Secretion and signaling activities of lipoprotein-associated hedgehog and non-sterol-modified hedgehog in flies and mammals

Conditional Deletion of Indian Hedgehog in Limb Mesenchyme Results in Complete Loss of Growth Plate Formation but Allows Mature Osteoblast Differentiation

Indian hedgehog (Ihh) is widely recognized as an essential factor for proper skeletal development. Previous in vivo studies using mutant Ihh mouse models were limited by perinatal lethality or carried out after a growth plate formed. Thus the important role of Ihh in mesenchymal cell differentiation has not been investigated. In this study, we established Prx1-Cre;Ihh(fl/fl) mice to ablate Ihh specifically in limb mesenchyme to allow us to observe the phenotype continuously from prenatal development to 3 weeks of age. Mutant mice displayed severe limb abnormalities characterized by complete lack of secondary ossification center and growth plate, indicating an essential role for Ihh in the development of these structures. Interestingly, we discovered that osteoblast differentiation and bone formation could occur in conditions of deficient Ihh. This is a novel finding that has not been observed because of the early lethality of previous Ihh mutants. Mature osteoblasts expressing osteocalcin could be detected in the center of mutant bones at postnatal day 10 (P10). Osteoclasts and blood vessel formation were also present, suggesting active bone remodeling. Histomorphometric analyses show a significant increase in osteoclast number with no major changes in bone formation rate at 3 weeks of age. Mutant long bones in the limbs were deformed, with cortices comprised of irregular woven bone. Also, there was a marked decrease in gene expression of osteoblastic and osteocytic markers. Moreover, mutant long bones displayed bone dysplasia in which we observed increased osteoclast activity and partially reduced osteoblastic and osteocytic differentiation that lead ultimately to loss of bone structures at 3 weeks of age. In summary, our data show for the first time, the presence of mature osteoblasts in long bones of the limbs despite the complete loss of growth plate formation due to Ihh deficiency. These data indicate an important function for Ihh in regulating limb mesenchymal cell differentiation.

Hedgehog Cholesterolysis: Specialized Gatekeeper to Oncogenic Signaling

Discussions of therapeutic suppression of hedgehog (Hh) signaling almost exclusively focus on receptor antagonism; however, hedgehog's biosynthesis represents a unique and potentially targetable aspect of this oncogenic signaling pathway. Here, we review a key biosynthetic step called cholesterolysis from the perspectives of structure/function and small molecule inhibition. Cholesterolysis, also called cholesteroylation, generates cholesterol-modified Hh ligand via autoprocessing of a hedgehog precursor protein. Post-translational modification by cholesterol appears to be restricted to proteins in the hedgehog family. The transformation is essential for Hh biological activity and upstream of signaling events. Despite its decisive role in generating ligand, cholesterolysis remains conspicuously unexplored as a therapeutic target.

Smoothened Regulation: A Tale of Two Signals

The G protein-coupled receptor (GPCR) Smoothened (Smo) is the signal transducer of the developmentally and therapeutically relevant Hedgehog (Hh) pathway. Although recent structural analyses have advanced our understanding of Smo biology, several questions remain. Chief among them are the identity of its natural ligand, the regulatory processes controlling its activation, and the mechanisms by which it signals to downstream effectors. In this review, we discuss recent discoveries from multiple model systems that have set the stage for solving these mysteries. We focus on the roles of distinct Smo functional domains, post-translational modifications, and trafficking, and conclude by discussing their contributions to signal output.

Sonic hedgehog processing and release are regulated by glypican heparan sulfate proteoglycans

All Hedgehog morphogens are released from producing cells, despite being synthesized as N- and C-terminally lipidated molecules, a modification that firmly tethers them to the cell membrane. We have previously shown that proteolytic removal of both lipidated peptides, called shedding, releases bioactive Sonic hedgehog (Shh) morphogens from the surface of transfected Bosc23 cells. Using in vivo knockdown together with in vitro cell culture studies, we now show that glypican heparan sulfate proteoglycans regulate this process, through their heparan sulfate chains, in a cell autonomous manner. Heparan sulfate specifically modifies Shh processing at the cell surface, and purified glycosaminoglycans enhance the proteolytic removal of N- and C-terminal Shh peptides under cell-free conditions. The most likely explanation for these observations is direct Shh processing in the extracellular compartment, suggesting that heparan sulfate acts as a scaffold or activator for Shh ligands and the factors required for their turnover. We also show that purified heparan sulfate isolated from specific cell types and tissues mediates the release of bioactive Shh from pancreatic cancer cells, revealing a previously unknown regulatory role for these versatile molecules in a pathological context.

Cholesterylation: a tail of hedgehog

Cholesterylation is a post-translational attachment of sterol to proteins. This modification has been a characteristic of a single family of hedgehog proteins (Hh). Hh is a well-established morphogenic molecule important in embryonic development. It was also found to be involved in the progression of many cancer types. Herein, we describe the mechanism of biosynthesis of cholesterylated Hh, the role of this unusual modification on protein functions and novel chemical probes, which could be used to specifically target this modification, both in vitro and in vivo.

Membrane bound O-acyltransferases and their inhibitors

Since the identification of the membrane-bound O-acyltransferase (MBOATs) protein family in the early 2000s, three distinct members [porcupine (PORCN), hedgehog (Hh) acyltransferase (HHAT) and ghrelin O-acyltransferase (GOAT)] have been shown to acylate specific proteins or peptides. In this review, topology determination, development of assays to measure enzymatic activities and discovery of small molecule inhibitors are compared and discussed for each of these enzymes.

Endocannabinoids are conserved inhibitors of the Hedgehog pathway

Hedgehog ligands control tissue development and homeostasis by alleviating repression of Smoothened, a seven-pass transmembrane protein. The Hedgehog receptor, Patched, is thought to regulate the availability of small lipophilic Smoothened repressors whose identity is unknown. Lipoproteins contain lipids required to repress Smoothened signaling in vivo. Here, using biochemical fractionation and lipid mass spectrometry, we identify these repressors as endocannabinoids. Endocannabinoids circulate in human and Drosophila lipoproteins and act directly on Smoothened at physiological concentrations to repress signaling in Drosophila and mammalian assays. Phytocannabinoids are also potent Smo inhibitors. These findings link organismal metabolism to local Hedgehog signaling and suggest previously unsuspected mechanisms for the physiological activities of cannabinoids.

Identification of a family of fatty-acid-speciated sonic hedgehog proteins, whose members display differential biological properties

Hedgehog (HH) proteins are proteolytically processed into a biologically active form that is covalently modified by cholesterol and palmitate. However, most studies of HH biogenesis have characterized protein from cells in which HH is overexpressed. We purified Sonic Hedgehog (SHH) from cells expressing physiologically relevant levels and showed that it was more potent than SHH isolated from overexpressing cells. Furthermore, the SHH in our preparations was modified with a diverse spectrum of fatty acids on its amino termini, and this spectrum of fatty acids varied dramatically depending on the growth conditions of the cells. The fatty acid composition of SHH affected its trafficking to lipid rafts as well as its potency. Our results suggest that HH proteins exist as a family of diverse lipid-speciated proteins that might be altered in different physiological and pathological contexts in order to regulate distinct properties of HH proteins.

The Hedgehog pathway: role in cell differentiation, polarity and proliferation

Hedgehog (Hh) is first described as a genetic mutation that has "spiked" phenotype in the cuticles of Drosophila in later 1970s. Since then, Hh signaling has been implicated in regulation of differentiation, proliferation, tissue polarity, stem cell population and carcinogenesis. The first link of Hh signaling to cancer was established through discovery of genetic mutations of Hh receptor gene PTCH1 being responsible for Gorlin syndrome in 1996. It was later shown that Hh signaling is associated with many types of cancer, including skin, leukemia, lung, brain and gastrointestinal cancers. Another important milestone for the Hh research field is the FDA approval for the clinical use of Hh inhibitor Erivedge/Vismodegib for treatment of locally advanced and metastatic basal cell carcinomas. However, recent clinical trials of Hh signaling inhibitors in pancreatic, colon and ovarian cancer all failed, indicating a real need for further understanding of Hh signaling in cancer. In this review, we will summarize recent progress in the Hh signaling mechanism and its role in human cancer.

Production of systemically circulating Hedgehog by the intestine couples nutrition to growth and development

Rodenfels et al. show that the Drosophila intestine responds to nutrient availability by regulating production of a circulating lipoprotein-associated form of Hedgehog (Hh). Levels of circulating Hh tune the rates of growth and developmental timing in a coordinated fashion. Circulating Hh is especially important during starvation, when it is also required for mobilization of fat body triacylglycerol stores.

In Drosophila larvae, growth and developmental timing are regulated by nutrition in a tightly coordinated fashion. The networks that couple these processes are far from understood. Here, we show that the intestine responds to nutrient availability by regulating production of a circulating lipoprotein-associated form of the signaling protein Hedgehog (Hh). Levels of circulating Hh tune the rates of growth and developmental timing in a coordinated fashion. Circulating Hh signals to the fat body to control larval growth. It regulates developmental timing by controlling ecdysteroid production in the prothoracic gland. Circulating Hh is especially important during starvation, when it is also required for mobilization of fat body triacylglycerol (TAG) stores. Thus, we demonstrate that Hh, previously known only for its local morphogenetic functions, also acts as a lipoprotein-associated endocrine hormone, coordinating the response of multiple tissues to nutrient availability.

The ESCRT machinery regulates the secretion and long-range activity of Hedgehog

The conserved family of Hedgehog (Hh) proteins acts as short- and long-range secreted morphogens, controlling tissue patterning and differentiation during embryonic development. Mature Hh carries hydrophobic palmitic acid and cholesterol modifications essential for its extracellular spreading. Various extracellular transportation mechanisms for Hh have been suggested, but the pathways actually used for Hh secretion and transport in vivo remain unclear. Here we show that Hh secretion in Drosophila wing imaginal discs is dependent on the endosomal sorting complex required for transport (ESCRT). In vivo the reduction of ESCRT activity in cells producing Hh leads to a retention of Hh at the external cell surface. Furthermore, we show that ESCRT activity in Hh-producing cells is required for long-range signalling. We also provide evidence that pools of Hh and ESCRT proteins are secreted together into the extracellular space in vivo and can subsequently be detected together at the surface of receiving cells. These findings uncover a new function for ESCRT proteins in controlling morphogen activity and reveal a new mechanism for the transport of secreted Hh across the tissue by extracellular vesicles, which is necessary for long-range target induction.

Metabolic insights from zebrafish genetics, physiology, and chemical biology

Metabolic diseases—atherosclerotic cardiovascular disease, type 2 diabetes mellitus, obesity, and non-alcoholic fatty liver disease––have reached pandemic proportions. Across gene, cell, organ, organism, and social-environmental scales, fundamental discoveries of the derangements that occur in these diseases are required to develop effective new treatments. Here we will review genetic, physiological, pathological and chemical biological discoveries in the emerging zebrafish model for studying metabolism and metabolic diseases. We present a synthesis of recent studies using forward and reverse genetic tools to make new contributions to our understanding of lipid trafficking, diabetes pathogenesis and complications, and to β-cell biology. The technical and physiological advantages and the pharmacological potential of this organism for discovery and validation of metabolic disease targets are stressed by our summary of recent findings. We conclude by arguing that metabolic research using zebrafish will benefit from adoption of conventional blood and tissue metabolite measurements, employment of modern imaging techniques, and development of more rigorous metabolic flux methods.

Vertebrate Hedgehog is secreted on two types of extracellular vesicles with different signaling properties

Hedgehog (Hh) is a secreted morphogen that elicits differentiation and patterning in developing tissues. Multiple proposed mechanisms to regulate Hh dispersion includes lipoprotein particles and exosomes. Here we report that vertebrate Sonic Hedgehog (Shh) is secreted on two types of extracellular-vesicles/exosomes, from human cell lines and primary chick notochord cells. Although largely overlapping in size as estimated from electron micrographs, the two exosomal fractions exhibited distinct protein and RNA composition. We have probed the functional properties of these vesicles using cell-based assays of Hh-elicited gene expression. Our results suggest that while both Shh-containing exo-vesicular fractions can activate an ectopic Gli-luciferase construct, only exosomes co-expressing Integrins can activate endogenous Shh target genes HNF3β and Olig2 during the differentiation of mouse ES cells to ventral neuronal progenitors. Taken together, our results demonstrate that primary vertebrate cells secrete Shh in distinct vesicular forms, and support a model where packaging of Shh along with other signaling proteins such as Integrins on exosomes modulates target gene activation. The existence of distinct classes of Shh-containing exosomes also suggests a previously unappreciated complexity for fine-tuning of Shh-mediated gradients and pattern formation.

Drosophila gains traction as a repurposed tool to investigate metabolism

The use of fruit flies has recently emerged as a powerful experimental paradigm to study the core aspects of energy metabolism. The fundamental need for lipid and carbohydrate processing and storage across species dictates that the central regulators that control metabolism are highly conserved through evolution. Accordingly, the Drosophila system is being used to identify human disease genes and has the potential to model successfully human disorders that center on excessive caloric intake and metabolic dysfunction, including diet-induced lipotoxicity and type 2 diabetes. We review here recent progress on this front and contend that increasing such efforts will yield unexpectedly high rates of experimental return, thereby leading to novel approaches in the treatment of obesity and its comorbidities.

Organismal proteostasis: role of cell-nonautonomous regulation and transcellular chaperone signaling

Protein quality control is regulated by the proteostasis network and cell stress response pathways to promote cellular health. In this review, van Oosten-Hawle and Morimoto cover recent advances in model systems that reveal how communication between subcellular compartments and across different cells and tissues maintains a functional proteome during stress. The authors propose that transcellular stress signaling provides a critical control mechanism for the proteostasis network to maintain organismal health and life span.

Protein quality control is essential in all organisms and regulated by the proteostasis network (PN) and cell stress response pathways that maintain a functional proteome to promote cellular health. In this review, we describe how metazoans employ multiple modes of cell-nonautonomous signaling across tissues to integrate and transmit the heat-shock response (HSR) for balanced expression of molecular chaperones. The HSR and other cell stress responses such as the unfolded protein response (UPR) can function autonomously in single-cell eukaryotes and tissue culture cells; however, within the context of a multicellular animal, the PN is regulated by cell-nonautonomous signaling through specific sensory neurons and by the process of transcellular chaperone signaling. These newly identified forms of stress signaling control the PN between neurons and nonneuronal somatic tissues to achieve balanced tissue expression of chaperones in response to environmental stress and to ensure that metastable aggregation-prone proteins expressed within any single tissue do not generate local proteotoxic risk. Transcellular chaperone signaling leads to the compensatory expression of chaperones in other somatic tissues of the animal, perhaps preventing the spread of proteotoxic damage. Thus, communication between subcellular compartments and across different cells and tissues maintains proteostasis when challenged by acute stress and upon chronic expression of metastable proteins. We propose that transcellular chaperone signaling provides a critical control step for the PN to maintain cellular and organismal health span.

New chemical probes targeting cholesterylation of Sonic Hedgehog in human cells and zebrafish

Alkynyl-cholesterol probes tag and track Hedgehog protein, illuminating the role of protein cholesterylation in secretion, transport complex formation and signalling, and enabling quantitative proteomic analysis, imaging, and detection of cholesterylation in developing zebrafish.

Sonic Hedgehog protein (Shh) is a morphogen molecule important in embryonic development and in the progression of many cancer types in which it is aberrantly overexpressed. Fully mature Shh requires attachment of cholesterol and palmitic acid to its C- and N-termini, respectively. The study of lipidated Shh has been challenging due to the limited array of tools available, and the roles of these posttranslational modifications are poorly understood. Herein, we describe the development and validation of optimised alkynyl sterol probes that efficiently tag Shh cholesterylation and enable its visualisation and analysis through bioorthogonal ligation to reporters. An optimised probe was shown to be an excellent cholesterol biomimetic in the context of Shh, enabling appropriate release of tagged Shh from signalling cells, formation of multimeric transport complexes and signalling. We have used this probe to determine the size of transport complexes of lipidated Shh in culture medium and expression levels of endogenous lipidated Shh in pancreatic ductal adenocarcinoma cell lines through quantitative chemical proteomics, as well as direct visualisation of the probe by fluorescence microscopy and detection of cholesterylated Hedgehog protein in developing zebrafish embryos. These sterol probes provide a set of novel and well-validated tools that can be used to investigate the role of lipidation on activity of Shh, and potentially other members of the Hedgehog protein family.

Signaling domain of Sonic Hedgehog as cannibalistic calcium-regulated zinc-peptidase

Sonic Hedgehog (Shh) is a representative of the evolutionary closely related class of Hedgehog proteins that have essential signaling functions in animal development. The N-terminal domain (ShhN) is also assigned to the group of LAS proteins (LAS = Lysostaphin type enzymes, D-Ala-D-Ala metalloproteases, Sonic Hedgehog), of which all members harbor a structurally well-defined Zn2+ center; however, it is remarkable that ShhN so far is the only LAS member without proven peptidase activity. Another unique feature of ShhN in the LAS group is a double-Ca2+ center close to the zinc. We have studied the effect of these calcium ions on ShhN structure, dynamics, and interactions. We find that the presence of calcium has a marked impact on ShhN properties, with the two calcium ions having different effects. The more strongly bound calcium ion significantly stabilizes the overall structure. Surprisingly, the binding of the second calcium ion switches the putative catalytic center from a state similar to LAS enzymes to a state that probably is catalytically inactive. We describe in detail the mechanics of the switch, including the effect on substrate co-ordinating residues and on the putative catalytic water molecule. The properties of the putative substrate binding site suggest that ShhN could degrade other ShhN molecules, e.g. by cleavage at highly conserved glycines in ShhN. To test experimentally the stability of ShhN against autodegradation, we compare two ShhN mutants in vitro: (1) a ShhN mutant unable to bind calcium but with putative catalytic center intact, and thus, according to our hypothesis, a constitutively active peptidase, and (2) a mutant carrying additionally mutation E177A, i.e., with the putative catalytically active residue knocked out. The in vitro results are consistent with ShhN being a cannibalistic zinc-peptidase. These experiments also reveal that the peptidase activity depends on pH.

Hedgehog and its circuitous journey from producing to target cells

The hedgehog (Hh) signaling protein has essential roles in the growth, development and regulation of many vertebrate and invertebrate organs. The processes that make Hh and prepare it for release from producing cells and that move it to target cells are both diverse and complex. This article reviews the essential features of these processes and highlights recent work that provides a novel framework to understand how these processes contribute to an integrated pathway.

Changes in morphology and function of adrenal cortex in mice fed a high-fat diet

BACKGROUND/OBJECTIVES

Obesity is a major risk factor for the development of type 2 diabetes and other debilitating diseases. Obesity and diabetes are intimately linked with altered levels of adrenal steroids. Elevated levels of these hormones induce insulin resistance and cause cardiovascular diseases. The mechanisms underlying obesity-related alterations in adrenal steroids are still not well understood. Here, we investigated how diet-induced obesity affects the morphology and function of the mouse adrenal cortex.

METHODS

We fed animals either a high-fat diet (HFD) or a normal diet (60% kcal from fat or 10% kcal from fat, respectively) for 18 weeks. We then assessed various aspects of adrenal gland morphology and function, as well as basal plasma concentrations of steroid hormones and ACTH.

RESULTS

We show that adrenal glands of mice fed a HFD release more corticosterone and aldosterone, resulting in higher plasma levels. This increase is driven by adrenal cortical hyperplasia, and by increased expression of multiple genes involved in steroidogenesis. We demonstrate that diet-induced obesity elevates Sonic hedgehog signaling in Gli1-positive progenitors, which populate the adrenal capsule and give rise to the steroidogenic cells of the adrenal cortex. Feeding animals with a HFD depletes Gli1-positive progenitors, as the adrenal cortex expands.

CONCLUSIONS

This work provides insight into how diet-induced obesity changes the biology of the adrenal gland. The association of these changes with increased Shh signaling suggests possible therapeutic strategies for obesity-related steroid hormone dysfunction.

Canonical and non-canonical Hedgehog signalling and the control of metabolism

Obesity and diabetes represent key healthcare challenges of our day, affecting upwards of one billion people worldwide. These individuals are at higher risk for cancer, stroke, blindness, heart and cardiovascular disease, and to date, have no effective long-term treatment options available. Recent and accumulating evidence has implicated the developmental morphogen Hedgehog and its downstream signalling in metabolic control. Generally thought to be quiescent in adults, Hedgehog is associated with several human cancers, and as such, has already emerged as a therapeutic target in oncology. Here, we attempt to give a comprehensive overview of the key signalling events associated with both canonical and non-canonical Hedgehog signalling, and highlight the increasingly complex regulatory modalities that appear to link Hedgehog and control metabolism. We highlight these key findings and discuss their impact for therapeutic development, cancer and metabolic disease.

Sulf1 influences the Shh morphogen gradient during the dorsal ventral patterning of the neural tube in Xenopus tropicalis

Genetic studies have established that heparan sulphate proteoglycans (HSPGs) are required for signalling by key developmental regulators, including Hedgehog, Wnt/Wg, FGF, and BMP/Dpp. Post-synthetic remodelling of heparan sulphate (HS) by Sulf1 has been shown to modulate these same signalling pathways. Sulf1 codes for an N-acetylglucosamine 6-O-endosulfatase, an enzyme that specifically removes the 6-O sulphate group from glucosamine in highly sulfated regions of HS chains. One striking aspect of Sulf1 expression in all vertebrates is its co-localisation with that of Sonic hedgehog in the floor plate of the neural tube. We show here that Sulf1 is required for normal specification of neural progenitors in the ventral neural tube, a process known to require a gradient of Shh activity. We use single-cell injection of mRNA coding for GFP-tagged Shh in early Xenopus embryos and find that Sulf1 restricts ligand diffusion. Moreover, we find that the endogenous distribution of Shh protein in Sulf1 knockdown embryos is altered, where a less steep ventral to dorsal gradient forms in the absence of Sulf1, resulting in more a diffuse distribution of Shh. These data point to an important role for Sulf1 in the ventral neural tube, and suggests a mechanism whereby Sulf1 activity shapes the Shh morphogen gradient by promoting ventral accumulation of high levels of Shh protein.

Hedgehog: Multiple Paths for Multiple Roles in Shaping the Brain and Spinal Cord

Since the discovery of the segment polarity gene Hedgehog in Drosophila three decades ago, our knowledge of Hedgehog signaling pathway has considerably improved and paved the way to a wide field of investigations in the developing and adult central nervous system. Its peculiar transduction mechanism together with its implication in tissue patterning, neural stem cell biology, and neural tissue homeostasis make Hedgehog pathway of interest in a high number of normal or pathological contexts. Consistent with its role during brain development, misregulation of Hedgehog signaling is associated with congenital diseases and tumorigenic processes while its recruitment in damaged neural tissue may be part of the repairing process. This review focuses on the most recent data regarding the Hedgehog pathway in the developing and adult central nervous system and also its relevance as a therapeutic target in brain and spinal cord diseases.

Wing tips: The wing disc as a platform for studying Hedgehog signaling

Hedgehog (Hh) signal transduction is necessary for the development of most mammalian tissues and can go awry and cause birth defects or cancer. Hh signaling was initially described in Drosophila, and much of what we know today about mammalian Hh signaling was directly guided by discoveries in the fly. Indeed, Hh signaling is a wonderful example of the use of non-vertebrate model organisms to make basic discoveries that lead to new disease treatment. The first pharmaceutical to treat hyperactive Hh signaling in Basal Cell Carcinoma was released in 2012, approximately 30 years after the isolation of Hh mutants in Drosophila. The study of Hh signaling has been greatly facilitated by the imaginal wing disc, a tissue with terrific experimental advantages. Studies using the wing disc have led to an understanding of Hh ligand processing, packaging into particles for transmission, secretion, reception, signal transduction, target gene activation, and tissue patterning. Here we describe the imaginal wing disc, how Hh patterns this tissue, and provide methods to use wing discs to study Hh signaling in Drosophila. The tools and approaches we highlight form the cornerstone of research efforts in many laboratories that use Drosophila to study Hh signaling, and are essential for ongoing discoveries.

Scube2 enhances proteolytic Shh processing from the surface of Shh-producing cells

All morphogens of the Hedgehog (Hh) family are synthesized as dual-lipidated proteins, which results in their firm attachment to the surface of the cell in which they were produced. Thus, Hh release into the extracellular space requires accessory protein activities. We suggested previously that the proteolytic removal of N- and C-terminal lipidated peptides (shedding) could be one such activity. More recently, the secreted glycoprotein Scube2 (signal peptide, cubulin domain, epidermal-growth-factor-like protein 2) was also implicated in the release of Shh from the cell membrane. This activity strictly depended on the CUB domains of Scube2, which derive their name from the complement serine proteases and from bone morphogenetic protein-1/tolloid metalloproteinases (C1r/C1s, Uegf and Bmp1). CUB domains function as regulators of proteolytic activity in these proteins. This suggested that sheddases and Scube2 might cooperate in Shh release. Here, we confirm that sheddases and Scube2 act cooperatively to increase the pool of soluble bioactive Shh, and that Scube2-dependent morphogen release is unequivocally linked to the proteolytic processing of lipidated Shh termini, resulting in truncated soluble Shh. Thus, Scube2 proteins act as protease enhancers in this setting, revealing newly identified Scube2 functions in Hh signaling regulation.

Lipoproteins and Hedgehog signalling--possible implications for the adrenal gland function

BACKGROUND

Metabolic syndrome is a common metabolic disorder that is associated with an increased risk of type 2 diabetes and cardiovascular diseases. Disturbances in adrenal steroid hormone production significantly contribute to the development of this disorder. Therefore, it is extremely important to fully understand the mechanisms governing adrenal gland function, both in physiological and pathological conditions.

RESULTS

Recently, Sonic hedgehog has emerged as an important regulator of adrenal development, with a possible role in adult gland homeostasis. Recent work of our group shows that lipoproteins are important regulators of Hedgehog signaling; they act as carriers for the spread of Hedgehog proteins, but also contain lipid(s) that inhibit the pathway.

CONCLUSIONS

We propose that lipoproteins may affect Sonic hedgehog signaling in the adult adrenal gland at multiple levels. Understanding the interplay between lipoprotein metabolism and adrenal Hedgehog signaling may improve our understanding of how adrenal gland disorders contribute to the metabolic syndrome.

Cholesterol-free and cholesterol-bound Hedgehog: Two sparring-partners working hand in hand in the Drosophila wing disc?

Hedgehog (Hh) is a signaling ligand conserved from flies to humans that is covalently bound to both palmitate and cholesterol moieties. These lipid modifications are crucial for Hh signaling. A recent article reports that in both flies and human-cultured cells a cholesterol-free form of Hh (SHh-N*/Hh-N*) is produced and secreted. In the Drosophila wing disc, Hh associated with Lipoproteins-lipophorin complexes (Lpp) would lead to the accumulation of Cubitus interruptus (Ci), the transcription factor in the Hh pathway but this would be insufficient to activate Hh target genes. On the other hand, Lpp-free Hh-N* would act in synergy with Lpp-associated Hh to eventually activate target gene expression. This suggests that Hh can be secreted in 2 different forms that would have distinct and synergic functions.

Suzanne Eaton: the beautiful logic of development

Eaton studies tissue patterning in flies and mammals.

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.