review, we discuss how signalling by the polarizing region is established and how limb bud morphogenesis is controlled by both long-range and signal relay mechanisms. We also discuss recent results showing that differential mesenchymal responsiveness to SHH signalling is established prior to its expression by the polarizing region.

Growth arrest specific gene 1: a fuel for driving growth in the cerebellum

Cell proliferation is an essential force to build up the size, shape, and function of an organ. This force is particularly prominent in the production of the cerebellar granule neurons, which represent 80% of all brain neurons. Extensive cell biological and tissue transplantation studies have uncovered both long-range diffusible and local cell-cell, contact-dependent growth cues for the granular neurons. The assignment of specific gene products to their contributions to the genesis of the granular neurons is greatly facilitated by in vitro culture assays and knock-out mouse analyses. Among them, the Growth arrest specific gene 1 (Gas1), a known negative regulator of the cell cycle, was shown to have profound influence on the production of the granule cells. Our aim here is to review the contributions of Gas1 and a few other selected genes and put them into a more comprehensive framework, though it may be speculative at times, of granule cell proliferation regulation.

Behavioural and immunohistochemical changes following supranigral administration of sonic hedgehog in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated common marmosets

Sonic hedgehog (SHH) has trophic actions on dopaminergic cell cultures and protects them from MPP(+) toxicity but its in vivo actions have not been explored. We now investigate the effects of unilateral supranigral administration of SHH on nigro-striatal function in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated common marmosets. SHH (0.1 or 1.0 microg) or vehicle was stereotaxically injected into the region of the right substantia nigra twice with an interval of 5 weeks between administrations. The first or second administration of low dose SHH (0.1 microg) did not significantly improve motor disability or locomotor activity compared to time-matched vehicle-treated animals. There was, however, an approximately 30% improvement in both motor disability and locomotor activity following the first administration of high dose SHH (1.0 microg). No further improvements occurred following the second high dose SHH treatment. Acute oral administration of L-3,4-dihydroxyphenylalanine (L-DOPA) produced a smaller increase in locomotor activity and greater reversal of motor disability in animals treated with SHH than occurred in vehicle-treated common marmosets. In the substantia nigra pars compacta, ipsilateral to SHH administration, the number of tyrosine hydroxylase-positive neurones was increased by 21% (P > 0.05) and 57% (P < 0.05) in low and high dose SHH groups respectively compared to the untreated contralateral hemisphere. There was no difference in the number of glial fibrillary acidic protein-positive cells. SHH may improve nigro-striatal function by restoring tyrosine hydroxylase positivity. This is reflected by an improvement in basal disability and a reduction in the lesion-induced response to L-DOPA.

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.

Sonic hedgehog signaling modulates activation of and cytokine production by human peripheral CD4+ T cells

Sonic hedgehog (Shh) is important in the growth and differentiation of a variety of cell types, including the development of T cells in the thymus. This prompted us to investigate whether Shh signaling is a functional component of the physiological response of human mature CD4(+) T cells following Ag recognition. In this study, we demonstrate that Shh and its receptor Patched (Ptc) are expressed on resting and activated human peripheral CD4(+) T cells. In approximately one-half of the randomly selected, anonymous blood donors tested, exposure of anti-CD3/28 Ab-activated CD4(+) T cells to the biologically active N-terminal Shh peptide increased the transcription of ptc, thereby demonstrating that Shh signaling had occurred. Furthermore, the addition of exogenous Shh amplified the production of IL-2, IFN-gamma, and IL-10 by activated CD4(+) T cells. The synthesis of IL-2 and IFN-gamma, but not IL-10, by CD4(+) T cells was down-regulated by the addition of neutralizing anti-Shh Ab. Cell surface expression of CD25 and CD69 on activated T cells was up-regulated by exogenous Shh, whereas in the presence of the neutralizing anti-Shh Ab expression it was reduced. Collectively, our findings demonstrate that Shh-mediated signaling is a physiological component of T cell responses, which acts to modulate CD4(+) T cell effector function.

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.

Morphogen transport along epithelia, an integrated trafficking problem

Graded signals are an important component of current models of pattern formation. Typically, a group of cells produces a signal that decays as it spreads through neighboring tissue. By contrast with endocrine signals, which spread systemically, patterning signals or morphogens have a restricted zone of influence, an area classically known as a field. The widely accepted model is that graded distribution of such signals allow cells to measure their position relative to the source. Although it provides a framework for understanding pattern formation, the concept of the morphogen raises many mechanistic issues. For example, how the distribution of a morphogen is established and maintained remains an outstanding issue. There is no doubt that signals are transported over distances of tens of cell diameters and that stable gradients do form. The question of how this is achieved has aroused the interest of many cell biologically minded developmental biologists.

Hedgehog-mediated patterning of the mammalian embryo requires transporter-like function of dispatched

The dispatched (disp) gene is required for long-range Hedgehog (Hh) signaling in Drosophila. Here, we demonstrate that one of two murine homologs, mDispA, can rescue disp function in Drosophila and is essential for all Hh patterning activities examined in the early mouse embryo. Embryonic fibroblasts lacking mDispA respond normally to exogenously provided Sonic hedgehog (Shh) signal, but are impaired in stimulation of other responding cells when expressing Shh. We have developed a biochemical assay that directly measures the activity of Disp proteins in release of soluble Hh proteins. This activity is disrupted by alteration of residues functionally conserved in Patched and in a related family of bacterial transmembrane transporters, thus suggesting similar mechanisms of action for all of these proteins.

Patched acts catalytically to suppress the activity of Smoothened

Mutations affecting the transmembrane proteins Patched (Ptc) or Smoothened (Smo) that trigger ligand-independent activity of the Hedgehog (Hh) signalling pathway are associated with human tumours such as basal cell carcinoma (BCC) and medulloblastoma. Despite extensive genetic studies demonstrating the importance of these receptor components in embryonic patterning and cancer, the mechanism by which Ptc regulates Smo is not understood. Here we report that Ptc and Smo are not significantly associated within Hh-responsive cells. Furthermore, we show that free Ptc (unbound by Hh) acts sub-stoichiometrically to suppress Smo activity and thus is critical in specifying the level of pathway activity. Patched is a twelve-transmembrane protein with homology to bacterial proton-driven transmembrane molecular transporters; we demonstrate that the function of Ptc is impaired by alterations of residues that are conserved in and required for function of these bacterial transporters. These results suggest that the Ptc tumour suppressor functions normally as a transmembrane molecular transporter, which acts indirectly to inhibit Smo activity, possibly through changes in distribution or concentration of a small molecule.

Sonic hedgehog promotes cell cycle progression in activated peripheral CD4(+) T lymphocytes

Sonic hedgehog (Shh) signaling is important in the growth and differentiation of many cell types and recently has been reported to play a role in T cell development in the thymus. This prompted us to investigate whether or not Shh contributes to the clonal expansion of peripheral CD4(+) T cells. In this study, we demonstrate that Shh and other components of the signaling pathway patched, smoothened, and Gli1 (glioma-associated oncogene) are expressed in peripheral CD4(+) T cells. The addition of the biologically active amino-terminal Shh peptide had no effect on resting CD4(+) T cells, but significantly enhanced proliferation of anti-CD3/28 Ab-activated CD4(+) T cells. This was not due to antiapoptotic effects, but by promoting entry of T cells into the S-G(2) proliferative phase of the cell cycle. Neutralizing anti-Shh Ab reduced T cell proliferation by inhibiting cell transition into the S-G(2) phase, suggesting that endogenously produced Shh plays a physiological role in the clonal expansion of T cells. Furthermore, we have observed a significant up-regulation of Shh and Gli1 (glioma-associated oncogene) mRNA in activated CD4(+) T cells with or without addition of exogenous Shh, which corresponds with maximal CD4(+) T cell proliferation, whereas bcl-2 was only up-regulated in activated cells in the presence of Shh. Our findings suggest that endogenously produced Shh may play a role in sustaining normal CD4(+) T cell proliferation and exogenously added Shh enhances this response.

Expression of the human poliovirus receptor/CD155 gene is activated by sonic hedgehog

The human poliovirus receptor/CD155 is a transmembrane glycoprotein belonging to the immunoglobulin superfamily. The ectodomain of CD155 mediates cell attachment to the extracellular matrix molecule vitronectin, while its intracellular domain interacts with the dynein light chain Tctex-1. CD155 is a primate-restricted gene that is expressed during development in mesenchymal tissues and ventrally derived structures within the CNS. Its function in adults is as yet unknown, but significantly, CD155 is aberrantly expressed in neuroectodermal tumors. We show that the expression of CD155 mRNA is up-regulated when human Ntera2 cells are treated with purified Sonic hedgehog (Shh) protein. Reporter gene expression driven by the CD155 core promoter is activated by Shh in transient co-transfection assays. Analysis of the CD155 core promoter indicates that an intact GLI binding site is required for Shh activation. In addition, overexpression of Gli1 or Gli3 potently activates reporter gene expression driven by the CD155 core promoter. These data identify the CD155 gene as a transcriptional target of Shh, a finding that has significance for the normal function of CD155 during development and the expression of CD155 in neuroectodermal tumors.

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.

Prediction of glycosylphosphatidylinositol-anchored proteins in Arabidopsis. A genomic analysis

Glycosylphosphatidylinositol (GPI) anchoring of proteins provides a potential mechanism for targeting to the plant plasma membrane and cell wall. However, relatively few such proteins have been identified. Here, we develop a procedure for database analysis to identify GPI-anchored proteins (GAP) based on their possession of common features. In a comprehensive search of the annotated Arabidopsis genome, we identified 167 novel putative GAP in addition to the 43 previously described candidates. Many of these 210 proteins show similarity to characterized cell surface proteins. The predicted GAP include homologs of beta-1,3-glucanases (16), metallo- and aspartyl proteases (13), glycerophosphodiesterases (6), phytocyanins (25), multi-copper oxidases (2), extensins (6), plasma membrane receptors (19), and lipid-transfer-proteins (18). Classical arabinogalactan (AG) proteins (13), AG peptides (9), fasciclin-like proteins (20), COBRA and 10 homologs, and novel potential signaling peptides that we name GAPEPs (8) were also identified. A further 34 proteins of unknown function were predicted to be GPI anchored. A surprising finding was that over 40% of the proteins identified here have probable AG glycosylation modules, suggesting that AG glycosylation of cell surface proteins is widespread. This analysis shows that GPI anchoring is likely to be a major modification in plants that is used to target a specific subset of proteins to the cell surface for extracellular matrix remodeling and signaling.

Do morphogen gradients arise by diffusion?

Many patterns of cell and tissue organization are specified during development by gradients of morphogens, substances that assign different cell fates at different concentrations. Gradients form by morphogen transport from a localized site, but whether this occurs by simple diffusion or by more elaborate mechanisms is unclear. We attempt to resolve this controversy by analyzing recent data in ways that appropriately capture the complexity of systems in which transport, receptor interaction, endo- and exocytosis, and degradation occur together. We find that diffusive mechanisms of morphogen transport are much more plausible-and nondiffusive mechanisms much less plausible-than has generally been argued. Moreover, we show that a class of experiments, endocytic blockade, thought to effectively distinguish between diffusive and nondiffusive transport models actually fails to draw useful distinctions.

TGFβ2 mediates the effects of Hedgehog on hypertrophic differentiation and PTHrP expression

The development of endochondral bones requires the coordination of signals from several cell types within the cartilage rudiment. A signaling cascade involving Indian hedgehog (Ihh) and parathyroid hormone related peptide (PTHrP) has been described in which hypertrophic differentiation is limited by a signal secreted from chondrocytes as they become committed to hypertrophy. In this negative-feedback loop, Ihh inhibits hypertrophic differentiation by regulating the expression of Pthrp, which in turn acts directly on chondrocytes in the growth plate that express the PTH/PTHrP receptor. Previously, we have shown that PTHrP also acts downstream of transforming growth factor β (TGFβ) in a common signaling cascade to regulate hypertrophic differentiation in embryonic mouse metatarsal organ cultures. As members of the TGFβ superfamily have been shown to mediate the effects of Hedgehog in several developmental systems, we proposed a model where TGFβ acts downstream of Ihh and upstream of PTHrP in a cascade of signals that regulate hypertrophic differentiation in the growth plate. This report tests the hypothesis that TGFβ signaling is required for the effects of Hedgehog on hypertrophic differentiation and expression of Pthrp. We show that Sonic hedgehog (Shh), a functional substitute for Ihh, stimulates expression of Tgfb2 and Tgfb3 mRNA in the perichondrium of embryonic mouse metatarsal bones grown in organ cultures and that TGFβ signaling in the perichondrium is required for inhibition of differentiation and regulation of Pthrp expression by Shh. The effects of Shh are specifically dependent on TGFβ2, as cultures from Tgfb3-null embryos respond to Shh but cultures from Tgfb2-null embryos do not. Taken together, these data suggest that TGFβ2 acts as a signal relay between Ihh and PTHrP in the regulation of cartilage hypertrophic differentiation.

The sterol-sensing domain: multiple families, a unique role?

The "sterol-sensing domain" (SSD) is conserved across phyla and is present in several membrane proteins, such as Patched (a Hedgehog receptor) and NPC-1 (the protein defective in Niemann-Pick type C1 disease). The role of the SSD is perhaps best understood from the standpoint of its involvement in cholesterol homeostasis. This article discusses how the SSD appears to function as a regulatory domain involved in linking vesicle trafficking and protein localization with such varied processes as cholesterol homeostasis, cell signalling and cytokinesis.

Sonic hedgehog is a potent inducer of rat oligodendrocyte development from cortical precursors in vitro

Sonic Hedgehog (Shh) induces oligodendrocyte development in the ventral neural tube and telencephalon but its role in oligodendrocyte generation in dorsal telencephalon is debated. Transcripts for Shh and its receptor complex were detected in subventricular zone and neocortex from E17 to birth. As Shh is not yet expressed in E15 neocortex, we grew E15 cortical precursors (CP) into neurospheres in the presence of recombinant Octyl-Shh (O-Shh). After sphere adhesion and removal of O-Shh, enhanced neurite outgrowth and cell migration were already observed at 3 h. Three days after O-Shh treatment, oligodendrocyte progenitors (OP) emerged and continued to increase in number for 7 days while the ratio of neuronal cells decreased compared to control. Shh selectively triggered mitosis of OP but not neuronal progenitors and enhanced growth of neonatal OP. Thus Shh in E15-17 embryonic neocortex can signal CP to adopt an oligodendrocyte fate and favors expansion of this lineage.

Distinct consequences of sterol sensor mutations in Drosophila and mouse patched homologs

The membrane protein Patched (Ptc) is a critical regulator of Hedgehog signaling. Ptc is among a family of proteins that contain a sterol sensor motif. The function of this domain is poorly understood, but some proteins that contain sterol sensors are involved in cholesterol homeostasis. In the SREBP cleavage-activating protein (SCAP), sterols inhibit the protein's activity through this domain. Mutations in two highly conserved residues in the SCAP sterol sensor have been identified that confer resistance to sterol regulation. We introduced the analogous mutations in the sterol sensor motif of fly Ptc and mouse Ptc1 and examined their effect on protein activity. In contrast to SCAP, the sterol sensor mutations had different affects on Drosophila Ptc; Ptc Y442C retained function, while Ptc D584N conferred dominant negative activity. In the wing imaginal disc, Ptc D584N overexpression induced Hedgehog targets by stabilizing Cubitus interruptus and inducing decapentaplegic. However, Ptc D584N did not induce collier, a gene that requires high levels of Hedgehog signaling. In mouse Ptc1, the Y438C and D585N mutations did not stimulate signaling in Shh-responsive cell lines but did complement murine ptc1(-/-) cells. The results suggest that mutations in sterol sensor motifs alter function differently between sterol sensor family members.

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.

Myf5 is a direct target of long-range Shh signaling and Gli regulation for muscle specification

Sonic hedgehog (Shh) is a secreted signaling molecule for tissue patterning and stem cell specification in vertebrate embryos. Shh mediates both long-range and short-range signaling responses in embryonic tissues through the activation and repression of target genes by its Gli transcription factor effectors. Despite the well-established functions of Shh signaling in development and human disease, developmental target genes of Gli regulation are virtually unknown. In this study, we investigate the role of Shh signaling in the control of Myf5, a skeletal muscle regulatory gene for specification of muscle stem cells in vertebrate embryos. In previous genetic studies, we showed that Shh is required for Myf5 expression in the specification of dorsal somite, epaxial muscle progenitors. However, these studies did not distinguish whether Myf5 is a direct target of Gli regulation through long-range Shh signaling, or alternatively, whether Myf5 regulation is a secondary response to Shh signaling. To address this question, we have used transgenic analysis with lacZ reporter genes to characterize an Myf5 transcription enhancer that controls the activation of Myf5 expression in the somite epaxial muscle progenitors in mouse embryos. This Myf5 epaxial somite (ES) enhancer is Shh-dependent, as shown by its complete inactivity in somites of homozygous Shh mutant embryos, and by its reduced activity in heterozygous Shh mutant embryos. Furthermore, Shh and downstream Shh signal transducers specifically induce ES enhancer/luciferase reporters in Shh-responsive 3T3 cells. A Gli-binding site located within the ES enhancer is required for enhancer activation by Shh signaling in transfected 3T3 cells and in epaxial somite progenitors in transgenic embryos. These findings establish that Myf5 is a direct target of long-range Shh signaling through positive regulation by Gli transcription factors, providing evidence that Shh signaling has a direct inductive function in cell lineage specification.

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.

"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).

Evidence that the WNT-inducible growth arrest-specific gene 1 encodes an antagonist of sonic hedgehog signaling in the somite

The dorsal-ventral polarity of the somite is controlled by antagonistic signals from the dorsal neural tube/surface ectoderm, mediated by WNTs, and from the ventral notochord, mediated by sonic hedgehog (SHH). Each factor can act over a distance greater than a somite diameter in vitro, suggesting they must limit each other's actions within their own patterning domains in vivo. We show here that the growth-arrest specific gene 1 (Gas1), which is expressed in the dorsal somite, is induced by WNTs and encodes a protein that can bind to SHH. Furthermore, ectopic expression of Gas1 in presomitic cells attenuates the response of these cells to SHH in vitro. Taken together, these data suggest that GAS1 functions to reduce the availability of active SHH within the dorsal somite.

Activated Notch2 signaling inhibits differentiation of cerebellar granule neuron precursors by maintaining proliferation

In the developing cerebellar cortex, granule neuron precursors (GNPs) proliferate and commence differentiation in a superficial zone, the external granule layer (EGL). The molecular basis of the transition from proliferating precursors to immature differentiating neurons remains unknown. Notch signaling is an evolutionarily conserved pathway regulating the differentiation of precursor cells of many lineages. Notch2 is specifically expressed in proliferating GNPs in the EGL. Treatment of GNPs with soluble Notch ligand Jagged1, or overexpression of activated Notch2 or its downstream target HES1, maintains precursor proliferation. The addition of GNP mitogens Jagged1 or Sonic Hedgehog (Shh) upregulates the expression of HES1, suggesting a role for HES1 in maintaining precursor proliferation.