Skinny hedgehog, an acyltransferase required for palmitoylation and activity of the hedgehog signal

Hedgehog on the move: a precise spatial control of Hedgehog dispersion shapes the gradient

Hedgehog (Hh) as morphogen directs cell differentiation during development activating various target genes in a concentration dependent manner. The mechanisms that permit controlled Hh dispersion and gradient formation remain controversial. New research in the Drosophila wing disc epithelium has revealed a crucial role of Hh recycling for its release and transportation from source cells. Lipid modifications on Hh mediate key interactions with different elements of the pathway, which balance the retention and release of the molecule through the basolateral side of the epithelium, allowing its tight spatial control. Dispersion of Hh is also determined by its hydrophobic nature, and the mechanisms that include membrane-tethered transport of Hh are increasingly proposed.

Absolute requirement of cholesterol binding for Hedgehog gradient formation in Drosophila

How morphogen gradients are shaped is a major question in developmental biology, but remains poorly understood. Hedgehog (Hh) is a locally secreted ligand that reaches cells at a distance and acts as a morphogen to pattern the Drosophila wing and the vertebrate neural tube. The proper patterning of both structures relies on the precise control over the slope of Hh activity gradient. A number of hypotheses have been proposed to explain Hh movement and hence graded activity of Hh. A crux to all these models is that the covalent binding of cholesterol to Hh N-terminus is essential to achieve the correct slope of the activity gradient. Still, the behavior of cholesterol-free Hh (Hh-N) remains controversial: cholesterol has been shown to either increase or restrict Hh range depending on the experimental setting. Here, in fly embryos and wing imaginal discs, we show that cholesterol-free Hh diffuses at a long-range. This unrestricted diffusion of cholesterol-free Hh leads to an absence of gradient while Hh signaling strength remains uncompromised. These data support a model where cholesterol addition restricts Hh diffusion and can transform a leveled signaling activity into a gradient. In addition, our data indicate that the receptor Patched is not able to sequester cholesterol-free Hh. We propose that a morphogen gradient does not necessarily stem from the active transfer of a poorly diffusing molecule, but can be achieved by the restriction of a highly diffusible ligand.

Smoothened antagonists: a promising new class of antitumor agents

BACKGROUND

Hedgehog signaling is essential for the development of most metazoans. In recent years, evidence has accumulated showing that many human tumors aberrantly re-activate this developmental signaling pathway and that interfering with it may provide a new strategy for the development of novel anti-cancer therapeutics. Smoothened is a G-protein coupled receptor-like protein that is essentially involved in hedgehog signal transduction and small molecule antagonists of Smoothened have started to show antitumor activity in preclinical models and in clinical trials.

OBJECTIVE

We critically review the role of hedgehog signaling in normal development and in human malignancies, the available drug discovery tools and the classes of small molecule inhibitors that are in development. We further aim to address the potential impact that pathway antagonists may have on the treatment options of cancer patients.

METHODS

Literature, patents and clinical trial results from the past 5 years were analyzed.

CONCLUSIONS

1) A large body of evidence suggests a frequent reactivation of hedgehog signaling in human cancer. 2) Smoothened is an attractive, highly druggable target with extensive preclinical and initial clinical validation in basal cell carcinoma. Several promising novel classes of Smoothened antagonists have been discovered and are being developed as anticancer agents. 3) Our knowledge of the biology of hedgehog signaling in cancer is still very incomplete and significant efforts will be required to understand how to use the emerging novel agents in the clinic.

Whole-genome-wide association study in the Bulgarian population reveals HHAT as schizophrenia susceptibility gene

OBJECTIVE

Schizophrenia, the most common major psychiatric disorder (or group of disorders), entails severe decline of higher functions, principally with alterations in cognitive functioning and reality perception. Both genetic and environmental factors are involved in its pathogenesis; however, its genetic background still needs to be clarified. The objective of the study was to reveal genetic markers associated with schizophrenia in the Bulgarian population.

METHODS

We have conducted a genome-wide association study using 554 496 single nucleotide polymorphisms (SNPs) in 188 affected and 376 unaffected Bulgarian individuals. Subsequently, the 100 candidate SNPs that revealed the smallest P-values were further evaluated in an additional set of 99 case and 328 control samples.

RESULTS

We found a significant association between schizophrenia and the intronic SNP rs7527939 in the HHAT gene (P-value of 6.49×10 with an odds ratio of 2.63, 95% confidence interval of 1.89-3.66). We also genotyped additional SNPs within a 58-kb linkage disequilibrium block surrounding the landmark SNP.

CONCLUSION

We suggest rs7527939 to be the strongest indicator of susceptibility to schizophrenia in the Bulgarian population within the HHAT locus.

How is digit identity determined during limb development?

Digit identity has been studied using the chick embryo as a model system for more than 40 years. Using this model system, several milestone findings have been reported, such as the apical ectodermal ridge (AER), the zone of polarizing activity (ZPA), the Shh gene, and the theory of morphogen and positional information. These experimental results and models provided context for understanding pattern formation in developmental biology. The focus of this review is on the determination of digit identity during limb development. First, the history of studies on digit identity determination is described, followed by descriptions of the molecular mechanisms and current models for determination of digit identity. Finally, future questions and remarkable points will be discussed.

An emerging role of Sonic hedgehog shedding as a modulator of heparan sulfate interactions

Major developmental morphogens of the Hedgehog (Hh) family act at short range and long range to direct cell fate decisions in vertebrate and invertebrate tissues. To this end, Hhs are released from local sources and act at a distance on target cells that express the Hh receptor Patched. However, morphogen secretion and spreading are not passive processes because all Hhs are synthesized as dually (N- and C-terminal) lipidated proteins that firmly tether to the surface of producing cells. On the cell surface, Hhs associate with each other and with heparan sulfate (HS) proteoglycans. This raises the question of how Hh solubilization and spreading is achieved. We recently discovered that Sonic hedgehog (Shh) is solubilized by proteolytic processing (shedding) of lipidated peptide termini in vitro. Because unprocessed N termini block Patched receptor binding sites in the cluster, we further suggested that their proteolytic removal is required for simultaneous Shh activation. In this work we confirm inactivity of unprocessed protein clusters and demonstrate restored biological Shh function upon distortion or removal of N-terminal amino acids and peptides. We further show that N-terminal Shh processing targets and inactivates the HS binding Cardin-Weintraub (CW) motif, resulting in soluble Shh clusters with their HS binding capacities strongly reduced. This may explain the ability of Shh to diffuse through the HS-containing extracellular matrix, whereas other HS-binding proteins are quickly immobilized. Our in vitro findings are supported by the presence of CW-processed Shh in murine brain samples, providing the first in vivo evidence for Shh shedding and subsequent solubilization of N-terminal-truncated proteins.

Identification of N-terminal residues of Sonic Hedgehog important for palmitoylation by Hedgehog acyltransferase

Sonic Hedgehog (Shh) is a secreted morphogen that regulates embryonic development. After removal of the signal peptide, Shh is processed to the mature, active form through autocleavage and a series of lipid modifications, including the attachment of palmitate. Covalent attachment of palmitate to the N-terminal cysteine of Shh is catalyzed by Hedgehog acyltransferase (Hhat) and is critical for proper signaling. The sequences within Shh that are responsible for palmitoylation by Hhat are not known. Here we show that the first six amino acids of mature Shh (CGPGRG) are sufficient for Hhat-mediated palmitoylation. Alanine scanning mutagenesis was used to determine the role of each amino acid and the positional sequence requirement in a cell-based Shh palmitoylation assay. Mutation of residues in the GPGR sequence to Ala had no effect on palmitoylation, provided that a positively charged residue was present within the first seven residues. The N-terminal position exhibited a strong but not exclusive requirement for Cys. Constructs with an N-terminal Ala were not palmitoylated. However, an N-terminal Ser served as a substrate for Hhat, but not the Drosophila melanogaster ortholog Rasp, highlighting a critical difference between the mammalian and fly enzymes. These findings define residues and regions within Shh that are necessary for its recognition as a substrate for Hhat-mediated palmitoylation. Finally, we report the results of a bioinformatics screen to identify other potential Hhat substrates encoded in the human genome.

Mutations in Hedgehog acyltransferase (Hhat) perturb Hedgehog signaling, resulting in severe acrania-holoprosencephaly-agnathia craniofacial defects

Holoprosencephaly (HPE) is a failure of the forebrain to bifurcate and is the most common structural malformation of the embryonic brain. Mutations in SHH underlie most familial (17%) cases of HPE; and, consistent with this, Shh is expressed in midline embryonic cells and tissues and their derivatives that are affected in HPE. It has long been recognized that a graded series of facial anomalies occurs within the clinical spectrum of HPE, as HPE is often found in patients together with other malformations such as acrania, anencephaly, and agnathia. However, it is not known if these phenotypes arise through a common etiology and pathogenesis. Here we demonstrate for the first time using mouse models that Hedgehog acyltransferase (Hhat) loss-of-function leads to holoprosencephaly together with acrania and agnathia, which mimics the severe condition observed in humans. Hhat is required for post-translational palmitoylation of Hedgehog (Hh) proteins; and, in the absence of Hhat, Hh secretion from producing cells is diminished. We show through downregulation of the Hh receptor Ptch1 that loss of Hhat perturbs long-range Hh signaling, which in turn disrupts Fgf, Bmp and Erk signaling. Collectively, this leads to abnormal patterning and extensive apoptosis within the craniofacial primordial, together with defects in cartilage and bone differentiation. Therefore our work shows that Hhat loss-of-function underscrores HPE; but more importantly it provides a mechanism for the co-occurrence of acrania, holoprosencephaly, and agnathia. Future genetic studies should include HHAT as a potential candidate in the etiology and pathogenesis of HPE and its associated disorders.

Craniofacial anomalies account for approximately one third of all birth defects, and holoprosencephaly (HPE) is the most common structural malformation of the embryonic brain. HPE is a failure of the forebrain to bifurcate and is a heterogeneous disorder that is often found in patients together with other craniofacial malformations. Currently, it is not known if these phenotypes arise through a common etiology and pathogenesis, as the genetic lesions responsible for HPE have only been identified in about 20% of affected individuals. Here we demonstrate for the first time that Hedgehog acyltransferase (Hhat) loss-of-function leads to holoprosencephaly together with acrania and agnathia, which highlights the importance of Hh signaling in complex craniofacial disorders.

Planar polarized protrusions break the symmetry of EGFR signaling during Drosophila bract cell fate induction

Secreted signaling molecules typically float in the outer leaflet of the plasma membrane or freely diffuse away from the signaling cell, suggesting that a signal should be sensed equally by all neighboring cells. However, we demonstrate that Spitz (Spi)-mediated epidermal growth factor receptor (EGFR) signaling is spatially biased to selectively determine the induction of a single bract cell on the proximal side of each mechanosensory organ on the Drosophila leg. Dynamic and oriented cellular protrusions emanating from the socket cell, the source of Spi, robustly favor the Spi/EGFR signaling response in a particular cell among equally competent neighbors. We propose that these protrusive structures enhance signaling by increasing contact between the signaling and responding cells. The planar polarized direction of the protrusions determines the direction of the signaling outcome. This asymmetric cell signaling serves as a developmental mechanism to generate spatially patterned cell fates.

Dispatched and scube mediate the efficient secretion of the cholesterol-modified hedgehog ligand

The Hedgehog (Hh) signaling pathway plays critical roles in metazoan development and in cancer. How the Hh ligand is secreted and spreads to distant cells is unclear, given its covalent modification with a hydrophobic cholesterol molecule, which makes it stick to membranes. We demonstrate that Hh ligand secretion from vertebrate cells is accomplished via two distinct and synergistic cholesterol-dependent binding events, mediated by two proteins that are essential for vertebrate Hh signaling: the membrane protein Dispatched (Disp) and a member of the Scube family of secreted proteins. Cholesterol modification is sufficient for a heterologous protein to interact with Scube and to be secreted in a Scube-dependent manner. Disp and Scube recognize different structural aspects of cholesterol similarly to how Niemann-Pick disease proteins 1 and 2 interact with cholesterol, suggesting a hand-off mechanism for transferring Hh from Disp to Scube. Thus, Disp and Scube cooperate to dramatically enhance the secretion and solubility of the cholesterol-modified Hh ligand.

Hedgehog signaling

The original hedgehog (hh) gene was found in Drosophila and named for the appearance of a mutant phenotype which causes an embryo to be covered with pointy denticles, thus resembling a hedgehog. The hedgehog family consists of sonic hedgehog (Shh), desert hedgehog (Dhh), and Indian hedgehog (Ihh). Shh is found in vertebrates and the best studied ligand of the hedgehog signaling pathway (Gilbert, 2000). It plays an important role in regulating vertebrate organogenesis, such as in the growth of digits on limbs and organization of the brain, and earlier studies also show that it is important during retinal development (for a review, see Amato et al., 2004). Hedgehog expression drives waves of neurogenesis in animal retina, although genetic programs that control its expression are poorly elucidated. Recently, a novel transcriptional cascade which involves the atonal and Iroquois gene family was proposed in the regulation of hedgehog waves during vertebrate retinal development (Choy et al., 2010). This chapter will focus on Shh by addressing its signaling mechanisms and roles in vertebrate eye development, as well as a novel finding in retinogenesis.

Palmitoylation of Hedgehog proteins

Hedgehog (Hh) proteins are secreted signaling proteins that contain amide-linked palmitate at the N-terminus and cholesterol at the C-terminus. Palmitoylation of Hh proteins is critical for effective long- and short-range signaling. The palmitoylation reaction occurs during transit of Hh through the secretory pathway, most likely in the lumen of the ER. Attachment of palmitate to Hh proteins is independent of cholesterol modification and autoprocessing and is catalyzed by Hhat (Hedgehog acyltransferase). Hhat is a member of the membrane bound O-acyltransferase (MBOAT) family, a subgroup of multipass membrane proteins that catalyze transfer of fatty acyl groups to lipids and proteins. Several classes of secreted proteins have recently been shown to be substrates for MBOAT acyltransferases, including Hh proteins and Spitz (palmitoylated by Hhat), Wg/Wnt proteins (modified with palmitate and/or palmitoleate by Porcupine) and ghrelin (octanoylated by ghrelin O-acyltransferase). These findings highlight protein fatty acylation as a mechanism that not only influences membrane binding of intracellular proteins but also regulates the signaling range and efficacy of secreted proteins.

The hedgehog processing pathway is required for NSCLC growth and survival

Considerable interest has been generated from the results of recent clinical trials using smoothened (SMO) antagonists to inhibit the growth of hedgehog (HH) signaling-dependent tumors. This interest is tempered by the discovery of SMO mutations mediating resistance, underscoring the rationale for developing therapeutic strategies that interrupt HH signaling at levels distinct from those inhibiting SMO function. Here, we demonstrate that HH-dependent non-small cell lung carcinoma (NSCLC) growth is sensitive to blockade of the HH pathway upstream of SMO, at the level of HH ligand processing. Individually, the use of different lentivirally delivered shRNA constructs targeting two functionally distinct HH-processing proteins, skinny hedgehog (SKN) or dispatched-1 (DISP-1), in NSCLC cell lines produced similar decreases in cell proliferation and increased cell death. Further, providing either an exogenous source of processed HH or a SMO agonist reverses these effects. The attenuation of HH processing, by knocking down either of these gene products, also abrogated tumor growth in mouse xenografts. Finally, we extended these findings to primary clinical specimens, showing that SKN is frequently overexpressed in NSCLC and that higher DISP-1 expression is associated with an unfavorable clinical outcome. Our results show a critical role for HH processing in HH-dependent tumors, identifies two potential druggable targets in the HH pathway, and suggest that similar therapeutic strategies could be explored to treat patients harboring HH ligand-dependent cancers.

Acyl-CoA:diacylglycerol acyltransferase: molecular biology, biochemistry and biotechnology

Triacylglycerol (TG) is a storage lipid which serves as an energy reservoir and a source of signalling molecules and substrates for membrane biogenesis. TG is essential for many physiological processes and its metabolism is widely conserved in nature. Acyl-CoA:diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) catalyzes the final step in the sn-glycerol-3-phosphate pathway leading to TG. DGAT activity resides mainly in two distinct membrane bound polypeptides, known as DGAT1 and DGAT2 which have been identified in numerous organisms. In addition, a few other enzymes also hold DGAT activity, including the DGAT-related acyl-CoA:monoacylglycerol acyltransferases (MGAT). Progress on understanding structure/function in DGATs has been limited by the lack of detailed three-dimensional structural information due to the hydrophobic properties of theses enzymes and difficulties associated with purification. This review examines several aspects of DGAT and MGAT genes and enzymes, including current knowledge on their gene structure, expression pattern, biochemical properties, membrane topology, functional motifs and subcellular localization. Recent progress in probing structural and functional aspects of DGAT1 and DGAT2, using a combination of molecular and biochemical techniques, is emphasized. Biotechnological applications involving DGAT enzymes ranging from obesity therapeutics to oilseed engineering are also discussed.

Scube/You activity mediates release of dually lipid-modified Hedgehog signal in soluble form

Owing to their covalent modification by cholesterol and palmitate, Hedgehog (Hh) signaling proteins are localized predominantly to the plasma membrane of expressing cells. Yet Hh proteins are also capable of mobilizing to and eliciting direct responses from distant cells. The zebrafish you gene, identified genetically >15 years ago, was more recently shown to encode a secreted glycoprotein that acts cell-nonautonomously in the Hh signaling pathway by an unknown mechanism. We investigated the function of the protein encoded by murine Scube2, an ortholog of you, and found that it mediates release in soluble form of the mature, cholesterol- and palmitate-modified Sonic hedgehog protein signal (ShhNp) when added to cultured cells or purified detergent-resistant membrane microdomains containing ShhNp. The efficiency of Scube2-mediated release of ShhNp is enhanced by the palmitate adduct of ShhNp and by coexpression in ShhNp-producing cells of mDispatchedA (mDispA), a transporter-like protein with a previously defined role in the release of lipid-modified Hh signals. The structural determinants of Scube2 required for its activity in cultured cell assays match those required for rescue of you mutant zebrafish embryos, and we thus conclude that the role of Scube/You proteins in Hh signaling in vivo is to facilitate the release and mobilization of Hh proteins for distant action.

Hedgehog secretion and signal transduction in vertebrates

Signaling by the Hedgehog (Hh) family of secreted proteins is essential for proper embryonic patterning and development. Dysregulation of Hh signaling is associated with a variety of human diseases ranging from developmental disorders such as holoprosencephaly to certain forms of cancer, including medulloblastoma and basal cell carcinoma. Genetic studies in flies and mice have shaped our understanding of Hh signaling and revealed that nearly all core components of the pathway are highly conserved. Although many aspects of the Drosophila Hh pathway are conserved in vertebrates, mechanistic differences between the two species have begun to emerge. Perhaps the most striking divergence in vertebrate Hh signaling is its dependence on the primary cilium, a vestigial organelle that is largely absent in flies. This minireview will provide an overview of Hh signaling and present recent insights into vertebrate Hh secretion, receptor binding, and signal transduction.

A genome-wide RNAi screen identifies regulators of cholesterol-modified hedgehog secretion in Drosophila

Hedgehog (Hh) proteins are secreted molecules that function as organizers in animal development. In addition to being palmitoylated, Hh is the only metazoan protein known to possess a covalently-linked cholesterol moiety. The absence of either modification severely disrupts the organization of numerous tissues during development. It is currently not known how lipid-modified Hh is secreted and released from producing cells. We have performed a genome-wide RNAi screen in Drosophila melanogaster cells to identify regulators of Hh secretion. We found that cholesterol-modified Hh secretion is strongly dependent on coat protein complex I (COPI) but not COPII vesicles, suggesting that cholesterol modification alters the movement of Hh through the early secretory pathway. We provide evidence that both proteolysis and cholesterol modification are necessary for the efficient trafficking of Hh through the ER and Golgi. Finally, we identified several putative regulators of protein secretion and demonstrate a role for some of these genes in Hh and Wingless (Wg) morphogen secretion in vivo. These data open new perspectives for studying how morphogen secretion is regulated, as well as provide insight into regulation of lipid-modified protein secretion.

Release and transportation of Hedgehog molecules

Secretion of the Hedgehog morphogen induces different cell fates over the short and long ranges during developmental patterning. Mature Hedgehog carries hydrophobic palmitic acid and cholesterol modifications essential for its correct spread. The long-range activity of Hedgehog raises questions about how a dually lipidated protein can spread in the hydrophilic environment of the extracellular space. There is compelling experimental evidence in favour of the existence of several different carriers for Hedgehog transportation, via very different routes. This suggests that different accessory proteins and cellular machineries may be involved in the specific release of Hedgehog. I suggest that Hh carriers may work in parallel within a given cell and that developmental context may condition the choice of Hh carrier in secreting cells.

Targeting protein lipidation in disease

Fatty acids and/or isoprenoids are covalently attached to a variety of disease-related proteins. The distinct chemical properties of each of these hydrophobic moieties allow lipid modification to serve as a mechanism to regulate protein structure, localization and function. This review highlights recent progress in identifying inhibitors of protein lipidation and their effects on human disease. Myristoylation inhibitors have shown promise in blocking the action of human pathogens. Although inhibitors that block prenylation of Ras proteins have not yet been successful for cancer treatment, they may be efficacious in the rare premature aging syndrome progeria. Agents that alter the palmitoylation status of Ras, Wnt and Hh proteins have recently been discovered, and represent the next generation of potential chemotherapeutics.

Ghrelin O-acyltransferase assays and inhibition

Ghrelin O-acyltransferase (GOAT) is responsible for catalyzing the attachment of the eight-carbon fatty acid octanoyl to the Ser3 side chain of the peptide ghrelin to generate the active form of this metabolic hormone. As such, GOAT is viewed as a potential therapeutic target for the treatment of obesity and diabetes mellitus. Here, we review recent progress in the development of cell and in vitro assays to measure GOAT action and the identification of several synthetic GOAT inhibitors. In particular, we discuss the design, synthesis, and characterization of the bisubstrate analog GO-CoA-Tat and its ability to modulate weight and blood glucose in mice. We also highlight current challenges and future research directions in our biomedical understanding of this fascinating ghrelin processing enzyme.

Regulation of developmental intercellular signalling by intracellular trafficking

Universal trafficking components within the cell can be recruited to coordinate and regulate the developmental signalling cascades. We will present ways in which the intracellular trafficking machinery is used to affect and modulate the outcome of signal transduction in developmental contexts, thus regulating multicellular development. Each of the signalling components must reach its proper intracellular destination, in a form that is properly folded and modified. In many instances, the ability to bring components together or segregate them into distinct compartments within the cell actually provides the switch mechanism to turn developmental signalling pathways on or off. The review will begin with a focus on the signal-sending cells, and the ways in which ligand trafficking can impinge on the signalling outcome, via processing, endocytosis and recycling. We will then turn to the signal-receiving cell, and discuss mechanisms by which endocytosis can affect the spatial features of the signal, and the compartmentalization of components downstream to the receptor.

Sonic hedgehog shedding results in functional activation of the solubilized protein

All Hedgehog (Hh) proteins are released from producing cells despite being synthesized as N- and C-terminally lipidated, membrane-tethered molecules. Thus, a cellular mechanism is needed for Hh solubilization. We previously suggested that a disintegrin and metalloprotease (ADAM)-mediated shedding of Sonic hedgehog (ShhNp) from its lipidated N and C termini results in protein solubilization. This finding, however, seemed at odds with the established role of N-terminal palmitoylation for ShhNp signaling activity. We now resolve this paradox by showing that N-palmitoylation of ShhNp N-terminal peptides is required for their proteolytic removal during solubilization. These peptides otherwise block ShhNp zinc coordination sites required for ShhNp binding to its receptor Patched (Ptc), explaining the essential yet indirect role of N-palmitoylation for ShhNp function. We suggest a functional model in which membrane-tethered multimeric ShhNp is at least partially autoinhibited in trans but is processed into fully active, soluble multimers upon palmitoylation-dependent cleavage of inhibitory N-terminal peptides.

Acyl-CoA:lysophosphatidylcholine acyltransferase I (Lpcat1) catalyzes histone protein O-palmitoylation to regulate mRNA synthesis

The enzyme acyl-CoA:lysophosphatidylcholine acyltransferase (Lpcat1) is a critical cytosolic enzyme needed for lung surfactant synthesis that catalyzes an acyltransferase reaction by adding a palmitate to the sn-2 position of lysophospholipids. Here we report that histone H4 protein is subject to palmitoylation catalyzed by Lpcat1 in a calcium-regulated manner. Cytosolic Lpcat1 was observed to shift into the nucleus in lung epithelia in response to exogenous Ca(2+). Nuclear Lpcat1 colocalizes with and binds to histone H4, where it catalyzes histone H4 palmitoylation. Mutagenesis studies demonstrated that Ser(47) within histone H4 serves as a putative acceptor site, indicative of Lpcat1-mediated O-palmitoylation. Lpcat1 knockdown or expression of a histone H4 Ser(47A) mutant protein in cells decreased cellular mRNA synthesis. These findings provide the first evidence of a protein substrate for Lpcat1 and reveal that histone lipidation may occur through its O-palmitoylation as a novel post-translational modification. This epigenetic modification regulates global gene transcriptional activity.

Lipid modifications of Sonic hedgehog ligand dictate cellular reception and signal response

Background

Sonic hedgehog (Shh) signaling regulates cell growth during embryonic development, tissue homeostasis and tumorigenesis. Concentration-dependent cellular responses to secreted Shh protein are essential for tissue patterning. Shh ligand is covalently modified by two lipid moieties, cholesterol and palmitate, and their hydrophobic properties are known to govern the cellular release and formation of soluble multimeric Shh complexes. However, the influences of the lipid moieties on cellular reception and signal response are not well understood.

Methodology/Principal Findings

We analyzed fully lipidated Shh and mutant forms to eliminate one or both adducts in NIH3T3 mouse embryonic fibroblasts. Quantitative measurements of recombinant Shh protein concentration, cellular localization, and signaling potency were integrated to determine the contributions of each lipid adduct on ligand cellular localization and signaling potency. We demonstrate that lipid modification is required for cell reception, that either adduct is sufficient to confer cellular association, that the cholesterol adduct anchors ligand to the plasma membrane and that the palmitate adduct augments ligand internalization. We further show that signaling potency correlates directly with cellular concentration of Shh ligand.

Conclusions/Significance

The findings of this study demonstrate that lipid modification of Shh determines cell concentration and potency, revealing complementary functions of hydrophobic modification in morphogen signaling by attenuating cellular release and augmenting reception of Shh protein in target tissues.

Mechanisms and functions of Hedgehog signalling across the metazoa

Hedgehog proteins constitute one of a small number of families of secreted signals that have a central role in the development of metazoans. Genetic analyses in flies, fish and mice have uncovered the major components of the pathway that transduces Hedgehog signals, and recent genome sequence projects have provided clues about its evolutionary origins. In this Review we provide an updated overview of the mechanisms and functions of this signalling pathway, highlighting the conserved and divergent features of the pathway, as well as some of the common themes in its deployment that have emerged from recent studies.

Drugging the cancer stem cell compartment: lessons learned from the hedgehog and Wnt signal transduction pathways

Cell-cell communication mediated by the secreted Hedgehog (Hh) and Wnt signaling molecules is essential to the coordination of cell fate decision making throughout the metazoan lifespan. From decades of genetically based interrogation, core components constituting the Hh and Wnt signal transduction pathways have been assembled, and a deep appreciation of how these signals elaborate distinct bodily tissues during development has been established. On the other hand, our incapacity to leverage similar genetic approaches to study adult organ systems has limited our understanding of how these molecules promote tissue renewal and regeneration through stem cell regulation. We discuss recent progress in the use of chemically based approaches to achieve control of these pathway activities in a broad range of biological studies and therapeutic contexts. In particular, we discuss the unique experimental opportunities that chemical modulators of these pathways afford in exploring the cancer stem cell hypothesis.

Molecular cloning and expression analysis of porcine ghrelin o-acyltransferase

The peptide hormone ghrelin is secreted in the stomach, with unique N-octanoylation at serine 3, which is a requirement for its functionality. These functions include growth hormone release, appetite stimulation, gastrointestinal motility, glucose regulation, and cell proliferation. The enzyme responsible for ghrelin acylation was recently identified as ghrelin O-acyltransferase (GOAT). In this study, porcine GOAT was cloned and characterized. A full-length cDNA of GOAT of 2013 bp was obtained, which included a 70-bp 5' UTR, a 635-bp 3' UTR, and a 1308-bp open reading frame encoding a protein of 415 amino acids. The GOAT and ghrelin mRNAs are co-expressed in stomach, pancreas, and duodenum at high levels. GOAT was also detected in liver, lung, brain, testis, spleen, kidney, heart, muscle, lipid, and ovary. Our results provide an important basis for further research on GOAT function and the relationship between ghrelin and GOAT.

Processing and turnover of the Hedgehog protein in the endoplasmic reticulum

Autocatalytic processing of the Hedgehog ligand from its precursor protein relies on protein disulfide isomerase and ER-associated degradation.

The Hedgehog (Hh) signaling pathway has important functions during metazoan development. The Hh ligand is generated from a precursor by self-cleavage, which requires a free cysteine in the C-terminal part of the protein and results in the production of the cholesterol-modified ligand and a C-terminal fragment. In this paper, we demonstrate that these reactions occur in the endoplasmic reticulum (ER). The catalytic cysteine needs to form a disulfide bridge with a conserved cysteine, which is subsequently reduced by protein disulfide isomerase. Generation of the C-terminal fragment is followed by its ER-associated degradation (ERAD), providing the first example of an endogenous luminal ERAD substrate that is constitutively degraded. This process requires the ubiquitin ligase Hrd1, its partner Sel1, the cytosolic adenosine triphosphatase p97, and degradation by the proteasome. Processing-defective mutants of Hh are degraded by the same ERAD components. Thus, processing of the Hh precursor competes with its rapid degradation, explaining the impaired Hh signaling of processing-defective mutants, such as those causing human holoprosencephaly.

Hedgehog signaling in hematopoiesis

The Hedgehog signaling pathway is highly conserved and plays an essential role in the embryonic development of a wide variety of organs. In adult tissues, such as the central nervous system, it may also be required for homeostasis and repair following injury. The role of Hedgehog signaling in regulating hematopoiesis is not entirely clear. Evidence has shown that Hedgehog signaling is required for both primitive hematopoiesis in the developing embryo, as well as for definitive hematopoiesis in the adult. However, several studies also suggest that Hedgehog pathway activity is completely dispensable in postnatal hematopoiesis. In this review, we discuss the current understanding of Hedgehog signaling in vertebrate hematopoiesis, as well as the contradictory findings that have been reported.

Metabolism strikes back: metabolic flux regulates cell signaling

Mammalian cells depend on growth factor signaling to take up nutrients; however, coordination of glucose and glutamine uptake has been a mystery. In this issue of Genes & Development, Wellen and colleagues (pp. 2784-2799) show that glucose flux through the hexosamine biosynthesis pathway regulates growth factor receptor glycosylation and enables glutamine consumption. This mechanism ensures that cells do not engage in anabolic metabolism when nutrients are limiting, and highlights how substrate availability for protein modifications can modulate cell signaling.

Processing-dependent trafficking of Sonic hedgehog to the regulated secretory pathway in neurons

Neurons are an important source of the secreted morphogen Sonic hedgehog (Shh), however, little is known about neuron-specific regulation of Shh transport and secretion. To study this process, we investigated the subcellular distribution of Shh in primary neurons and differentiated cells of a neuroendocrine cell line by fluorescence microscopy and biochemical fractionation. In retinal ganglion cells, endogenous Shh was distributed as intra- and extracellular puncta at the soma, dendrites, axons and neurite terminals. Shh(+) puncta move bidirectionally and colocalize with markers of synaptic vesicles (SVs) and dense core granules. Lipid modification and proteolysis were required for Shh sorting to SVs and cell surface association. Finally, consistent with its association with regulated secretory vesicles, Shh secretion could be induced under depolarizing conditions. Taken together, these observations suggest that long-range Shh transport and signalling in neurons involves trafficking to the regulated secretory pathway and cell surface accumulation of Shh on axons and suggests a link between neuronal activity and Shh release.

Hedgehog signaling regulates E-cadherin expression for the maintenance of the actin cytoskeleton and tight junctions

In the stomach, strictly regulated cell adherens junctions are crucial in determining epithelial cell differentiation. Sonic Hedgehog (Shh) regulates epithelial cell differentiation in the adult stomach. We sought to identify whether Shh plays a role in regulating adherens junction protein E-cadherin as a mechanism for epithelial cell differentiation. Mouse nontumorigenic gastric epithelial (IMGE-5) cells treated with Hedgehog signaling inhibitor cyclopamine and anti-Shh 5E1 antibody or transduced with short hairpin RNA against Skinny Hedgehog (IMGE-5(Ski)) were cultured. A mouse model expressing a parietal cell-specific deletion of Shh (HKCre/Shh(KO)) was used to identify further changes in adherens and tight junctions. Inhibition of Hedgehog signaling in IMGE-5 cells caused loss of E-cadherin expression accompanied by disruption of F-actin cortical expression and relocalization of zonula occludens-1 (ZO-1). Loss of E-cadherin was also associated with increased proliferation in IMGE-5(Ski) cells and increased expression of the mucous neck cell lineage marker MUC6. Compared with membrane-expressed E-cadherin and ZO-1 protein in controls, dissociation of E-cadherin/β-catenin and ZO-1/occludin protein complexes was observed in HKCre/Shh(KO) mice. In conclusion, we demonstrate that Hedgehog signaling regulates E-cadherin expression that is required for the maintenance of F-actin cortical expression and stability of tight junction protein ZO-1.

The Drosophila protein palmitoylome: characterizing palmitoyl-thioesterases and DHHC palmitoyl-transferases

Palmitoylation is the post-translational addition of a palmitate moiety to a cysteine residue through a covalent thioester bond. The addition and removal of this modification is controlled by both palmitoyl acyl-transferases and thioesterases. Using bioinformatic analysis, we identified 22 DHHC family palmitoyl acyl-transferase homologs in the Drosophila genome. We used in situ hybridization,RT-PCR, and published FlyAtlas microarray data to characterize the expression patterns of all 22 fly homologs. Our results indicate that all are expressed genes, but several, including CG1407, CG4676, CG5620, CG6017/dHIP14, CG6618, CG6627 and CG17257 appear to be enriched in neural tissues suggesting that they are important for neural function. Furthermore, we have found that several may be expressed in a sex-specific manner with adult male specific expression of CG4483 and CG17195. Using tagged versions of the DHHC genes, we demonstrate that fly DHHC proteins are primarily located in either the Golgi Apparatus or Endoplasmic Reticulum in S2 cells, except for CG1407, which was found on the plasma membrane. We also characterized the subcellular localization and expression of the three known thioesterases: Palmitoyl-protein Thioesterase 1 (Ppt1), Palmitoyl-protein Thioesterase 2 (Ppt2)and Acyl-protein Thioesterase 1 (APT1). Our results indicate that Ppt1 and Ppt2 are the major lysosomal thioesterases while APT1 is the likely cytoplasmic thioesterase. Finally, in vivo rescue experiments show that Ppt2 expression cannot rescue the neural inclusion phenotypes associated with loss of Ppt1, further supporting distinct functions and substrates for these two thioesterases. These results will serve as the basis for a more complete understanding of the protein palmitoylome's normal cellular functions in the fly and will lead to further insights into the molecular etiology of diseases associated with the mis-regulation of palmitoylation.

Protection from lipopolysaccharide-induced pulmonary microvascular endothelial cell injury by activation of hedgehog signaling pathway

Pulmonary microvascular endothelial cells (PMVECs) are critically involved in the pathogenesis of acute lung injury. Hedgehog signaling pathway plays a fundamental role in embryonic development as well as adult morphogenesis and carcinogenesis. As the priming protein of hedgehog signaling pathway, sonic hedgehog (Shh) may recently be advantage for decreasing endothelial injury and promoting the repair of endothelial barrier function. To investigate the expression and role of hedgehog signal pathway in PMVECs injured by lipopolysaccharide (LPS), cells were divided into six groups: control group, LPS group, rhShh group, LPS+rhShh group, rhShh+cyclopamine group, and LPS+rhShh+cyclopamine group. Real time RT-PCR and Western blotting were used to detect the mRNA and protein expression of hedgehog signal molecules including Shh, Patched-1 (Ptc-1) and Gli1 in nucleus. The activity of PMVECs was examined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. In this study, we found that Shh, Ptch1, and Gli1 were expressed in rat PMVECs and their expression decreased when cells were treated by LPS. In the other hand, LPS inhibited the activity of rat PMVECs and caused the cells injury. Activation of Hedgehog signaling pathway by Shh could elevate the activity of PMVECs with pretreatment by LPS. Therefore, hedgehog signaling pathway should play a protective role on injury PMVECs by LPS.

Hedgehog signaling update

In vertebrate hedgehog signaling, hedgehog ligands are processed to become bilipidated and then multimerize, which allows them to leave the signaling cell via Dispatched 1 and become transported via glypicans and megalin to the responding cells. Hedgehog then interacts with a complex of Patched 1 and Cdo/Boc, which activates endocytic Smoothened to the cilium. Patched 1 regulates the activity of Smoothened (1) via Vitamin D3, which inhibits Smoothened in the absence of hedgehog ligand or (2) via oxysterols, which activate Smoothened in the presence of hedgehog ligand. Hedgehog ligands also interact with Hip1, Patched 2, and Gas1, which regulate the range as well as the level of hedgehog signaling. In vertebrates, Smoothened is shortened at its C-terminal end and lacks most of the phosphorylation sites of importance in Drosophila. Cos2, also of importance in Drosophila, plays no role in mammalian transduction, nor do its homologs Kif7 and Kif27. The cilium may provide a function analogous to that of Cos2 by linking Smoothened to the modulation of Gli transcription factors. Disorders associated with the hedgehog signaling network follow, including nevoid basal cell carcinoma syndrome, holoprosencephaly, Smith-Lemli-Opitz syndrome, Greig cephalopolysyndactyly syndrome, Pallister-Hall syndrome, Carpenter syndrome, and Rubinstein-Taybi syndrome.

Hedgehog signaling update

In vertebrate hedgehog signaling, hedgehog ligands are processed to become bilipidated and then multimerize, which allows them to leave the signaling cell via Dispatched 1 and become transported via glypicans and megalin to the responding cells. Hedgehog then interacts with a complex of Patched 1 and Cdo/Boc, which activates endocytic Smoothened to the cilium. Patched 1 regulates the activity of Smoothened (1) via Vitamin D3, which inhibits Smoothened in the absence of hedgehog ligand or (2) via oxysterols, which activate Smoothened in the presence of hedgehog ligand. Hedgehog ligands also interact with Hip1, Patched 2, and Gas1, which regulate the range as well as the level of hedgehog signaling. In vertebrates, Smoothened is shortened at its C-terminal end and lacks most of the phosphorylation sites of importance in Drosophila. Cos2, also of importance in Drosophila, plays no role in mammalian transduction, nor do its homologs Kif7 and Kif27. The cilium may provide a function analogous to that of Cos2 by linking Smoothened to the modulation of Gli transcription factors. Disorders associated with the hedgehog signaling network follow, including nevoid basal cell carcinoma syndrome, holoprosencephaly, Smith-Lemli-Opitz syndrome, Greig cephalopolysyndactyly syndrome, Pallister-Hall syndrome, Carpenter syndrome, and Rubinstein-Taybi syndrome.

The role of Hedgehog signaling in fibrogenic liver repair

Repair of adult liver, like many tissues, involves the coordinated response of a number of different cell types. In adult livers, fibroblastic cells, ductular cells, inflammatory cells, and progenitor cells contribute to this process. Our studies demonstrate that the fates of such cells are dictated, at least in part, by Hedgehog, a fetal morphogenic pathway that was once thought to be active mainly during embryogenesis. Studies of injured adult human and rodent livers demonstrate that injury-related activation of the Hedgehog pathway modulates several important aspects of repair, including the growth of hepatic progenitor populations, hepatic accumulation of myofibroblasts, repair-related inflammatory responses, vascular remodeling, liver fibrosis and hepatocarcinogenesis. These findings identify the Hedgehog pathway as a potentially important target for biomarker development and therapeutic manipulation, and emphasize the need for further research to advance knowledge about how this pathway is regulated by and interacts with other signals that regulate adult liver repair.

Interactions between Hedgehog proteins and their binding partners come into view

Hedgehog (Hh) proteins are secreted signaling molecules that mediate essential tissue-patterning events during embryonic development and function in tissue homeostasis and regeneration throughout life. Hh signaling is regulated by multiple mechanisms, including covalent lipid modification of the Hh protein and interactions with multiple protein and glycan partners. Unraveling the nature and effects of these interactions has proven challenging, but recent structural and biophysical studies of Hh proteins and active fragments of heparin, Ihog, Cdo, Boc, Hedgehog-interacting protein (Hhip), Patched (Ptc), and the monoclonal antibody 5E1 have added a new level of molecular detail to our understanding of how Hh signal response and distribution are regulated within tissues. We review these results and discuss their implications for understanding Hh signaling in normal and disease states.

Drug discovery approaches to target Wnt signaling in cancer stem cells

Cancer stem cells (CSCs) represent a unique subset of cells within a tumor that possess self-renewal capacity and pluripotency, and can drive tumor initiation and maintenance. First identified in hematological malignancies, CSCs are now thought to play an important role in a wide variety of solid tumors such as NSCLC, breast and colorectal cancer. The role of CSCs in driving tumor formation illustrates the dysregulation of differentiation in tumorigenesis. The Wnt, Notch and Hedgehog (HH) pathways are developmental pathways that are commonly activated in many types of cancer. While substantial progress has been made in developing therapeutics targeting Notch and HH, the Wnt pathway has remained an elusive therapeutic target. This review will focus on the clinical relevance of the Wnt pathway in CSCs and tumor cell biology, as well as points of therapeutic intervention and recent advances in targeting Wnt/β-catenin signaling.

Aberrant forebrain signaling during early development underlies the generation of holoprosencephaly and coloboma

In this review, we highlight recent literature concerning the signaling mechanisms underlying the development of two neural birth defects, holoprosencephaly and coloboma. Holoprosencephaly, the most common forebrain defect, occurs when the cerebral hemispheres fail to separate and is typically associated with mispatterning of embryonic midline tissue. Coloboma results when the choroid fissure in the eye fails to close. It is clear that Sonic hedgehog (Shh) signaling regulates both forebrain and eye development, with defects in Shh, or components of the Shh signaling cascade leading to the generation of both birth defects. In addition, other intercellular signaling pathways are known factors in the incidence of holoprosencephaly and coloboma. This review will outline recent advances in our understanding of forebrain and eye embryonic pattern formation, with a focus on zebrafish studies of Shh and retinoic acid pathways. Given the clear overlap in the mechanisms that generate both diseases, we propose that holoprosencephaly and coloboma can represent mild and severe aspects of single phenotypic spectrum resulting from aberrant forebrain development. This article is part of a Special Issue entitled Zebrafish Models of Neurological Diseases.

The role of the acyl modification, palmitoylation, in Arabidopsis stem cell regulation

Proper control of stem cell populations is key for the development of all multicellular organisms. In Arabidopsis, stem cells are located primarily in the shoot, root and floral meristems where they undergo complex regulation. The Arabidopsis shoot and root meristems are regulated by the related WUS and WOX5 pathways, respectively. Previous studies established that these pathways share the signal transduction components POLTERGEIST (POL) and PLL1. Our latest study in Plant Cell revealed key roles for acyl modifications and lipid binding in the regulation of these two type 2C protein phosphatases. Specifically, POL and PLL1 were shown to localize to the plasma membrane in a myristioylation- and palmitoylation-dependent manner, POL and PLL1 were shown to bind to membrane lipids, and POL activity was found to be stimulated in vitro by the phospholipid PI(4)P. Here, we will discuss what is currently known in Arabidopsis and other organisms about the mechanisms of palmitoylation and provide additional evidence supporting that POL and PLL1 are palmitoylated, including describing the identification of a putative Arabidopsis palmitoyl transferase as a PLL1 interactor.

Mutations causing Greenberg dysplasia but not Pelger anomaly uncouple enzymatic from structural functions of a nuclear membrane protein

The lamin B receptor (LBR) is an inner nuclear membrane protein with a structural function interacting with chromatin and lamins, and an enzymatic function as a sterol reductase. Heterozygous LBR mutations cause nuclear hyposegmentation in neutrophils (Pelger anomaly), while homozygous mutations cause prenatal death with skeletal defects and abnormal sterol metabolism (Greenberg dysplasia). It has remained unclear whether the lethality in Greenberg dysplasia is due to cholesterol defects or altered nuclear morphology.To answer this question we characterized two LBR missense mutations and showed that they cause Greenberg dysplasia. Both mutations affect residues that are evolutionary conserved among sterol reductases. In contrast to wildtype LBR, both mutations failed to rescue C14 sterol reductase deficient yeast, indicating an enzymatic defect. We found no Pelger anomaly in the carrier parent excluding marked effects on nuclear structure. We studied Lbr in mouse embryos and demonstrate expression in skin and the developing skeletal system consistent with sites of histological changes in Greenberg dysplasia. Unexpectedly we found in disease-relevant cell types not only nuclear but also cytoplasmatic LBR localization. The cytoplasmatic LBR staining co-localized with ER-markers and is thus consistent with the sites of endogeneous sterol synthesis. We conclude that LBR missense mutations can abolish sterol reductase activity, causing lethal Greenberg dysplasia but not Pelger anomaly. The findings separate the metabolic from the structural function and indicate that the sterol reductase activity is essential for human intrauterine development.

dHIP14-dependent palmitoylation promotes secretion of the BMP antagonist Sog

Analysis of diverse signaling systems has revealed that one important level of control is regulation of membrane trafficking of ligands and receptors. The activities of some ligands are also regulated by whether they are membrane bound or secreted. In Drosophila, several morphogenetic signals that play critical roles in development have been found to be subject to such regulation. For example, activity of the Hedgehog (Hh) is regulated by Raspberry, which palmitoylates Hh. Similarly, the palmitoylases Porcupine and Raspberry increase the activities of Wingless (Wg) and the EGF-ligand Spitz (Spi), respectively. In contrast to its vertebrate homologues, which have typical N-terminal signal sequences, the precursor form of Drosophila Hh contains an internal type-II secretory signal motif. The Short Gastrulation (Sog) protein is another secreted Drosophila protein that contains a type-II signal and differs from its vertebrate ortholog Chordin which contains a standard signal peptide. In this study, we examine the regulation of Sog secretion and regulation by dHIP14, the ortholog of a mammalian palmitoylase first identified as Huntington Interacting Protein (HIP). We show that dHIP14 binds to Sog and that Sog is palmitoylated. In S2 cells, dHIP14 promotes secretion of Sog as well as stabilizing a membrane associated form of Sog. We examined the requirement for candidate cysteine residues in the N-terminal predicted cytoplasmic domain of Sog and find that Cys27, one of two adjacent cysteines (Cys27 and Cys28), is essential for the full activity of dHIP14 and its effect on Sog. Finally, we find that dHIP14 promotes the activity of Sog in vivo. These studies highlight the growing importance of lipid modification in regulating signaling at the level of ligand production and localization.

Identification of conserved regions and residues within Hedgehog acyltransferase critical for palmitoylation of Sonic Hedgehog

Background

Sonic hedgehog (Shh) is a palmitoylated protein that plays key roles in mammalian development and human cancers. Palmitoylation of Shh is required for effective long and short range Shh-mediated signaling. Attachment of palmitate to Shh is catalyzed by Hedgehog acyltransferase (Hhat), a member of the membrane bound O-acyl transferase (MBOAT) family of multipass membrane proteins. The extremely hydrophobic composition of MBOAT proteins has limited their biochemical characterization. Except for mutagenesis of two conserved residues, there has been no structure-function analysis of Hhat, and the regions of the protein required for Shh palmitoylation are unknown.

Methodology/Principal Findings

Here we undertake a systematic approach to identify residues within Hhat that are required for protein stability and/or enzymatic activity. We also identify a second, novel MBOAT homology region (residues 196–234) that is required for Hhat activity. In total, ten deletion mutants and eleven point mutants were generated and analyzed. Truncations at the N- and C-termini of Hhat yielded inactive proteins with reduced stability. Four Hhat mutants with deletions within predicted loop regions and five point mutants retained stability but lost palmitoylation activity. We purified two point mutants, W378A and H379A, with defective Hhat activity. Kinetic analyses revealed alterations in apparent K_m and V_max for Shh and/or palmitoyl CoA, changes that likely explain the catalytic defects observed for these mutants.

Conclusions/Significance

This study has pinpointed specific regions and multiple residues that regulate Hhat stability and catalysis. Our findings should be applicable to other MBOAT proteins that mediate lipid modification of Wnt proteins and ghrelin, and should serve as a model for understanding how secreted morphogens are modified by palmitoyl acyltransferases.

Mechanism and evolution of cytosolic Hedgehog signal transduction

Hedgehog (Hh) signaling is required for embryonic patterning and postnatal physiology in invertebrates and vertebrates. With the revelation that the primary cilium is crucial for mammalian Hh signaling, the prevailing view that Hh signal transduction mechanisms are conserved across species has been challenged. However, more recent progress on elucidating the function of core Hh pathway cytosolic regulators in Drosophila, zebrafish and mice has confirmed that the essential logic of Hh transduction is similar between species. Here, we review Hh signaling events at the membrane and in the cytosol, and focus on parallel and divergent functions of cytosolic Hh regulators in Drosophila and mammals.