The Drosophila smoothened gene encodes a seven-pass membrane protein, a putative receptor for the hedgehog signal

Genomic characterization of Wilms' tumor suppressor 1 targets in nephron progenitor cells during kidney development

The Wilms' tumor suppressor 1 (WT1) gene encodes a DNA- and RNA-binding protein that plays an essential role in nephron progenitor differentiation during renal development. To identify WT1 target genes that might regulate nephron progenitor differentiation in vivo, we performed chromatin immunoprecipitation (ChIP) coupled to mouse promoter microarray (ChIP-chip) using chromatin prepared from embryonic mouse kidney tissue. We identified 1663 genes bound by WT1, 86% of which contain a previously identified, conserved, high-affinity WT1 binding site. To investigate functional interactions between WT1 and candidate target genes in nephron progenitors, we used a novel, modified WT1 morpholino loss-of-function model in embryonic mouse kidney explants to knock down WT1 expression in nephron progenitors ex vivo. Low doses of WT1 morpholino resulted in reduced WT1 target gene expression specifically in nephron progenitors, whereas high doses of WT1 morpholino arrested kidney explant development and were associated with increased nephron progenitor cell apoptosis, reminiscent of the phenotype observed in Wt1(-/-) embryos. Collectively, our results provide a comprehensive description of endogenous WT1 target genes in nephron progenitor cells in vivo, as well as insights into the transcriptional signaling networks controlled by WT1 that might direct nephron progenitor fate during renal development.

Wnt won the war: antagonistic role of Wnt over Shh controls dorso-ventral patterning of the vertebrate neural tube

The spinal cord has been used as a model to dissect the mechanisms that govern the patterning of tissues during animal development, since the principles that rule the dorso-ventral patterning of the neural tube are applicable to other systems. Signals that determine the dorso-ventral axis of the spinal cord include Sonic hedgehog (Shh), acting as a bona fide morphogenetic signal to determine ventral progenitor identities, and members of the Bmp and the Wnt families, acting in the dorsal neural tube. Although Wnts have been initially recognized as important in proliferation of neural progenitor cells, their role in the dorso-ventral patterning has been controversial. In this review, we discuss recent reports that show an important contribution of the Wnt canonical pathway in dorso-ventral pattern formation. These data allow building a model by which the ventralizing activity of Shh is antagonized by Wnt activity through the expression of Gli3, a potent inhibitor of the Shh pathway. Therefore, antagonistic interactions between canonical Wnt, promoting dorsal identities, and Shh pathways, inducing ventral ones, would define the dorso-ventral patterning of the developing central nervous system.

Evaluating Smoothened as a G-protein-coupled receptor for Hedgehog signalling

The Hedgehog signalling pathway controls numerous developmental processes. In response to Hedgehog, Smoothened (Smo), a seven-pass transmembrane protein, orchestrates pathway signalling and controls transcription factor activation. In the absence of Hedgehog, the receptor Patched indirectly inhibits Smo in a catalytic manner. Many questions surrounding Smo activation and signalling remain. Recent findings in Drosophila and vertebrate systems have provided strong evidence that Smo acts as a G-protein-coupled receptor. We discuss the role and regulation of Smo and reassess similarities between Smo and G-protein-coupled receptors. We also examine recently identified members of the invertebrate and vertebrate Smo signalling cascades that are typical components of G-protein-coupled receptor pathways. Greater understanding of the mechanisms of Smo activation and its signalling pathways will allow implementation of novel strategies to target disorders related to disruption of Hh signalling.

A quantification of pathway components supports a novel model of Hedgehog signal transduction

The secreted protein Hedgehog (Hh) plays a critical instructional role during metazoan development. In Drosophila, Hh signaling is interpreted by a set of conserved, downstream effectors that differentially localize and interact to regulate the stability and activity of the transcription factor Cubitus interruptus. Two essential models that integrate genetic, cell biological, and biochemical information have been proposed to explain how these signaling components relate to one another within the cellular context. As the molar ratios of the signaling effectors required in each of these models are quite different, quantitating the cellular ratio of pathway components could distinguish these two models. Here, we address this important question using a set of purified protein standards to perform a quantitative analysis of Drosophila cell lysates for each downstream pathway component. We determine each component's steady-state concentration within a given cell, demonstrate the molar ratio of Hh signaling effectors differs more than two orders of magnitude and that this ratio is conserved in vivo. We find that the G-protein-coupled transmembrane protein Smoothened, an activating component, is present in limiting amounts, while a negative pathway regulator, Suppressor of Fused, is present in vast molar excess. Interestingly, despite large differences in the steady-state ratio, all downstream signaling components exist in an equimolar membrane-associated complex. We use these quantitative results to re-evaluate the current models of Hh signaling and now propose a novel model of signaling that accounts for the stoichiometric differences observed between various Hh pathway components.

Crystal structure of the frizzled-like cysteine-rich domain of the receptor tyrosine kinase MuSK

Muscle-specific kinase (MuSK) is an essential receptor tyrosine kinase for the establishment and maintenance of the neuromuscular junction (NMJ). Activation of MuSK by agrin, a neuronally derived heparan-sulfate proteoglycan, and LRP4 (low-density lipoprotein receptor-related protein-4), the agrin receptor, leads to clustering of acetylcholine receptors on the postsynaptic side of the NMJ. The ectodomain of MuSK comprises three immunoglobulin-like domains and a cysteine-rich domain (Fz-CRD) related to those in Frizzled proteins, the receptors for Wnts. Here, we report the crystal structure of the MuSK Fz-CRD at 2.1 A resolution. The structure reveals a five-disulfide-bridged domain similar to CRDs of Frizzled proteins but with a divergent C-terminal region. An asymmetric dimer present in the crystal structure implicates surface hydrophobic residues that may function in homotypic or heterotypic interactions to mediate co-clustering of MuSK, rapsyn, and acetylcholine receptors at the NMJ.

Distinct effects of Hedgehog signaling on neuronal fate specification and cell cycle progression in the embryonic mouse retina

Cell-extrinsic signals can profoundly influence the production of various neurons from common progenitors. Yet mechanisms by which extrinsic signals coordinate progenitor cell proliferation, cell cycle exit, and cell fate choices are not well understood. Here, we address whether Hedgehog (Hh) signals independently regulate progenitor proliferation and neuronal fate decisions in the embryonic mouse retina. Conditional ablation of the essential Hh signaling component Smoothened (Smo) in proliferating progenitors, rather than in nascent postmitotic neurons, leads to a dramatic increase of retinal ganglion cells (RGCs) and a mild increase of cone photoreceptor precursors without significantly affecting other early-born neuronal cell types. In addition, Smo-deficient progenitors exhibit aberrant expression of cell cycle regulators and delayed G(1)/S transition, especially during the late embryonic stages, resulting in a reduced progenitor pool by birth. Deficiency in Smo function also causes reduced expression of the basic helix-loop-helix transcription repressor Hes1 and preferential elevation of the proneural gene Math5. In Smo and Math5 double knock-out mutants, the enhanced RGC production observed in Smo-deficient retinas is abolished, whereas defects in the G(1)/S transition persist, suggesting that Math5 mediates the Hh effect on neuronal fate specification but not on cell proliferation. These findings demonstrate that Hh signals regulate progenitor pool expansion primarily by promoting cell cycle progression and influence cell cycle exit and neuronal fates by controlling specific proneural genes. Together, these distinct cellular effects of Hh signaling in neural progenitor cells coordinate a balanced production of diverse neuronal cell types.

olig1 Expression identifies developing oligodendrocytes in zebrafish and requires hedgehog and notch signaling

Myelin, the isolating sheath around large diameter axons, is formed in the central nervous system (CNS) by oligodendrocytes. We isolated the zebrafish ortholog of olig1, a bHLH transcription factor, and describe the origin and development of oligodendrocytes in the zebrafish brain. Olig1:mem-eGFP transgenic animals demonstrate the highly dynamic nature of oligodendrocyte membrane processes, providing a tool for studying in vivo oligodendrocyte development. Formation of oligodendrocytes and initiation of olig1 expression are under the control of long-range hedgehog and notch signaling while maintenance of olig1 expression only depends on hedgehog. Over-expression of olig1 did not affect myelin formation in the brain and combined over-expression of olig1 and olig2 could not rescue loss of hedgehog signaling, indicating that critical factors other than olig1 and olig2 are necessary. Lastly, knockdown of Olig1 in an Olig2-sensitized background did result in defects in CNS myelination, indicating a functional overlap between Olig1 and Olig2 proteins.

Sonic hedgehog mediates BDNF-induced neuroprotection against mitochondrial inhibitor 3-nitropropionic acid

Sonic hedgehog (SHH), a morphogen critical for embryogenesis, has also been shown to be neuroprotective. We have recently reported that pretreatment of rat cortical neurons for 8 h with brain-derived neurotrophic factor (BDNF; 100 ng/ml) affords protection against neurotoxicity of 3-nitropropionic acid (3-NP; 2.5 mM for 24 h), a mitochondrial complex II inhibitor. However, whether SHH is involved in BDNF-mediated neuroprotection remains unknown. Herein we tested whether BDNF induces SHH expression and if so, whether BDNF induction of SHH contributes to the observed neuroprotective effects. We found BDNF (100 ng/ml) increased SHH expression at both mRNA and protein levels. BDNF protection against 3-NP was abolished by cyclopamine (CPM; 5 microM), the SHH pathway inhibitor. Preconditioning of cortical neurons with N-terminal fragment of SHH (SHH-N; 0.1-1 ng/ml) was sufficient to confer resistance. These results indicate that BDNF induces SHH expression, which contributes to neuroprotection against 3-NP toxicity in rat cortical neurons.

Tow (Target of Wingless), a novel repressor of the Hedgehog pathway in Drosophila

Hedgehog (Hh) signalling plays a crucial role in the development and patterning of many tissues in both vertebrates and invertebrates. Aberrations in this pathway lead to severe developmental defects and cancer. Hh signal transduction in receiving cells is a well studied phenomenon; however questions still remain concerning the mechanism of repression of the pathway activator Smoothened (Smo) in the absence of Hh. Here we describe a novel repressor of the Hh pathway, Target of Wingless (Tow). Tow represents the Drosophila homolog of a conserved uncharacterised protein family. We show that Tow acts in Hh receiving cells, where its overexpression represses all levels of Hh signalling, and that this repression occurs upstream or at the level of Smo and downstream of the Hh receptor Patched (Ptc). In addition, we find that like Ptc, overexpression of Tow causes an accumulation of lipophorin in the wing disc. We demonstrate that loss of tow enhances different ptc alleles in a similar manner to another pathway repressor, Suppressor of Fused (SuFu), possibly through mediating Ptc dependant lipophorin internalisation. Combined, these results demonstrate that Tow is an important novel regulator of the Hh pathway in the wing imaginal disc, and may shed light on the mechanism of Ptc repression of Smo.

G protein Galphai functions immediately downstream of Smoothened in Hedgehog signalling

The hedgehog (Hh) signalling pathway has an evolutionarily conserved role in patterning fields of cells during metazoan development, and is inappropriately activated in cancer. Hh pathway activity is absolutely dependent on signalling by the seven-transmembrane protein smoothened (Smo), which is regulated by the Hh receptor patched (Ptc). Smo signals to an intracellular multi-protein complex containing the Kinesin related protein Costal2 (Cos2), the protein kinase Fused (Fu) and the transcription factor Cubitus interruptus (Ci). In the absence of Hh, this complex regulates the cleavage of full-length Ci to a truncated repressor protein, Ci75, in a process that is dependent on the proteasome and priming phosphorylations by Protein kinase A (PKA). Binding of Hh to Ptc blocks Ptc-mediated Smo inhibition, allowing Smo to signal to the intracellular components to attenuate Ci cleavage. Because of its homology with the Frizzled family of G-protein-coupled receptors (GPCR), a likely candidate for an immediate Smo effector would be a heterotrimeric G protein. However, the role that G proteins may have in Hh signal transduction is unclear and quite controversial, which has led to widespread speculation that Smo signals through a variety of novel G-protein-independent mechanisms. Here we present in vitro and in vivo evidence in Drosophila that Smo activates a G protein to modulate intracellular cyclic AMP levels in response to Hh. Our results demonstrate that Smo functions as a canonical GPCR, which signals through Galphai to regulate Hh pathway activation.

Hedgehog: functions and mechanisms

The Hedgehog (Hh) family of proteins control cell growth, survival, and fate, and pattern almost every aspect of the vertebrate body plan. The use of a single morphogen for such a wide variety of functions is possible because cellular responses to Hh depend on the type of responding cell, the dose of Hh received, and the time cells are exposed to Hh. The Hh gradient is shaped by several proteins that are specifically required for Hh processing, secretion, and transport through tissues. The mechanism of cellular response, in turn, incorporates multiple feedback loops that fine-tune the level of signal sensed by the responding cells. Germline mutations that subtly affect Hh pathway activity are associated with developmental disorders, whereas somatic mutations activating the pathway have been linked to multiple forms of human cancer. This review focuses broadly on our current understanding of Hh signaling, from mechanisms of action to cellular and developmental functions. In addition, we review the role of Hh in the pathogenesis of human disease and the possibilities for therapeutic intervention.

Ventral specification and perturbed boundary formation in the mouse midbrain in the absence of Hedgehog signaling

Although Hedgehog (HH) signaling plays a critical role in patterning the ventral midbrain, its role in early midbrain specification is not known. We examined the midbrains of sonic hedgehog (Shh) and smoothened (Smo) mutant mice where HH signaling is respectively attenuated and eliminated. We show that some ventral (Evx1+) cell fates are specified in the Shh-/- mouse in a Ptc1- and Gli1-independent manner. HH-independent ventral midbrain induction was further confirmed by the presence of a Pax7-negative ventral midbrain territory in both Shh-/- and Smo-/- mice at and before embryonic day (E) 8.5. Midbrain signaling centers are severely disrupted in the Shh-/- mutant. Interestingly, dorsal markers are up-regulated (Wnt1, Gdf7, Pax7), down-regulated (Lfng), or otherwise altered (Zic1) in the Shh-/- midbrain. Together with the increased cell death seen specifically in Shh-/- dorsal midbrains (E8.5-E9), our results suggest specific regulation of dorsal patterning by SHH, rather than a simple deregulation due to its absence.

Pattern formation in the vertebrate neural tube: a sonic hedgehog morphogen-regulated transcriptional network

Neuronal subtype specification in the vertebrate neural tube is one of the best-studied examples of embryonic pattern formation. Distinct neuronal subtypes are generated in a precise spatial order from progenitor cells according to their location along the anterior-posterior and dorsal-ventral axes. Underpinning this organization is a complex network of multiple extrinsic and intrinsic factors. This review focuses on the molecular mechanisms and general strategies at play in ventral regions of the forming spinal cord, where sonic hedgehog-based morphogen signaling is a key determinant. We discuss recent advances in our understanding of these events and highlight unresolved questions.

Insulin receptor substrate 1 is an effector of sonic hedgehog mitogenic signaling in cerebellar neural precursors

Sonic hedgehog (SHH) and insulin-like growth factor (IGF) signaling are essential for development of many tissues and are implicated in medulloblastoma, the most common solid pediatric malignancy. Cerebellar granule neuron precursors (CGNPs), proposed cells-of-origin for specific classes of medulloblastomas, require SHH and IGF signaling for proliferation and survival during development of the cerebellum. We asked whether SHH regulates IGF pathway components in proliferating CGNPs. We report that SHH-treated CGNPs showed increased levels of insulin receptor substrate 1 (IRS1) protein, which was also present in the germinal layer of the developing mouse cerebellum and in mouse SHH-induced medulloblastomas. Previous roles for IRS1, an oncogenic protein that is essential for IGF-mediated proliferation in other cell types, have not been described in SHH-mediated CGNP proliferation. We found that IRS1 overexpression can maintain CGNP proliferation in the absence of SHH. Furthermore, lentivirus-mediated knock down experiments have shown that IRS1 activity is required for CGNP proliferation in slice explants and dissociated cultures. Contrary to traditional models for SHH signaling that focus on gene transcription, SHH stimulation does not regulate Irs1 transcription but rather stabilizes IRS1 protein by interfering with mTOR-dependent IRS1 turnover and possibly affects Irs1 mRNA translation. Thus, we have identified IRS1 as a novel effector of SHH mitogenic signaling that may serve as a future target for medulloblastoma therapies. Our findings also indicate a previously unreported interaction between the SHH and mTOR pathways, and provide an example of a non-classical means for SHH-mediated protein regulation during development.

Costal2 functions as a kinesin-like protein in the hedgehog signal transduction pathway

The Hedgehog (Hh) signaling pathway initiates an evolutionarily conserved developmental program required for the proper patterning of many tissues [1]. Although Costal2 (Cos2) is a requisite component of the Hh pathway, its mechanistic role is not well understood. Because of its primary sequence, Cos2 was initially predicted to function as a kinesin-like protein [2]. However, evidence showing that Cos2 function might require kinesin-like properties has been lacking [2-6]. Thus, the prevailing dogma in the field is that Cos2 functions solely as a scaffolding protein [7, 8]. Here, we show that Cos2 motility is required for its biological function and that this motility may be Hh regulated. We show that Cos2 motility requires an active motor domain, ATP, and microtubules. Additionally, Cos2 recruits and transports other components of the Hh signaling pathway, including the transcription factor Cubitus interruptus (Ci). Drosophila expressing cos2 mutations that encode proteins that lack motility are attenuated in their ability to regulate Ci activity and exhibit phenotypes consistent with attenuated Cos2 function [9]. Combined, these results demonstrate that Cos2 motility plays an important role in its function, regulating the amounts and activity of Ci that ultimately interpret the level of Hh to which cells are exposed.

Indian hedgehog signaling from endothelial cells is required for sclera and retinal pigment epithelium development in the mouse eye

The development of extraocular orbital structures, in particular the choroid and sclera, is regulated by a complex series of interactions between neuroectoderm, neural crest and mesoderm derivatives, although in many instances the signals that mediate these interactions are not known. In this study we have investigated the function of Indian hedgehog (Ihh) in the developing mammalian eye. We show that Ihh is expressed in a population of non-pigmented cells located in the developing choroid adjacent to the RPE. The analysis of Hh mutant mice demonstrates that the RPE and developing scleral mesenchyme are direct targets of Ihh signaling and that Ihh is required for the normal pigmentation pattern of the RPE and the condensation of mesenchymal cells to form the sclera. Our findings also indicate that Ihh signals indirectly to promote proliferation and photoreceptor specification in the neural retina. This study identifies Ihh as a novel choroid-derived signal that regulates RPE, sclera and neural retina development.

Hedgehog and Dpp signaling induce cadherin Cad86C expression in the morphogenetic furrow during Drosophila eye development

During Drosophila eye development, cell differentiation is preceded by the formation of a morphogenetic furrow, which progresses across the epithelium from posterior to anterior. Cells within the morphogenetic furrow are apically constricted and shortened along their apical-basal axis. However, how these cell shape changes and, thus, the progression of the morphogenetic furrow are controlled is not well understood. Here we show that cells simultaneously lacking Hedgehog and Dpp signal transduction fail to shorten and do not enter the morphogenetic furrow. Moreover, we have identified a gene, cadherin Cad86C, which is highly expressed in cells of the leading flank of the morphogenetic furrow. Ectopic activation of either the Hedgehog or Dpp signal transduction pathway results in elevated Cad86C expression. Conversely, simultaneous loss of both Hedgehog and Dpp signal transduction leads to decreased Cad86C expression. Finally, ectopic expression of Cad86C in either eye-antennal imaginal discs or wing imaginal discs results in apical constriction and shortening of cells. We conclude that Hedgehog and Dpp signaling promote the shortening of cells within the morphogenetic furrow. Induction of Cad86C expression might be one mechanism through which Hedgehog and Dpp promote these cell shape changes.

Hedgehog signaling pathway function conserved in Tribolium segmentation

In Drosophila, maintenance of parasegmental boundaries and formation of segmental grooves depend on interactions between segment polarity genes. Wingless and Engrailed appear to have similar roles in both short and long germ segmentation, but relatively little is known about the extent to which Hedgehog signaling is conserved. In a companion study to the Tribolium genome project, we analyzed the expression and function of hedgehog, smoothened, patched, and cubitus interruptus orthologs during segmentation in Tribolium. Their expression was largely conserved between Drosophila and Tribolium. Parental RNAi analysis of positive regulators of the pathway (Tc-hh, Tc-smo, or Tc-ci) resulted in small spherical cuticles with little or no evidence of segmental grooves. Segmental Engrailed expression in these embryos was initiated but not maintained. Wingless-independent Engrailed expression in the CNS was maintained and became highly compacted during germ band retraction, providing evidence that derivatives from every segment were present in these small spherical embryos. On the other hand, RNAi analysis of a negative regulator (Tc-ptc) resulted in embryos with ectopic segmental grooves visible during germband elongation but not discernible in the first instar larval cuticles. These transient grooves formed adjacent to Engrailed expressing cells that encircled wider than normal wg domains in the Tc-ptc RNAi embryos. These results suggest that the en–wg–hh gene circuit is functionally conserved in the maintenance of segmental boundaries during germ band retraction and groove formation in Tribolium and that the segment polarity genes form a robust genetic regulatory module in the segmentation of this short germ insect.

Hh and Wnt signaling regulate formation of olig2+ neurons in the zebrafish cerebellum

The cerebellum, which forms from anterior hindbrain, coordinates motor movements and balance. Sensory input from the periphery is relayed and modulated by cerebellar interneurons, which are organized in layers. The mechanisms that specify the different neurons of the cerebellum and direct its layered organization remain poorly understood. Drawing from investigations of spinal cord, we hypothesized that the embryonic cerebellum is patterned on the dorsoventral axis by opposing morphogens. We tested this using zebrafish. Here we show that expression of olig2, which encodes a bHLH transcription factor, marks a distinct subset of neurons with similarities to eurydendroid neurons, the principal efferent neurons of the teleost cerebellum. In combination with other markers, olig2 reveals a dorsoventral organization of cerebellar neurons in embryos. Disruption of Hedgehog signaling, which patterns the ventral neural tube, produced a two-fold increase in the number of olig2(+) neurons. By contrast, olig2(+) neurons did not develop in embryos deficient for Wnt signaling, which patterns dorsal neural tube, nor did they develop in embryos deficient for both Hedgehog and Wnt signaling. Our data indicate that Hedgehog and Wnt work in opposition across the dorsoventral axis of the cerebellum to regulate formation of olig2(+) neurons. Specifically, we propose that Hedgehog limits the range of Wnt signaling, which is necessary for olig2(+) neuron development.

Proliferative and cell fate effects of Hedgehog signaling in the vertebrate retina

The retina is an excellent system for delving into the question of how cell fate, number and organization are regulated in the central nervous system. Multipotential progenitor cells in the immature retina proliferate, exit the cell cycle and generate neurons and one glial cell type in a prescribed temporal sequence. While some aspects of progenitor behavior are controlled cell intrinsically, extrinsic signals present in the retina environment have been shown to impact on proliferation, differentiation and cell fate of progenitors. Intercellular signaling proteins of the Hedgehog (Hh) family regulate several aspects of visual system development in vertebrates--ranging from early eye field patterning to retinal and optic nerve development. This review highlights the role of Hh signaling on retinal progenitor proliferation and diversification.

Upregulation of Indian hedgehog gene in the uterine epithelium by leukemia inhibitory factor during mouse implantation

Leukemia inhibitory factor (LIF) and Indian hedgehog (Ihh) are essential for embryo implantation in mice and are regulated by the actions of 17beta-estradiol (E2) and progesterone, respectively. The present study examined the effect of LIF on Ihh and Ihh-related factors in the uterine luminal epithelium during the implantation period using a DNA microarray. Expression of Ihh mRNA reached its peak on the forth day of pregnancy, and progesterone receptor (Pgr) mRNA decreased on the fifth day of pregnancy in wildtype mice. On the other hand, these changes in expression were not seen in LIF-/- mice. Ihh and Pgr mRNA were upregulated by LIF injection in delayed implantation mice. This up-regulation of Pgr was transient and preceded an increase of Ihh mRNA. Ihh mRNA also increased after E2 injection in delayed implantation mice of the LIF-/- genotype. E2 did not affect transcription of Pgr mRNA in the uterine luminal epithelium of delayed implantation LIF-/- mice. Using an antibody against the C-terminal epitope of Ihh, unprocessed Ihh proteins, but not C-terminal peptides, by autoproteolytic cleavage of Ihh were detected by western blot analysis. Unprocessed Ihh did not show quantitative changes between the wildtype and LIF-/- mice during the implantation period. Transcription of hedgehog acyltransferase was not influenced by LIF and E2 injection. In conclusion, LIF, which has a crucial role in E2 action for initiation of implantation, caused transient induction of Pgr mRNA and subsequent upregulation of Ihh mRNA, which mediates progesterone-Pgr actions for successful implantation.

Hedgehog regulates smoothened activity by inducing a conformational switch

Hedgehog (HH) morphogen is essential for metazoan development. The seven-transmembrane protein smoothened (SMO) transduces the HH signal across the plasma membrane, but how SMO is activated remains poorly understood. In Drosophila melanogaster, HH induces phosphorylation at multiple Ser/Thr residues in the SMO carboxy-terminal cytoplasmic tail, leading to its cell surface accumulation and activation. Here we provide evidence that phosphorylation activates SMO by inducing a conformational switch. This occurs by antagonizing multiple Arg clusters in the SMO cytoplasmic tail. The Arg clusters inhibit SMO by blocking its cell surface expression and keeping it in an inactive conformation that is maintained by intramolecular electrostatic interactions. HH-induced phosphorylation disrupts the interaction, and induces a conformational switch and dimerization of SMO cytoplasmic tails, which is essential for pathway activation. Increasing the number of mutations in the Arg clusters progressively activates SMO. Hence, by employing multiple Arg clusters as inhibitory elements counteracted by differential phosphorylation, SMO acts as a rheostat to translate graded HH signals into distinct responses.

Commercially available recombinant sonic hedgehog up-regulates Ptc and modulates the cytokine and chemokine expression of human macrophages: an effect mediated by endotoxin contamination?

The Sonic hedgehog (Shh) signalling pathway plays an important role in developmental patterning and proliferation. Recent evidence suggests that Shh also plays a role in the development of the immune system. Here, we demonstrate that components of the Shh signalling pathway are expressed in human macrophages and that the receptor for Shh, Ptc, is up-regulated by a commercially available recombinant preparation of Shh (CArShh). Further, we report that the addition of CArShh up-regulates the production of IL-6, IL-8, MCP-1, IP-10, MIG and RANTES by macrophages, an effect enhanced by the presence of fetal calf serum in the culture medium. In contrast, TGF-beta, TNF-alpha, IL-1b, IL-12 and IL-10 production were not modulated by CArShh and VEGF was minimally up-regulated even in the presence of serum. The up-regulation of these cytokines and chemokines was abrogated by CD14 inhibition and polymixin B, but not reliably inhibited by the specific Shh pathway inhibitor cyclopamine. These results suggest that, although components of the Shh signalling pathway are expressed in macrophages, the modulation of macrophage cytokine and chemokine effector function seen in response to commercially available rShh results from low levels of endotoxin contained within the CArShh preparations employed to explore the effects of Shh in vitro.

Sonic hedgehog induces transcription-independent cytoskeletal rearrangement and migration regulated by arachidonate metabolites

Sonic hedgehog (Shh) is a morphogen pivotal for development and tissue maintenance. Biological effects of Shh are mediated through a pathway that involves binding to patched1 (Ptch1), thereby releasing Smoothened (Smo) from inhibition resulting in the activation of Gli transcription factors, which mediate the induction of Shh target genes. Here, we describe a novel signal transduction pathway for Shh, which is transcription/translation-independent, SuFu insensitive, and consequently independent of Gli-mediated induction of transcription. Through this alternative pathway Shh, transduced via Smo, induced altered cell morphology together with lamellipodia formation. Migration assays demonstrate that this cytoskeletal rearrangement mediates the migratory response to Shh. This Shh-induced, Smo mediated migration utilizes and requires the metabolism of arachidonic acid through the 5-lipoxygenase pathway. These data provide a link between a seemingly novel Gli-independent Hh signaling pathway and the leukotriene metabolism, and might explain the developmental abnormalities observed in both patients with defective leukotriene metabolism as well as in rodent models of defective Rho family GTPase signaling.

Interaction of L-Arginine-methyl ester and Sonic hedgehog in liver ischemia-reperfusion injury in the rats

AIM: To investigate the role of Sonic hedgehog (Shh) on the course of liver ischemia and reperfusion (I/R) in rats, and the interaction between treatment with nitric oxide donor L-Arginine-methyl ester (L-Arg) and up-regulation of Shh expression.

METHODS: A total of 30 male Sprague-Dawley rats weighing 220-240 g were used in this study. Sham-control group (G1, n = 10): a sham operation was performed (except for liver I/R). I/R-untreated group (G2, n = 10): rats underwent liver ischemia for 1 h followed by reperfusion for 45 min. I/R-L-Arg group (G3, n = 10): after performing the same surgical procedure as in group 2, animals were treated with L-Arg. Liver tissues were taken for determination of malondialdehyde (MDA) levels, and biochemical and histological evaluations were made.

RESULTS: Plasma alanine aminotransferase (ALT), aspartate aminotransferase (AST), lactate dehydrogenase (LDH) and γ-glutamyltranspeptidase (GGT) activities were higher in group 2 than in group 3. MDA values and the hepatic injury score decreased in the L-Arg treated group compared to the I/R-untreated group. In group 2, the hepatocytes were swollen with marked vacuolization. Group 3 rats showed well-preserved liver parenchyma, with hepatocytes extending from the central vein. The morphology of the hepatocytes and the sinusoidal structures was normal, without any signs of congestion. Mild Shh positive immunostaining was detected in group 2 animals. The expression of immunoreactive cells was increased markedly in liver tissue from I/R-L-Arg rats.

CONCLUSION: Our findings suggest that Shh molecules are critical factors in the pathophysiology of inflammatory liver injury induced by I/R. In addition, NO plays an important role in the immunohistochemical expression of these molecules.

Regulation of ventral midbrain patterning by Hedgehog signaling

In the developing ventral midbrain, the signaling molecule sonic hedgehog(SHH) is sufficient to specify a striped pattern of cell fates (midbrain arcs). Here, we asked whether and precisely how hedgehog (HH) signaling might be necessary for ventral midbrain patterning. By blocking HH signaling by in ovo misexpression of Ptc1Δloop2,we show that HH signaling is necessary and can act directly at a distance to specify midbrain cell fates. Ventral midbrain progenitors extinguish their dependence upon HH in a spatiotemporally complex manner, completing cell-fate specification at the periphery by Hamburger and Hamilton stage 13. Thus,patterning at the lateral periphery of the ventral midbrain is accomplished early, when the midbrain is small and the HH signal needs to travel relatively short distances (approximately 30 cell diameters). Interestingly, single-cell injections demonstrate that patterning in the midbrain occurs within the context of cortex-like radial columns of cells that can share HH blockade and are cytoplasmically connected by gap junctions. HH blockade results in increased cell scatter, disrupting the spatial coherence of the midbrain arc pattern. Finally, HH signaling is required for the integrity and the signaling properties of the boundaries of the midbrain (e.g. the midbrain-hindbrain boundary, the dorsoventral boundary), its perturbations resulting in abnormal cell mixing across `leaky' borders.

Tamoxifen-inducible gene deletion reveals a distinct cell type associated with trabecular bone, and direct regulation of PTHrP expression and chondrocyte morphology by Ihh in growth region cartilage

Indian hedgehog (Ihh) controls multiple aspects of endochondral skeletal development by signaling to both chondrocytes and perichondrial cells. Previous efforts to delineate direct effects of Ihh on chondrocytes by Col2-Cre-mediated ablation of Smoothened (Smo, encoding a transmembrane protein indispensable for Ihh signaling) has been only partially successful, due to the inability to discriminate between chondrocytes and perichondrial cells. Here we report a transgenic line (Col2-Cre) expressing under the control of the Colalpha1(II) promoter an inert form of Cre that is activatable by exogenous tamoxifen (TM); TM administration at proper times during embryogenesis induced Cre activity in chondrocytes but not in the perichondrium. By using this mouse line, we deleted Smo within subsets of chondrocytes without affecting the perichondrium and found that Smo removal led to localized disruption of the expression of parathyroid hormone-related protein (PTHrP) and the morphology of chondrocytes. Unexpectedly, TM invariably induced Cre activity in a subset of cells associated with the trabecular bone surface of long bones. These cells, when genetically marked and cultured in vitro, were capable of producing bone nodules. Expression of the Col2-Cre transgene in these cells likely reflected the endogenous Colalpha1(II) promoter activity as similar cells were found to express the IIA isoform of Colalpha1(II) mRNA endogenously. In summary, the present study has not only provided evidence that Ihh signaling directly controls PTHrP expression and chondrocyte morphology in the growth region cartilage, but has also uncovered a distinct cell type associated with the trabecular bone that appears to possess osteogenic potential.

Pathways of signal transduction employed by vertebrate Hedgehogs

Signalling by Hh (Hedgehog) proteins is among the most actively studied receptor-mediated phenomena relevant to development and post-embryonic homoeostatic events. The impact of signalling by the Hh proteins is profound, and work pertaining to the presentation of these proteins and the pathways engaged by them continues to yield unique insights into basic aspects of morphogenic signalling. We review here the mechanisms of signalling relevant to the actions of Hh proteins in vertebrates. We emphasize findings within the past several years on the recognition of, in particular, Sonic hedgehog by target cells, pathways of transduction employed by the seven-pass transmembrane protein Smoothened and end points of action, as manifest in the regulation of the Gli transcription factors. Topics of extended interest are those regarding the employment of heterotrimeric G-proteins and G-protein-coupled receptor kinases by Smoothened. We also address the pathways, insofar as known, linking Smoothened to the expression and stability of Gli1, Gli2 and Gli3. The mechanisms by which Hh proteins signal have few, if any, parallels. It is becoming clear in vertebrates, however, that several facets of signalling are shared in common with other venues of signalling. The challenge in understanding both the actions of Hh proteins and the overlapping forms of regulation will be in understanding, in molecular terms, both common and divergent signalling events.

Orchestrating ontogenesis: variations on a theme by sonic hedgehog

Embryonic development is an emergent process in which increasing complexity is generated by sequential cellular interactions. Recently, it has become clear that such interactions are mediated by just a few families of signalling molecules; but how does this limited repertoire elicit the diversity of form that is characteristic of multicellular organisms? Here we review the various ways in which a member of one such family, the sonic hedgehog (SHH) protein, is deployed during embryonic development. These examples of SHH function provide paradigms for inductive interactions that should help to inform attempts to recapitulate cellular programming and organogenesis in vitro.

Molecular mechanisms involved in progesterone receptor regulation of uterine function

The ovarian steroid hormone progesterone is a major regulator of uterine function. The actions of this hormone is mediated through its cognate receptor, the progesterone receptor, Pgr. Ablation of the Pgr has shown that this receptor is critical for all female reproductive functions including the ability of the uterus to support and maintain the development of the implanting mouse embryo. High density DNA microarray analysis has identified direct and indirect targets of Pgr action. One of the targets of Pgr action is a member of the Hedgehog morphogen Indian Hedgehog, Ihh. Ihh and members of the Hh signaling cascade show a coordinate expression pattern in the mouse uterus during the preimplantation period of pregnancy. The expression of Ihh and its receptor Patched-1, Ptc1, as well as, down stream targets of Ihh-Ptch1 signaling, such as the orphan nuclear receptor COUP-TF II show that this morphogen pathway mediates communication between the uterine epithelial and stromal compartments. The members of the Ihh signaling axis may function to coordinate the proliferation, vascularization and differentiation of the uterine stroma during pregnancy. This analysis demonstrates that progesterone regulates uterine function in the mouse by coordinating the signals from the uterine epithelium to stroma in the preimplantation mouse uterus.

Uncovering the uncharacterized and unexpected: unbiased phenotype-driven screens in the mouse

Phenotype-based chemical mutagenesis screens for mouse mutations have undergone a transformation in the past five years from a potential approach to a practical tool. This change has been driven by the relative ease of identifying causative mutations now that the complete genome sequence is available. These unbiased screens make it possible to identify genes, gene functions and processes that are uniquely important to mammals. In addition, because chemical mutagenesis generally induces point mutations, these alleles often uncover previously unappreciated functions of known proteins. Here we provide examples of the success stories from forward genetic screens, emphasizing the examples that illustrate the discovery of mammalian-specific processes that could not be discovered in other model organisms. As the efficiency of sequencing and mutation detection continues to improve, it is likely that forward genetic screens will provide an even more important part of the repertoire of mouse genetics in the future.

The last 59 amino acids of Smoothened cytoplasmic tail directly bind the protein kinase Fused and negatively regulate the Hedgehog pathway

The Hedgehog (HH) signaling pathway is crucial for the development of many organisms and its inappropriate activation is involved in numerous cancers. HH signal controls the traffic and activity of the seven-pass transmembrane protein Smoothened (SMO), leading to the transcriptional regulation of HH-responsive genes. In Drosophila, the intracellular transduction events following SMO activation depend on cytoplasmic multimeric complexes that include the Fused (FU) protein kinase. Here we show that the regulatory domain of FU physically interacts with the last 52 amino acids of SMO and that the two proteins colocalize in vivo to vesicles. The deletion of this region of SMO leads to a constitutive activation of SMO, promoting the ectopic transcription of HH target genes. This activation is partially dependent of FU activity. Thus, we identify a novel link between SMO and the cytoplasmic complex(es) and reveal a negative role of the SMO C-terminal region that interacts with FU. We propose that FU could act as a switch, activator in presence of HH signal or inhibitor in absence of HH.

The C-terminal tail of the Hedgehog receptor Patched regulates both localization and turnover

Patched (Ptc) is a membrane protein whose function in Hedgehog (Hh) signal transduction has been conserved among metazoans and whose malfunction has been implicated in human cancers. Genetic analysis has shown that Ptc negatively regulates Hh signal transduction, but its activity and structure are not known. We investigated the functional and structural properties of Drosophila Ptc and its C-terminal domain (CTD), 183 residues that are predicted to reside in the cytoplasm. Our results show that Ptc, as well as truncated Ptc deleted of its CTD, forms a stable trimer. This observation is consistent with the proposal that Ptc is structurally similar to trimeric transporters. The CTD itself trimerizes and is required for both Ptc internalization and turnover. Two mutant forms of the CTD, one that disrupts trimerization and the other that mutates the target sequence of the Nedd4 ubiquitin ligase, stabilize Ptc but do not prevent internalization and sequestration of Hh. Ptc deleted of its CTD is stable and localizes to the plasma membrane. These data show that degradation of Ptc is regulated at a step subsequent to endocytosis, although endocytosis is a likely prerequisite. We also show that the CTD of mouse Ptc regulates turnover.

Activation of heterotrimeric G proteins by Smoothened

The mechanisms by which the activation of Smoothened (Smo), a protein essential to the actions of the Hedgehog family of secreted proteins, is translated into signals that converge on the Gli transcription factors are not fully understood. The seven-transmembrane structure of Smo has long implied the utilization of heterotrimeric GTP-binding regulatory proteins (G proteins); however, evidence in this regard has been indirect and contradictory. In the current study we evaluated the capacity of mammalian Smo to couple to G proteins directly. We found that Smo, by virtue of what appears to be constitutive activity, activates all members of the G(i) family but does not activate members of the G(s), G(q), and G(12) families. The activation is suppressed by cyclopamine and other inhibitors of Hedgehog signaling and is enhanced by the Smo agonist purmorphamine. Activation of G(i) by Smo is essential in the activation of Gli in fibroblasts, because disruption of coupling to G(i) with pertussis toxin inhibits the activation of Gli by Sonic hedgehog and a constitutively active form of Smo (SmoM2). However, G(i) does not provide a sufficient signal because a truncated form of Smo, although capable of activating G(i), does not effect activation of Gli. Rescue of pertussis toxin-inhibited activation of Gli by Sonic hedgehog can be achieved with a constitutively active Galpha(i)-subunit. The data suggest that Smo is in fact the source of two signals relevant to the activation of Gli: one involving G(i) and the other involving events at Smo's C-tail independent of G(i).

Roadkill attenuates Hedgehog responses through degradation of Cubitus interruptus

The final step in Hedgehog (Hh) signal transduction is post-translational regulation of the transcription factor, Cubitus interruptus (Ci). Ci resides in the cytoplasm in a latent form, where Hh regulates its processing into a transcriptional repressor or its nuclear access as a transcriptional activator. Levels of latent Ci are controlled by degradation, with different pathways activated in response to different levels of Hh. Here, we describe the roadkill (rdx) gene, which is expressed in response to Hh. The Rdx protein belongs to a conserved family of proteins that serve as substrate adaptors for Cullin3-mediated ubiquitylation. Overexpression of rdx reduced Ci levels and decreased both transcriptional activation and repression mediated by Ci. Loss of rdx allowed excessive accumulation of Ci. rdx manipulation in the eye revealed a novel role for Hh in the organization and survival of pigment and cone cells. These studies identify rdx as a limiting factor in a feedback loop that attenuates Hh responses through reducing levels of Ci. The existence of human orthologs for Rdx raises the possibility that this novel feedback loop also modulates Hh responses in humans.

Independent regulation of skeletal growth by Ihh and IGF signaling

The insulin-like growth factors (IGFs) play a major role in regulating the systemic growth of mammals. However, it is unclear to what extent their systemic and/or local functions act in concert with other local growth factors controlling the sizes of individual organs. We have specifically addressed whether growth control of the skeleton by IGFs interacts genetically with that by Indian hedgehog (Ihh), a locally produced growth signal for the endochondral skeleton. Here, we report that disruption of both IGF and Ihh signaling resulted in additive reduction in the size of the embryonic skeleton. Thus, IGF and Ihh signaling appear to control the growth of the skeleton in parallel pathways.

The ihog cell-surface proteins bind Hedgehog and mediate pathway activation

The ihog gene (interference hedgehog), identified by RNA interference in Drosophila cultured cells, encodes a type 1 membrane protein shown here to bind and to mediate response to the active Hedgehog (Hh) protein signal. ihog mutations produce defects characteristic of Hh signaling loss in embryos and imaginal discs, and epistasis analysis places ihog action at or upstream of the negatively acting receptor component, Patched (Ptc). The first of two extracellular fibronectin type III (FNIII) domains of the Ihog protein mediates a specific interaction with Hh protein in vitro, but the second FNIII domain is additionally required for in vivo signaling activity and for Ihog-enhanced binding of Hh protein to cells coexpressing Ptc. Other members of the Ihog family, including Drosophila Boi and mammalian CDO and BOC, also interact with Hh ligands via a specific FNIII domain, thus identifying an evolutionarily conserved family of membrane proteins that function in Hh signal response.

Signaling from Smo to Ci/Gli: conservation and divergence of Hedgehog pathways from Drosophila to vertebrates

Although the framework of the Hedgehog (Hh) signaling pathway is evolutionarily conserved, recent studies indicate that fundamental differences exist between Drosophila and vertebrates in the way signals are transduced from the membrane protein Smoothened (Smo) to the Ci/Gli transcription factors. For example, Smo structure and the roles of fused and Suppressor of fused have diverged. Recently, many vertebrate-specific components have been identified that act between Smo and Gli. These include intra-flagellar transport proteins, which link vertebrate Hh signaling to cilia. Because abnormal Hh signaling can cause birth defects and cancer, these vertebrate-specific components may have roles in human health.

Left-right asymmetry in the vertebrate embryo: from early information to higher-level integration

Although vertebrates seem to be essentially bilaterally symmetrical on the exterior, there are numerous interior left-right asymmetries in the disposition and placement of internal organs. These asymmetries are established during embryogenesis by complex epigenetic and genetic cascades. Recent studies in a range of model organisms have made important progress in understanding how this laterality information is generated and conveyed to large regions of the embryo. Both commonalities and divergences are emerging in the mechanisms that different vertebrates use in left-right axis specification. Recent evidence also provides intriguing links between the establishment of left-right asymmetries and the symmetrical elongation of the anterior-posterior axis.

Regulation of Ci and Su(fu) nuclear import in Drosophila

The Hedgehog (Hh) signal transduction pathway plays a central role in the development of invertebrates and vertebrates. While much is known about the pathway, the role of Suppressor of fused (Su(fu)), a component of the pathway's signaling complex has remained enigmatic. Previous studies have linked Su(fu) to the cytoplasmic sequestration of the zinc finger transcription factor, Cubitus interruptus (Ci), while other studies suggest a role in modulating target gene expression. In examining the cell biology of the pathway, we have found that like its vertebrate homologue, Drosophila Su(fu) enters the nucleus. Furthermore, we find that the nuclear import of Su(fu) occurs in concert with that of Ci in response to Hh signaling. Here, we examine the mechanism by which Su(fu) regulates Ci import by investigating the importance of the Ci nuclear localization signal (NLS) and the effect of adding an additional NLS. Finally, we demonstrate that Ci can bring Su(fu) with it to a multimerized Ci DNA binding site. These results provide a basis for understanding the dual roles played by Su(fu) in the regulation of Ci.

Mouse Rab23 regulates Hedgehog signaling from Smoothened to Gli proteins

Sonic hedgehog (Shh) signaling is required for the growth and patterning of many tissues in vertebrate embryos, but important aspects of the Shh signal transduction pathway are poorly understood. For example, the vesicle transport protein Rab23 is a cell autonomous negative regulator of Shh signaling, but the process affected by Rab23 has not been defined. Here, we demonstrate that Rab23 acts upstream of Gli transcription factors in patterning neural cell types in the spinal cord. Double mutant analysis indicates that the primary target of Rab23 is the Gli2 activator and that Rab23 and Gli3 repressor have additive effects on patterning. Analysis of Gli3 protein suggests that Rab23 also has a role in promoting the production of Gli3 repressor. Although the membrane proteins Patched and Smoothened change subcellular localization in response to Shh, double mutant analysis demonstrates that Rab23 does not work through either Patched or Smoothened. Instead, Rab23 appears to regulate subcellular localization of essential components of the Hedgehog pathway that act downstream of Smoothened and upstream of Gli proteins.

The highly ordered assembly of retinal axons and their synaptic partners is regulated by Hedgehog/Single-minded in the Drosophila visual system

During development of the Drosophila visual center, photoreceptor cells extend their axons (R axons) to the lamina ganglion layer, and trigger proliferation and differentiation of synaptic partners (lamina neurons) by delivering the inductive signal Hedgehog (Hh). This inductive mechanism helps to establish an orderly arrangement of connections between the R axons and lamina neurons, termed a retinotopic map because it results in positioning the lamina neurons in close vicinity to the corresponding R axons. We found that the bHLH-PAS transcription factor Single-minded (Sim) is induced by Hh in the lamina neurons and is required for the association of lamina neurons with R axons. In sim mutant brains, lamina neurons undergo the first step of differentiation but fail to associate with R axons. As a result, lamina neurons are set aside from R axons. The data reveal a novel mechanism for regulation of the interaction between axons and neuronal cell bodies that establishes precise neuronal networks.

Regulation of the Drosophila transcription factor, Cubitus interruptus, by two conserved domains

Hedgehog signaling is required for the development of many organisms, including Drosophila. In flies, Hh patterns the embryonic epidermis and larval imaginal discs by regulating the transcription factor, Cubitus interruptus (Ci). To date, three levels of regulation have been identified: proteolytic processing into a repressor, nuclear import, and activation. In this report, we characterize the function of two Ci domains that are conserved in the vertebrate homologues, GLI1, GLI2, and GLI3. One domain includes the first two of five C(2)-H(2) zinc-fingers. While conserved in all members of the GLI/Ci family, the first two fingers do not appear to make significant contacts with the DNA target sequence. Ci protein lacking this region is still able to interact with the cytoplasmic complex and activate transcription in embryos and wing imaginal discs, but it is no longer processed into the repressor form. The second domain, termed NR for "N-terminal Regulatory", binds Suppressor of Fused. Deletion of this region has little effect on embryonic patterning, but compromises cytoplasmic retention of Ci. Analysis of the amino acid sequence of this domain identifies 11 perfectly conserved serines and one tyrosine. We propose that this region may be modified, possibly by phosphorylation, to regulate Ci nuclear import.

Sonic hedgehog expression in the development of hindgut in ETU-exposed fetal rats

Sonic hedgehog (Shh) has been shown to be involved in the morphogenesis of many organ systems including the notochord, floor plate and limbs, as well as in the development of the left-right axis in vertebrates. Recent evidence suggests the Shh cascade plays a crucial role in the development of the foregut and hindgut. We have previously shown that prenatal exposure of fetal rats to ethylenethiourea (ETU) induces hindgut malformations and other abnormalities of the VACTERL association. The aim of this study was to determine the pattern of expression of Shh and its downstream genes during hindgut development in ETU-exposed embryos with anorectal malformations (ARMs). Pregnant Sprague-Dawley rats were mated together overnight and a positive vaginal plug was marked as gD0. On gD10, 1% ETU (125 mg/kg) was given to the experimental group and controls received the same volume of saline. Embryos were collected from both groups at gD12-16. The developing hindgut of each embryo was dissected under magnification and snap frozen. Highly purified RNA was isolated from each hindgut and first strand cDNA was prepared with appropriate negative controls. Reverse transcriptase (RT) polymerase chain reaction (PCR) was done to determine the transcripts of Shh in each sample and quantitative real-time PCR was carried out to show relative quantitative expression of Shh at each time point. Shh was detected in all samples confirming that Shh is active during the process of hindgut development in fetal rats. Relative quantitation demonstrated that Shh expression shows time-dependent changes in the developing hindgut of ETU-exposed rat embryos, and when results were compared with control samples, there was significant decrease in expression on gD14 and 15, when the cloaca normally separates into the rectum and urethra occurs in the rat fetus. The misregulated expression of Shh in the hindgut of ETU-exposed rat embryos suggests that ETU may interfere with Shh signalling. Downregulation at the time of cloacal separation into rectum and urethra indicates that Shh plays a crucial role in the development of hindgut.

Cardiac and CNS defects in a mouse with targeted disruption of suppressor of fused

The hedgehog (Hh) pathway is conserved from Drosophila to humans and plays a key role in embryonic development. In addition, activation of the pathway in somatic cells contributes to cancer development in several tissues. Suppressor of fused is a negative regulator of Hh signaling. Targeted disruption of the murine suppressor of fused gene (Sufu) led to a phenotype that included neural tube defects and lethality at mid-gestation (9.0-10.5 dpc). This phenotype resembled that caused by loss of patched (Ptch1), another negative regulator of the Hh pathway. Consistent with this finding, Ptch1 and Sufu mutants displayed excess Hh signaling and resultant altered dorsoventral patterning of the neural tube. Sufu mutants also had abnormal cardiac looping, indicating a defect in the determination of left-right asymmetry. Marked expansion of nodal expression in 7.5 dpc embryos and variable degrees of node dysmorphology in 7.75 dpc embryos suggested that the pathogenesis of the cardiac developmental abnormalities was related to node development. Other mutants of the Hh pathway, such as Shh, Smo and Shh/Ihh compound mutants, also have laterality defects. In contrast to Ptch1 heterozygous mice, Sufu heterozygotes had no developmental defects and no apparent tumor predisposition. The resemblance of Sufu homozygotes to Ptch1 homozygotes is consistent with mouse Sufu being a conserved negative modulator of Hh signaling.

Dysregulation of the Hedgehog pathway in human hepatocarcinogenesis

Hedgehog (Hh) pathway activation promotes tumors in several endodermally derived tissues, but its role in the pathogenesis of hepatocellular carcinoma (HCC) is unknown. Although normal hepatocytes lack Hh signaling, activation of the Hh pathway in endodermal progenitors is required for liver development. Thus, we hypothesized that hepatocarcinogenesis may involve regulation of Hh signaling. This pathway is activated when Hh ligand binds to its receptor, Patched (PTC). In an unoccupied state, PTC normally functions as a tumor suppressor that inhibits Smoothened (SMO), a proto-oncoprotein, from activating downstream components and transcription of target genes. Here we show that in HCCs, overexpression of the Smo proto-oncogene, as well as an increase in the stoichiometric ratio of Smo to Ptc mRNA levels, correlated with tumor size, a prognostic indicator in HCC biology. In one tumor we identified a novel Smo mutation in an evolutionarily conserved residue. We also demonstrated that HCC cell lines (HepG2 and Hep3B) expressed Hh pathway components and activated Hh transcriptional targets. In Hep3B cells, cyclopamine, an inhibitor of wild-type SMO, had no effect, but KAAD-cyclopamine, a blocker of oncogenic SMO, inhibited Hh signaling activity by 50%, decreased expression of the hepatocarcinogenic oncogene, c-myc, by 8-fold, and inhibited the growth rate of Hep3B cells by 94%. These data support our hypothesis that Hh signaling is dysregulated in human hepatocarcinogenesis. We demonstrate that overexpression and/or tumorigenic activation of the Smo proto-oncogene mediates c-myc overexpression which plays a critical role in hepatocarcinogenesis and suggests that Smo is a prognostic factor in HCC tumorigenesis.