Primary Cilia and Mammalian Hedgehog Signaling.
TL;DR: The genetic and developmental studies used to deduce how Hedgehog signal transduction is linked to cilia and the complex effects that perturbation of cilia structure can have on Hh signaling are summarized.
Abstract: It has been a decade since it was discovered that primary cilia have an essential role in Hedgehog (Hh) signaling in mammals. This discovery came from screens in the mouse that identified a set of genes that are required for both normal Hh signaling and for the formation of primary cilia. Since then, dozens of mouse mutations have been identified that disrupt cilia in a variety of ways and have complex effects on Hedgehog signaling. Here, we summarize the genetic and developmental studies used to deduce how Hedgehog signal transduction is linked to cilia and the complex effects that perturbation of cilia structure can have on Hh signaling. We conclude by describing the current status of our understanding of the cell-type-specific regulation of ciliogenesis and how that determines the ability of cells to respond to Hedgehog ligands.
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TL;DR: Investigating ciliopathies has helped to understand the molecular mechanisms by which the cilium-associated basal body functions in early ciliogenesis, as well as how the transition zone functions in ciliary gating, and how intraflagellar transport enables cargo trafficking and signalling.
Abstract: Motile and non-motile (primary) cilia are nearly ubiquitous cellular organelles. The dysfunction of cilia causes diseases known as ciliopathies. The number of reported ciliopathies (currently 35) is increasing, as is the number of established (187) and candidate (241) ciliopathy-associated genes. The characterization of ciliopathy-associated proteins and phenotypes has improved our knowledge of ciliary functions. In particular, investigating ciliopathies has helped us to understand the molecular mechanisms by which the cilium-associated basal body functions in early ciliogenesis, as well as how the transition zone functions in ciliary gating, and how intraflagellar transport enables cargo trafficking and signalling. Both basic biological and clinical studies are uncovering novel ciliopathies and the ciliary proteins involved. The assignment of these proteins to different ciliary structures, processes and ciliopathy subclasses (first order and second order) provides insights into how this versatile organelle is built, compartmentalized and functions in diverse ways that are essential for human health.
1,017 citations
TL;DR: Cilia, and primary cilia in particular, are important signalling organelles with established roles in odorant, light and Hedgehog morphogen signal transduction and are enriched in signalling receptors and effectors and in specific lipids.
Abstract: The primary cilium is a hair-like surface-exposed organelle of the eukaryotic cell that decodes a variety of signals - such as odorants, light and Hedgehog morphogens - by altering the local concentrations and activities of signalling proteins. Signalling within the cilium is conveyed through a diverse array of second messengers, including conventional signalling molecules (such as cAMP) and some unusual intermediates (such as sterols). Diffusion barriers at the ciliary base establish the unique composition of this signalling compartment, and cilia adapt their proteome to signalling demands through regulated protein trafficking. Much progress has been made on the molecular understanding of regulated ciliary trafficking, which encompasses not only exchanges between the cilium and the rest of the cell but also the shedding of signalling factors into extracellular vesicles.
275 citations
TL;DR: This review of emerging studies are being conducted in order to find new inhibitors for Shh signaling pathway, which could be safely used in clinical trials and highlight some clinical trials that use the Shh pathway as a target for treating brain cancer.
Abstract: Hedgehog (Hh) signaling pathway plays an essential role during vertebrate embryonic development and tumorigenesis. It is already known that Sonic hedgehog (Shh) pathway is important for the evolution of radio and chemo-resistance of several types of tumors. Most of the brain tumors are resistant to chemotherapeutic drugs, consequently, they have a poor prognosis. So, a better knowledge of the Shh pathway opens an opportunity for targeted therapies against brain tumors considering a multi-factorial molecular overview. Therefore, emerging studies are being conducted in order to find new inhibitors for Shh signaling pathway, which could be safely used in clinical trials. Shh can signal through a canonical and non-canonical way, and it also has important points of interaction with other pathways during brain tumorigenesis. So, a better knowledge of Shh signaling pathway opens an avenue of possibilities for the treatment of not only for brain tumors but also for other types of cancers. In this review, we will also highlight some clinical trials that use the Shh pathway as a target for treating brain cancer.
237 citations
Cites background from "Primary Cilia and Mammalian Hedgeho..."
...Further, Hh signaling is disrupted in diverse types of cancer [1, 2]....
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TL;DR: It is concluded that exit from cilia is a two-step process in which BBSome/Arl6 trains first move activated GPCRs through the transition zone before a periciliary barrier can be crossed.
Abstract: A diffusion barrier at the transition zone enables the compartmentalization of signaling molecules by cilia. The BBSome and the small guanosine triphosphatase Arl6, which triggers BBSome coat polymerization, are required for the exit of activated signaling receptors from cilia, but how diffusion barriers are crossed when membrane proteins exit cilia remains to be determined. In this study, we found that activation of the ciliary G protein-coupled receptors (GPCRs) Smoothened and SSTR3 drove the Arl6-dependent assembly of large, highly processive, and cargo-laden retrograde BBSome trains. Single-molecule imaging revealed that the assembly of BBSome trains enables the lateral transport of ciliary GPCRs across the transition zone. However, the removal of activated GPCRs from cilia was inefficient because a second periciliary diffusion barrier was infrequently crossed. We conclude that exit from cilia is a two-step process in which BBSome/Arl6 trains first move activated GPCRs through the transition zone before a periciliary barrier can be crossed.
190 citations
Cites background from "Primary Cilia and Mammalian Hedgeho..."
...Upon Hedgehog pathway activation, the seven-transmembrane-protein Smoothened accumulates in cilia, and ciliary exit of the G protein–coupled receptor (GPCR) GPR161 ensures the appropriate transduction of Hedgehog signals (Bangs and Anderson, 2017; Nager et al., 2017)....
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TL;DR: This review discusses the mechanisms by which the HH signal is received by patched on target cells, transduced across the cell membrane by smoothened, and transmitted to the nucleus by GLI proteins to influence gene-expression programs.
Abstract: Signaling pathways that mediate cell-cell communication are essential for collective cell behaviors in multicellular systems. The hedgehog (HH) pathway, first discovered and elucidated in Drosophila, is one of these iconic signaling systems that plays many roles during embryogenesis and in adults; abnormal HH signaling can lead to birth defects and cancer. We review recent structural and biochemical studies that have advanced our understanding of the vertebrate HH pathway, focusing on the mechanisms by which the HH signal is received by patched on target cells, transduced across the cell membrane by smoothened, and transmitted to the nucleus by GLI proteins to influence gene-expression programs.
161 citations
References
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TL;DR: The phenotypes of the mutant embryos indicate that the process of segmentation involves at least three levels of spatial organization: the entire egg as developmental unit, a repeat unit with the length of two segments, and the individual segment.
Abstract: In systematic searches for embryonic lethal mutants of Drosophila melanogaster we have identified 15 loci which when mutated alter the segmental pattern of the larva. These loci probably represent the majority of such genes in Drosophila. The phenotypes of the mutant embryos indicate that the process of segmentation involves at least three levels of spatial organization: the entire egg as developmental unit, a repeat unit with the length of two segments, and the individual segment.
4,170 citations
"Primary Cilia and Mammalian Hedgeho..." refers background in this paper
...The Hh gene was discovered in Drosophila based on the striking phenotype of fly larvae that lack Hh—the mutants do not develop the segmented anterior-to-posterior body plan and have ectopic denticles resembling a Hedgehog (Nüsslein-Volhard and Wieschaus 1980)....
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...The Hh gene was discovered in Drosophila based on the striking phenotype of fly larvae that lack Hh—the mutants do not develop the segmented anterior-to-posterior body plan and have ectopic denticles resembling a Hedgehog (Nüsslein-Volhard and Wieschaus 1980)....
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TL;DR: In their screen for mutations that disrupt the Drosophila larval body plan, these authors identified several that cause the duplication of denticles and an accompanying loss of naked cuticle, characteristic of the posterior half of each segment.
Abstract: Since their isolation in the early 1990s, members of the Hedgehog family of intercellular signaling proteins have come to be recognized as key mediators of many fundamental processes in embryonic development. Their activities are central to the growth, patterning, and morphogenesis of many different regions within the body plans of vertebrates and insects, and most likely other invertebrates. In some contexts, Hedgehog signals act as morphogens in the dose-dependent induction of distinct cell fates within a target field, in others as mitogens regulating cell proliferation or as inducing factors controlling the form of a developing organ. These diverse functions of Hedgehog proteins raise many intriguing questions about their mode of operation. How do these proteins move between or across fields of cells? How are their activities modulated and transduced? What are their intracellular targets? In this article we review some well-established paradigms of Hedgehog function inDrosophila and vertebrate development and survey the current understanding of the synthesis, modification, and transduction of Hedgehog proteins. Embryological studies over much of the last century that relied primarily on the physical manipulation of cells within the developing embryo or fragments of the embryo in culture, provided many compelling examples for the primacy of cell–cell interactions in regulating invertebrate and vertebrate development. The subsequent identification of many of the signaling factors that mediate cellular communication has led to two general conclusions. First, although there are many important signals, most of these fall into a few large families of secreted peptide factors: theWnt (Wodarz and Nusse 1998), fibroblast growth factor (Szebenyi and Fallon 1999), TGFsuperfamily (Massague and Chen 2000), plateletderived growth factor (Betsholtz et al. 2001), ephrin (Bruckner and Klein 1998), and Hedgehog families. Second, parallel studies in invertebrate and vertebrate systems have shown that although the final outcome might look quite different (e.g., a fly vs. a mouse), there is a striking conservation in the deployment of members of the same signaling families to regulate development of these seemingly quite different organisms. This review focuses on one of the most intriguing examples of this phenomenon, that of the Hedgehog family. As with many of the advances in our understanding of the genetic regulation of animal development, hedgehog (hh) genes owe their discovery to the pioneering work of Nusslein-Volhard and Wieschaus (1980). In their screen for mutations that disrupt the Drosophila larval body plan, these authors identified several that cause the duplication of denticles (spiky cuticular processes that decorate the anterior half of each body segment) and an accompanying loss of naked cuticle, characteristic of the posterior half of each segment (see Fig. 1). The ensuing appearance of a continuous lawn of denticles projecting from the larval cuticle evidently suggested the spines of a hedgehog to the discoverers, hence the origin of the name of one of these genes. Other loci identified by mutants with this phenotype included armadillo, gooseberry, and wingless (wg). In contrast, animals mutant for the aptly named naked gene showed the converse phenotype, with denticle belts replaced by naked cuticle in every segment. On the basis of these mutant phenotypes, Nusslein-Volhard and Wieschaus (1980) proposed that these so-called segment-polarity genes regulate pattern within each of the segments of the larval body, individual genes acting within distinct subregions of the emerging segmental pattern. The first important breakthrough in unraveling how segment-polarity genes act came in the mid-1980s with the cloning of two members of the class, wingless and engrailed (en). Wg was shown to be the ortholog of the vertebrate proto-oncogene int1 (subsequently renamed Wnt1 and the founder member of the Wnt family of secreted peptide factors; Rijsewijk et al. 1987), whereas the sequence of en revealed that it encodes a homeodomaincontaining transcription factor (Fjose et al. 1985; Poole et al. 1985). Intriguingly, the two genes were found to be expressed in adjacent narrow stripes of cells in each segment (Martinez Arias et al. 1988). A close spatial relationship between Wnt1 and En expression domains was also reported in the primordial midbrain and hindbrain of the vertebrate embryo (McMahon et al. 1992). AnalyWe dedicate this review to the memory of our dear friend and colleague Rosa Beddington, whose encouragement led to our initial collaboration. 3Corresponding authors. E-MAIL p.w.ingham@sheffield.ac.uk; FAX 0114-222-288. E-MAIL amcmahon@biosun.harvard.edu; FAX (617) 496-3763. Article and publication are at http://www.genesdev.org/cgi/doi/10.1101/ gad.938601.
2,919 citations
TL;DR: It is shown that mammalian Smoothened (Smo), a seven-transmembrane protein essential for Hh signalling, is expressed on the primary cilium, and Hh-dependent translocation to cilia is essential for Smo activity, suggesting that Smo acts at thePrimary cilia.
Abstract: The unanticipated involvement of several intraflagellar transport proteins in the mammalian Hedgehog (Hh) pathway has hinted at a functional connection between cilia and Hh signal transduction. Here we show that mammalian Smoothened (Smo), a seven-transmembrane protein essential for Hh signalling, is expressed on the primary cilium. This ciliary expression is regulated by Hh pathway activity; Sonic hedgehog or activating mutations in Smo promote ciliary localization, whereas the Smo antagonist cyclopamine inhibits ciliary localization. The translocation of Smo to primary cilia depends upon a conserved hydrophobic and basic residue sequence homologous to a domain previously shown to be required for the ciliary localization of seven-transmembrane proteins in Caenorhabditis elegans. Mutation of this domain not only prevents ciliary localization but also eliminates Smo activity both in cultured cells and in zebrafish embryos. Thus, Hh-dependent translocation to cilia is essential for Smo activity, suggesting that Smo acts at the primary cilium.
1,372 citations
"Primary Cilia and Mammalian Hedgeho..." refers background in this paper
...The first demonstration of cilia localization came for the membrane protein SMO, which accumulates in the cilium within an hour following stimulation of the Hh pathway (Corbit et al. 2005; Rohatgi et al. 2007)....
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...A common hydrophobic and basic residue motif following the seventh transmembrane domain at the carboxyl terminus of SMO is required for localization of SMO to the cilium in the presence of SHH (Händel et al. 1999; Brailov et al. 2000; Dwyer et al. 2001; Corbit et al. 2005; Aanstad et al. 2009)....
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...A common hydrophobic and basic residue motif following the seventh transmembrane domain at the carboxyl terminus of SMO is required for localization of SMO to the cilium in the presence of SHH (Händel et al. 1999; Brailov et al. 2000; Dwyer et al. 2001; Corbit et al. 2005; Aanstad et al. 2009)....
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TL;DR: The role of primary cilia in regulation of Patched1 (Ptc1), the receptor for Sonic Hedgehog (Shh), is investigated and it is found that Ptc1 localized to cilia and inhibited Smoothened (Smo) by preventing its accumulation within cilia.
Abstract: Primary cilia are essential for transduction of the Hedgehog (Hh) signal in mammals. We investigated the role of primary cilia in regulation of Patched1 (Ptc1), the receptor for Sonic Hedgehog (Shh). Ptc1 localized to cilia and inhibited Smoothened (Smo) by preventing its accumulation within cilia. When Shh bound to Ptc1, Ptc1 left the cilia, leading to accumulation of Smo and activation of signaling. Thus, primary cilia sense Shh and transduce signals that play critical roles in development, carcinogenesis, and stem cell function.
1,358 citations
"Primary Cilia and Mammalian Hedgeho..." refers background in this paper
...The first demonstration of cilia localization came for the membrane protein SMO, which accumulates in the cilium within an hour following stimulation of the Hh pathway (Corbit et al. 2005; Rohatgi et al. 2007)....
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...As total SMO protein levels are unaltered, accumulation of SMO in the cilium is a consequence of translocation of a ready-made pool of SMO and not from newly synthesized protein (Rohatgi et al. 2007)....
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...During the same interval after exposure to ligand when PTCH moves out of the cilium, SMO translocates into the cilium (Rohatgi et al. 2007)....
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TL;DR: Genetic analysis shows that Wim, Polaris and the IFT motor protein Kif3a are required for Hedgehog signalling at a step downstream of Patched1 (the Hedgehog receptor) and upstream of direct targets of hedgehog signalling.
Abstract: Intraflagellar transport (IFT) proteins were first identified as essential factors for the growth and maintenance of flagella in the single-celled alga Chlamydomonas reinhardtii. In a screen for embryonic patterning mutations induced by ethylnitrosourea, here we identify two mouse mutants, wimple (wim) and flexo (fxo), that lack ventral neural cell types and show other phenotypes characteristic of defects in Sonic hedgehog signalling. Both mutations disrupt IFT proteins: the wim mutation is an allele of the previously uncharacterized mouse homologue of IFT172; and fxo is a new hypomorphic allele of polaris, the mouse homologue of IFT88. Genetic analysis shows that Wim, Polaris and the IFT motor protein Kif3a are required for Hedgehog signalling at a step downstream of Patched1 (the Hedgehog receptor) and upstream of direct targets of Hedgehog signalling. Our data show that IFT machinery has an essential and vertebrate-specific role in Hedgehog signal transduction.
1,345 citations
"Primary Cilia and Mammalian Hedgeho..." refers background in this paper
...…and limb) Human phenotype Kif3a Anterograde IFT motor Loss of SHH-dependent ventral neural cell types; polydactyly; midgestation lethality (Huangfu et al. 2003) Dync2h1 Retrograde IFT motor Partial loss of ventral neural cell types; polydactyly; midgestation lethality (Huangfu and…...
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...(Polaris; Tg737) IFT-B Loss of SHH-dependent ventral neural cell types; midgestation lethality; polydactyly (Huangfu et al. 2003) Ift172 IFT-B Loss of SHH-dependent ventral neural cell types; midgestation lethality (Huangfu et al. 2003) Jeune (Halbritter et al. 2013) Ttc26 (hop) IFT-B Partial loss…...
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...polydactyly (Huangfu et al. 2003) Ift172 IFT-B Loss of SHH-dependent ventral neural cell types; midgestation lethality (Huangfu...
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...2) (Huangfu et al. 2003)....
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...…midgestation lethality; polydactyly (Huangfu et al. 2003) Ift172 IFT-B Loss of SHH-dependent ventral neural cell types; midgestation lethality (Huangfu et al. 2003) Jeune (Halbritter et al. 2013) Ttc26 (hop) IFT-B Partial loss of function allele only; abnormal neural patterning (Swiderski et…...
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