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The '''hedgehog signaling pathway''' is one of the key regulators of animal development conserved from [[Drosophila|flies]] to [[Homo sapiens|humans]]. The pathway takes its name from its [[polypeptide]] ligand, an intercellular signaling molecule called Hedgehog (''Hh'') found in fruit flies of the genus [[Drosophila]]. ''Hh'' is one of Drosophila's segment polarity gene products, involved in establishing the basis of the fly [[body plan]]. The molecule remains important during later stages of [[embryogenesis]] and [[Metamorphosis (biology)|metamorphosis]].

Mammals have three Hedgehog homologues, of which [[Sonic hedgehog]] is the best studied. The pathway is equally important during vertebrate embryonic development. In [[knockout mice]] lacking components of the pathway, the [[brain]], [[skeleton]], [[musculature]], [[gastrointestinal tract]] and [[lungs]] fail to develop correctly. Recent studies point to the role of hedgehog signaling in regulating [[adult stem cell]]s involved in maintenance and regeneration of adult [[biological tissue|tissues]]. The pathway has also been implicated in the development of some [[cancer]]s. Drugs that specifically target hedgehog signaling to fight this disease are being actively developed by a number of [[pharmaceutical company|pharmaceutical companies]].

==Discovery==
[[Image:Denticlebands.png|thumb|right|'''Figure 1.''' Normal and Hedgehog mutant larvae.]]
In 1995 [[Edward B. Lewis]], [[Christiane Nüsslein-Volhard]] and [[Eric F. Wieschaus]] were awarded a [[Nobel Prize in Physiology or Medicine|Nobel Prize]] for their work producing and studying genetic mutations in [[Drosophila embryogenesis]].<ref name="Nobel">1995 Nobel Prize for discovery of [http://nobelprize.org/nobel_prizes/medicine/laureates/1995/press.html the genetic control of early embryonic development]</ref> In the 1970s, a fundamental problem in [[developmental biology]] was to understand how a relatively simple egg can give rise to a complex [[Segmentation (biology)|segmented]] body plan. Nüsslein-Volhard and Wieschaus attempted to isolate mutations in genes that control development of the segmented anteror-posterior body axis of the fly;<ref name="NVW1980">Nusslein-Volhard C and Wieschaus E (1980) "Mutations affecting segment number and polarity in Drosophila" in ''[[Nature (journal)|Nature]]'' Volume 287, pages 795-801.{{Entrez Pubmed|6776413}}</ref> their "saturation mutagenesis" technique resulted in the discovery of a group of genes involved in the development of [[Morphogenesis#Anterior posterior axis patterning in Drosophila|body segmentation]].

The ''Drosophila'' hedgehog gene was identified as one of several genes important for creating the differences between the anterior and posterior parts of individual body segments. Some hedgehog mutants result in abnormally-shaped embryos that are unusually short and stubby compared to [[wild type]] embryos. The function of the hedgehog segment polarity gene has been studied in terms of its influence on the normally polarized distribution of larval cuticular [[Dermal denticle|denticles]] as well as features on adult appendages such as legs and antennae.<ref name="Mohler">Mohler J (1988) "Requirements for hedgehog, a segmental polarity gene, in patterning larval and adult cuticle of Drosophila" in ''Genetics'' Volume 120, pages 1061-1072.{{Entrez Pubmed|3147217}}</ref> Rather than the normal pattern of denticles, hedgehog mutant larvae tend to have "solid lawns" of denticles (Figure 1). The appearance of the stubby and "hairy" larvae inspired the name 'hedgehog' (see: [[Hedgehog|Hedgehog, the animal]]).

==Fruit fly==
[[Image:HedgehogCiR.png|thumb|left|'''Figure 2'''. Production of the CiR transcriptional repressor when Hh is not bound to Patched. In the diagram, "P" represents  [[phosphate]].]]
[[Image:HedgehogActive.png|thumb|right|'''Figure 3'''. When Hh is bound to Patched (PTCH), Ci protein is able to act as a transcription factor in the nucleus.]]
===Mechanism===
Insect cells express a full size zinc-finger [[transcription factor]] Cubitus interruptus (Ci), which forms a complex with the [[kinesin]]-like protein Costal-2 (Cos2) and is localized in the cytoplasm bound to cellular [[microtubule]]s (Figure 2). The complex targets the Ci protein for [[proteosome]]-dependent cleavage, which generates a fragment (CiR) that functions as a transcriptional repressor. CiR builds up in the cell and [[diffusion|diffuses]] into the [[cell nucleus|nucleus]], where it acts as a co-[[repressor]] for Hh target genes<ref name="Collins2005">{{cite online journal| author=Collins  RT and Cohen SM | year=2005 | title=A genetic screen in Drosophila for identifying novel components of the hedgehog signaling pathway | journal=Genetics | volume=170 | pages=173-184}} {{Entrez Pubmed|15744048}}</ref>. The steps leading to Ci protein proteolysis include [[phosphorylation]] of Ci protein by several [[protein kinase]]s; [[CAMP-dependent protein kinase|PKA]], [[GSK-3|GSK3β]] and [[Casein kinase 1|CK1]] (Figure 2)<ref name="Lum">{{cite online journal |author=Lum L and Beachy PA | year=2004 | title=The Hedgehog response network: sensors, switches, and routers | journal=Science | volume=304 | pages=1755-1759}} {{Entrez Pubmed|15205520}}</ref>. The ''Drosophila'' protein Slimb is part of an [[Ubiquitin#Ubiquitylation|SCF complex]] that targets proteins for [[Ubiquitin|ubiquitylation]]. Slimb binds to phosphorylated Ci protein.

In the absence of Hh (Figure 2), a cell-surface transmembrane protein called [[PTCH1|Patched]] (PTCH) acts to prevent high expression and activity of a [[7TM Receptor|7 membrane spanning receptor]]<ref name="Chen">Chen W, Ren XR, Nelson CD, Barak LS, Chen JK, Beachy PA, de Sauvage F and Lefkowitz RJ (2004) "Activity-dependent internalization of smoothened mediated by beta-arrestin 2 and GRK2" in
''[[Science (journal)|Science]]'' Volume 306, pages 2257-2260.{{Entrez Pubmed|15618519}}</ref> called [[Smoothened]] (SMO). Patched has sequence similarity to known membrane transport proteins. When extracellular Hh is present (Figure 3), it binds to and inhibits Patched, allowing Smoothened to accumulate and inhibit the proteolytic cleavage of the Ci protein. This process most likely involves the direct interaction of Smoothened and Costal-2 and may involve sequestration of the Ci protein-containing complex to a microdomain where the steps leading to Ci protein proteolysis are disrupted<ref name="Collins2005" />. The mechanism by which Hh binding to Patched leads to increased levels of Smoothened is not clear (Step 1 in Figure 3). Following binding of Hh to Patched, Smoothened levels increase greatly over the level maintained in cells when Patched is not bound to Hh<ref name="Alcedo2000">Alcedo J, Zou Y and Noll M (2000) "Posttranscriptional regulation of smoothened is part of a self-correcting mechanism in the Hedgehog signaling system" in ''Molecular Cell'' Volume 6, pages 457-465. {{Entrez Pubmed|10983991}}</ref>. It has been suggested that phosphorylation of Smoothened plays a role in Hh-dependent regulation of Smoothened levels<ref name="Apionishev ">Apionishev S, Katanayeva NM, Marks SA, Kalderon D and Tomlinson A (2005) "Drosophila Smoothened phosphorylation sites essential for Hedgehog signal transduction" ''Nature Cell Biology'' Volume 7, page 86-92. {{Entrez Pubmed|15592457}}</ref>.

In cells with Hh-activated Patched (Figure 3), the intact Ci protein accumulates in the cell cytoplasm and levels of CiR decrease, allowing transcription of some genes such as decapentaplegic (dpp, a member of the [[Bone morphogenetic protein|BMP]] growth factor family). For other Hh-regulated genes, expression requires not only loss of CiR but also the positive action of uncleaved Ci acting as a transcriptional activator <ref name="Lum" />. Costal-2 is normally important for holding Ci protein in the cytoplasm, but interaction of Smoothened with Costal-2 allows some intact Ci protein to go to the nucleus. The ''Drosophila'' protein Fused (Fu in Figure 3) is a protein kinase that binds to Costal-2. Fused can inhibit Suppressor of Fused (SUFU), which in turn interacts with Ci to regulate gene transcription in some cell types<ref name="Ho2005">Ho KS, Suyama K, Fish M and Scott MP (2005) "Differential regulation of Hedgehog target gene transcription by Costal2 and Suppressor of Fused" in ''Development'' Volume 132, pages 1401-1412. {{Entrez Pubmed|15750186}}</ref>.

===Role===
[[Image:Wingless.png|thumb|left|'''Figure 4.''' Interactions between Wingless and Hedgehog.]]
Hedgehog has roles in larval body segment development and in formation of adult appendages. During the formation of body segments in the developing ''Drosophila'' embryo, stripes of cells that synthesize the [[transcription factor]] Engrailed can also express the cell-to-cell signaling protein Hedgehog (green in Figure 4). Hedgehog is not free to move very far from the cells that make it and so it only activates a thin stripe of cells adjacent to the Engrailed-expressing cells. Only cells to one side of the Engrailed-expressing cells are competent to respond to Hedgehog following interaction of Hh with the receptor protein Patched (blue in Figure 4). 

Cells with Hh-activated Patched receptor synthesize the [[Wnt signaling pathway|Wingless]] protein (red in Figure 4). If a ''Drosophila'' embryo is altered so as to produce Hh in all cells, all of the competent cells respond and form a broader band of Wingless-expressing cells in each segment. The ''wingless'' gene has an upstream transcription regulatory region that binds the Ci transcription factor in a Hh-dependent fashion resulting in an increase in ''wingless'' transcription (interaction 2 in Figure 3) in a stripe of cells adjacent to the stripe of Hh-producing cells<ref name="VonOhlen"> Von Ohlen T, Lessing D, Nusse R and Hooper JE (1997) "Hedgehog signaling regulates transcription through cubitus interruptus, a sequence-specific DNA binding protein" in ''[[Proceedings of the National Academy of Sciences|Proceedings of the National Academy of Sciences of the United States of America]]'' Volume 94, pages 2404-2409. {{Entrez Pubmed|9122207}}</ref>.

Wingless protein acts as an extracelluar signal and patterns the adjacent rows of cells by activating its cell surface receptor Frizzled. Wingless acts on Engrailed-expressing cells to stabilize the stripes of Engrailed expression. Wingless is a member of the [[Wnt signaling pathway|Wnt]] family of cell-to-cell signaling proteins. The reciprocal signaling by Hedgehog and Wingless stabilizes the boundary between parasegments (Figure 4, top). The effects of Wingless and Hedgehog on other stripes of cells in each segment establishes a positional code that accounts for the distinct anatomical features along the anterior-posterior axis of the segments <ref name="Ingham2001">{{cite online journal | author=Ingham PW and McMahon AP | year=2001 | title=Hedgehog signaling in animal development: paradigms and principles | journal=Genes Dev | volume=15 | issue=23 | pages=3059-3087}} {{Entrez Pubmed|11731473}}<BR>Ingham and McMahon review the roles of hedgehog proteins in development; [http://www.genesdev.org/cgi/content/full/15/23/3059/T1 this table] provides a summary.</ref>.

The Wingless protein is called "wingless" because of the [[phenotype]] of some ''wingless'' fly mutants. Wingless and Hedgehog functioned together during [[Metamorphosis (biology)|metamorphosis]] to coordinate wing formation. Hedgehog is expressed in the posterior part of developing ''Drosophila'' limbs. Hedgehog also participates in the coordination of eye, brain, gonad, gut and tracheal development.

==Vertebrates==
===Mechanism===
[[Image:Sonic_hedgehog_pathway.png|thumb|right|300px|'''Figure 5.''' Overview of [[Sonic hedgehog]] signaling. 

[http://pid.nci.nih.gov/search/pathway_landing.shtml?pathway_id=200047&source=NCI-Nature%20Curated&what=graphic&gif=on&ppage=1 Click here for a more detailed diagram] ]]
[[Sonic hedgehog]] (SHH) is the best studied [[ligand]] of the vertebrate pathway. Most of what is known about hedgehog signaling has been established by studying SHH. It is [[translation (genetics)|translated]] as a ~45kDa precursor and undergoes [[autocatalytic]] processing to produce an ~20kDa N-terminal signaling domain (referred to as SHH-N) and a ~25kDa C-terminal domain with no known signaling role (1 on figure 5). During the cleavage, a [[cholesterol]] molecule is added to the carboxyl end of the N-terminal domain, which is involved in trafficking, secretion and receptor interaction of the ligand. SHH can signal in an [[autocrine]] fashion, affecting the cells in which it is produced. [[Secretion]] and consequent [[paracrine]] hedgehog signaling require the participation of Dispatched protein(2). 

When SHH reaches its target cell, it binds to the [[PTCH1|Patched-1]] (PTCH1) receptor(3). In the absence of ligand, PTCH1 inhibits Smoothened (SMO), a downstream protein in the pathway(4). It has been suggested that SMO is regulated by a small molecule, the cellular localisation of which is controlled by PTCH<ref>{{cite online journal| author=Taipale J, Cooper MK, Maiti T, and Beachy PA | year=2002 | title=Patched acts catalytically to suppress the activity of Smoothened | journal=Nature | volume=418 | pages=892-897 | id=12192414}}</ref>. PTCH1 has homology to Niemann-Pick disease, type C1 ([[NPC1]]) that is known to transport lipophilic molecules across a membrane. <ref>{{ cite online journal| author=Davies,J.P,J.P., Chen,F.W., and Ioannou,Y.A | year=2000 | title=Transmembrane molecular pump activity of Niemann-Pick C1 protein.| journal=Science| volume=5500 | pages=2295-2298 | id=1125140}}</ref> PTCH1 has a sterol sensing domain (SSD), which has been shown to be essential for suppression of Smo activity. <ref>{{ cite online journal| author=Strutt,H., Thomas,C., Nakano,Y., Stark,D., Neave,B., Taylor,A.M., and Ingham,P.W. | year=2001 | title=Mutations in the sterol-sensing domain of Patched suggest a role for vesicular trafficking in Smoothened regulation.| journal=Curr Biol.| volume=8 | pages=608-613| id=11369206}}</ref>A current theory of how PTCH regulates SMO is by removing oxysterols from SMO. PTCH acts like a sterol pump and remove oxysterols that have been created by 7-dehydrocholesterol reductase. <ref>{{cite online journal| author=Corcoran RB, Scott MP | year=2006 | title= Oxysterols stimulate Sonic hedgehog signal transduction and proliferation of medulloblastoma cells.| journal=Proc Natl Acad Sci U S A | volume=418 | pages=8408-8413 | id=167075754}}</ref>Upon binding of a Hh protein or a mutation in the SSD of PTCH the pump is turned off allowing oxysterols to accumulate around SMO.  
[[Image:PTCH_Neg_SMO.png|thumb|left|300px|'''Figure 6.''' Overview of PTCH/SMO signaling.]] 
[[Image:PTCH_Pos_SMO.png|thumb|left|300px|'''Figure 7.''' Overview of PTCH/SMO signaling.]]
This accumulation of sterols allows SMO to become active or stay on the membrane for a longer period of time.  This hypothesis is supported by the existence of a number of small molecule [[agonist]]s and [[antagonist]]s of the pathway that act on SMO. The binding of SHH relieves SMO inhibition, leading to activation of the GLI transcription factors(5): the [[Activator (genetics)|activators]] [[Gli1]] and [[Gli2]] and the [[repressor]] [[Gli3]]. The sequence of molecular events that connect SMO to GLIs is poorly understood. Activated GLI accumulates in the nucleus(6) and controls the transcription of hedgehog target genes(7). PTCH1 has recently been reported to repress transcription of hedgehog target genes through a mechanism independent of [[Smoothened]].<ref name="Rahnama">{{cite online journal| author=Rahnama F, Shimokawa T, Lauth M, Finta C, Kogerman P, Teglund S, Toftgard R and Zaphiropoulos PG | year=2006 | title=Inhibition of GLI1 gene activation by Patched1 | journal=Biochem J | volume=394 | pages=19-26}} {{Entrez Pubmed|16229683}}</ref>

In addition to PTCH1, mammals have another hedgehog receptor PTCH2 whose sequence identity with PTCH1 is 54%.<ref name="PTCH2">{{cite online journal| author=Carpenter D, Stone DM, Brush J, Ryan A, Armanini M, Frantz G, Rosenthal A and de Sauvage FJ | year=1998 | title=Characterization of two patched receptors for the vertebrate hedgehog protein family | journal=PNAS | volume=95 | issue=23 | pages=13630-13634}} {{Entrez Pubmed|9811851}}</ref> All three mammalian hedgehogs bind both receptors with similar affinity, so PTCH1 and PTCH2 cannot discriminate between the ligands. They do, however, differ in their expression patterns. PTCH2 is expressed at much higher levels in the [[testis]] and mediates desert hedgehog signaling there.<ref name="PTCH2" /> It appears to have a distinct downstream signaling role from PTCH1. In the absence of ligand binding PTCH2 has a decreased ability to inhibit the activity of SMO.<ref>{{cite online journal| author=Rahnama F, Toftgard R and Zaphiropoulos PG | year=2004 | title=Distinct roles of PTCH2 splice variants in Hedgehog signalling | journal=Biochem J | volume=378(Pt 2) | pages=325-334}} {{Entrez Pubmed|14613484}}</ref> Furthermore, overexpression of PTCH2 does not replace mutated PTCH1 in [[basal cell carcinoma]].<ref>{{cite online journal| author=Zaphiropoulos PG, Unden AB, Rahnama F, Hollingsworth RE and Toftgard R | year=1999 | title=PTCH2, a novel human patched gene, undergoing alternative splicing and up-regulated in basal cell carcinomas | journal=Cancer Res | volume=59 | issue=4 | pages=787-792}} {{Entrez Pubmed|10029063}}</ref>

In invertebrates, just as in ''Drosophila'', the binding of hedgehog to PTCH leads to internalisation and sequestration of the ligand.<ref>{{cite online journal| author=Incardona JP, Lee JH, Robertson CP, Enga K, Kapur RP and Roelink H | year=2000 | title=Receptor-mediated endocytosis of soluble and membrane-tethered Sonic hedgehog by Patched-1 | journal=PNAS | volume=97 | issue=22 | pages=12044-12049}} {{Entrez Pubmed|11027307}}</ref> Consequently ''in vivo'' the passage of hedgehog over a receptive field that expresses the receptor leads to attenuation of the signal, an effect called ligand-dependent antagonism (LDA). In contrast to ''Drosophila'', vertebrates possess another level of hedgehog regulation through LDA mediated by Hh-interacting protein 1 (HHIP1). HHIP1 also sequesters hedgehog ligands, but unlike PTCH, it has no effect on the activity of SMO.<ref>{{cite online journal | author=Jeong J and McMahon AP | year=2005 |title=Growth and pattern of the mammalian neural tube are governed by partially overlapping feedback activities of the hedgehog antagonists patched 1 and Hhip1 | journal=Development | volume=132 | issue=1 | pages=143-154}} {{Entrez Pubmed|15576403}}</ref>

===Role===
Members of the hedgehog family play key roles in a wide variety of developmental processes.<ref name="Ingham2001"/> One of the best studied examples is the action of [[Sonic hedgehog]] during development of the vertebrate limb. The classic experiments of Saunders and Gasseling in 1968 on the development of the chick limb bud formed the basis of the [[morphogen]] concept. They showed that identity of the digits in the chick limb was determined by a diffusible factor produced by the zone of polarizing activity (ZPA), a small region of tissue at the [[posterior]] margin of the limb. Mammalian development appeared to follow the same pattern. This diffusible factor was later shown to be [[Sonic hedgehog]]. However, precisely how SHH determines digit identity remained elusive until recently. The current model, proposed by Harfe ''et al''<ref>{{cite online journal | author=Harfe BD, Scherz PJ, Nissim S, Tian H, McMahon AP and Tabin CJ | year=2004 | title=Evidence for an expansion-based temporal Shh gradient in specifying vertebrate digit identities | journal=Cell | volume=118 | issue=4 | pages=517-528}} {{Entrez Pubmed|15315763}}</ref>, states that both the concentration and the time of exposure to SHH determines, which digit the tissue will develop into in the [[mouse]] [[embryo]] (figure 6).

[[Image:Limb bud.png|thumb|left|300px|'''Figure 6.''' [[Sonic hedgehog]] specifies digit identity in mammalian development.]]
Digits V, IV and part of III arise directly from cells that express SHH during [[embryogenesis]]. In these cells SHH signals in an [[autocrine]] fashion and these digits develop correctly in the absence of DISP, which is required for extracellular diffusion of the ligand. These digits differ in the length of time that SHH continues to be expressed. The most posterior digit V develops from cells that express the ligand for the longest period of time. Digit IV cells express SHH for a shorter time, and digit III cells shorter still. Digit II develops from cells that are exposed to moderate concentrations of extracellular SHH. Finally, digit I development does not require SHH. It is, in a sense, the default program of limb bud cells.

Hedgehog signaling remains important in the adult. [[Sonic hedgehog]] has been shown to promote the proliferation of [[adult stem cell]]s from various tissues, including primitive [[hematopoietic]] cells<ref>{{cite online journal | author=Bhardwaj G, Murdoch B, Wu D, Baker DP, Williams KP, Chadwick K, Ling LE, Karanu FN and Bhatia M | year=2001 | title=Sonic hedgehog induces the proliferation of primitive human haematopoetic cells via BMP regulation | journal=Nat Immunol | volume=2 | issue=2 | pages=172-180}} {{Entrez Pubmed|11175816}}</ref>, mammary<ref>{{cite online journal | author=Liu S, Dontu G, Mantle ID, Patel S, Ahn NS, Jackson KW, Suri P and Wicha MS | year=2006 | title=Hedgehog signaling and Bmi-1 regulate self-renewal of normal and malignant human mammary stem cells | journal=Cancer Res | volume=66 | issue=12 | pages=6063-6071}} {{Entrez Pubmed|16778178}}</ref> and neural<ref>{{cite online journal | author=Ahn S and Joyner AL | year=2005 | title=In vivo analysis of quiescent adult neural stem cells responding to Sonic hedgehog | journal=Nature | volume=437 | issue=7060 | pages=894-897}} {{Entrez Pubmed|16208373}}</ref> stem cells. Activation of the hedgehog pathway is required for transition of the [[hair follicle]] from the resting to the growth phase.<ref>{{cite online journal | author=Paladini RD, Saleh J, Qian C, Xu GX and Rubin LL | year=2005 | title=Modulation of hair growth with small molecule agonists of the hedgehog signaling pathway | journal=J Invest Dermatol | volume=125 | issue=4 | pages=638-646}} {{Entrez Pubmed|16185261}}</ref> Curis Inc. together with [[Procter & Gamble]] are developing a hedgehog [[agonist]] to be used as a drug for treatment of hair growth disorders.<ref>[http://www.curis.com/product_detail.php?id=5 http://www.curis.com/product_detail.php?id=5], retrieved on 25 June 2006.</ref>
This failed due to toxicities found in animal models.<ref>[http://phx.corporate-ir.net/phoenix.zhtml?c=123198&p=irol-newsArticle&ID=997941&highlight=], retrieved on May 9, 2007.</ref>

===Human disease===
Disruption of hedgehog signaling during embryonic development, either through deleterious mutation or consumption of [[teratogen]]s by the gestating mother, can lead to severe developmental abnormalities. [[Holoprosencephaly]], the failure of the embryonic [[prosencephalon]] to divide to form cerebral hemispheres, occurs with a frequency of about 1 in 16,000 live births and about 1 in 200 spontaneous abortions in humans and is commonly linked to mutations in genes involved in the hedgehog pathway, including ''SHH'' and ''PTCH''.<ref>OMIM Holoprosencephaly, updated 16 May 2006 {{OMIM|236100}}</ref> [[Cyclopia]], one of the most severe defects of [[holoprosencephaly]], results if the pathway inhibitor [[cyclopamine]] is consumed by gestating mammals.<ref>{{cite online journal| author=Keeler RF | year=1978 | title=Cyclopamine and related steroidal alkaloid teratogens: their occurrence, structural relationship, and biologic effects | journal=Lipids | volume=13 | issue=10 | pages=708-715}} {{Entrez Pubmed|723484}}</ref>

Activation of the hedgehog pathway has been implicated in the development of [[cancer]]s in various organs, including [[brain]], [[lung]], [[mammary gland]], [[prostate]] and [[skin]]. [[Basal cell carcinoma]], the most common form of cancerous [[malignancy]], has the closest association with hedgehog signaling. Loss-of-function mutations in [[PTCH1|Patched]] and activating mutations in [[Smoothened]] have been identified in patients with this disease.<ref>{{cite online journal| author=Xie J, Murone M, Luoh SM, Ryan A, Gu Q, Zhang C, Bonifas JM, Lam CW, Hynes M, Goddard A et al | year=1998 | title=Activating Smoothened mutations in sporadic basal-cell carcinoma | journal=Nature | volume=391 | issue=6662 | pages=90-92}} {{Entrez Pubmed|9422511}}</ref> Abnormal activation of the pathway probably leads to development of disease through transformation of [[adult stem cell]]s into [[cancer stem cell]]s that give rise to the tumor. Cancer researchers hope that specific inhibitors of hedgehog signaling will provide an efficient therapy for a wide range of malignancies.<ref>{{cite online journal| author=Chen JK, Taipale J, Young KE, Maiti T and Beachy PA | year=2002 | title=Small molecule modulation of Smoothened activity | journal=PNAS | volume=99 | issue=22 | pages=14071-14076}} {{Entrez Pubmed|12391318}}</ref>

Biotech companies are also attempting to turn this pathway on after a patient has a stroke or heart attack.  Since the pathway has been implicated in a number of lethal cancers Curis and Wyeth have devised a stable hedgehog protein that can cross the blood brain barrier.<ref name="Curis">Curis pipeline details [http://www.curis.com Stroke and Heart Attack]</ref>  In pre-clinical animal models it has shown that the pathway is up regulated upon a stroke or heart attack event.  The pathway provides a protective barrier against cell death and ischemia.  Agonizing the pathway this way allows the PTCH to be up regulated providing a negative feedback system.  This might help minimize the side effects.  

'''Targeting the Hedgehog Pathway'''

The most common way to target this pathway is modulate SMO.  Antagonist and agonist of SMO have already shown to effect the pathway regulation downstream.  PTCH<ref>{{cite online journal| author=Nakamura M, Kubo M, Yanai K, Mikami Y, Ikebe M, Nagai S, Yamaguchi K, Tanaka M, Katano M. | year=2007 | title=Anti-patched-1 antibodies suppress hedgehog signaling pathway and pancreatic cancer proliferation. | journal=Anticancer Res.  | Volume | issue=6A | pages=3743-7.}} {{Entrez Pubmed|17970037}}</ref> and Gli3 (5E1)<ref>{{cite online journal| author=Hunt R, Bragina O, Drews M, Kasak L, Timmusk S, Valkna A, Kogerman P, Järvekülg L. | year=2007 | title=Generation and characterization of mouse monoclonal antibody 5E1 against human transcription factor GLI3. | journal=Hybridoma  | Volume | issue=4 | pages=231-40}} {{Entrez Pubmed|17725385}}</ref> antibodies are also a way to regulate the pathway.  A downstream effector and strong transcriptional activator siRNA Gli1 has been used to inhibit cell growth and promote apoptosis<ref>{{cite online journal| author=Stecca B, Mas C, Ruiz i Altaba A. | year=2005 | title=Interference with HH-GLI signaling inhibits prostate cancer. | journal=Trends Mol Med.  | Volume | issue=5 | pages=199-203}} {{Entrez Pubmed|15882606}}</ref>.   
    

'''Hedgehog Pathway and Metastasis'''

Activation of the Hedgehog pathway leads to an increase in Snail protein expression and a decrease in [[E-cadherin]] and [[Tight junction|Tight Junctions]]. <ref>{{cite online journal| author=Xingnan Li, Wentao Deng, Clinton D. Nail, Sarah K. Bailey, Matthias H. Kraus, J. Michael Ruppert, and Susan M. Lobo-Ruppert1 | year=2006 | title=Snail induction is an early response to Gli1 that determines the efficiency of epithelial transformation | journal=Oncogene. | volume=99 | issue=4 | pages=609-621}} {{Entrez Pubmed|16158046}}</ref>.

'''Hedgehog Pathway and Tumor Regulation'''

Activation of the Hedgehog pathway leads to an increase in Angiogenic Factors (angiopoietin-1 and angiopoietin-2)<ref>{{cite online journal| Lee SW, Moskowitz MA, Sims JR.| year=2007 | title=Sonic hedgehog inversely regulates the expression of angiopoietin-1 and angiopoietin-2 in fibroblasts. | journal=Int J Mol Med. | volume=99 | issue=3 | pages=445-51}} {{Entrez Pubmed|17273793}}</ref>, Cyclins (cyclin D1 and B1))<ref>{{cite online journal| Adolphe C, Hetherington R, Ellis T, Wainwright B.| year=2006 | title=Patched1 functions as a gatekeeper by promoting cell cycle progression. | journal= Cancer Res. | volume=99 | issue=5 | pages=2081-8.}} {{Entrez Pubmed|16489008}}</ref>, anti-apoptotic genes and a decrease in apoptotic genes (Fas)<ref>{{cite online journal| Athar M, Li C, Tang X, Chi S, Zhang X, Kim AL, Tyring SK, Kopelovich L, Hebert J, Epstein EH Jr, Bickers DR, Xie J.| year=2004 | title=Inhibition of smoothened signaling prevents ultraviolet B-induced basal cell carcinomas through regulation of Fas expression and apoptosis. | journal= Cancer Res. | volume=99 | issue=5 | pages=7545-52.}} {{Entrez Pubmed|15492281}}</ref>.

==Evolution==
[[Image:Hedgehog ligand evolution.png|thumb|right|300px|'''Figure 7'''. [[Phylogenetic]] relationship of hedgehog ligands (based on Ingham and McMahon, 2001).]]
Hedgehog-like genes, 2 Patched homologs and Patched-related genes exist in the worm ''[[Caenorhabditis elegans|C. elegans]]'' <ref name="Zugasti2005">Zugasti O, Rajan J and Kuwabara PE (2005) "The function and expansion of the Patched- and Hedgehog-related homologs in C. elegans" in ''Genome Research'' Volume 15, pages 1402-1410.{{Entrez Pubmed|16204193}}</ref> <ref name="Novatchkova2005">Novatchkova M, Wildpaner M, Schweizer F and Eisenhaber F (2005) "PhyloDome--visualization of taxonomic distributions of domains occurring in eukaryote protein sequence sets" in ''Nucleic Acids Research'' Volume 33(Web Server issue):W121-5.{{Entrez Pubmed|15980439}}</ref>. These genes have been shown to code for proteins that have roles in ''C. elegans'' development<ref name="Zugasti2005" />. The hedgehog-like and Patched-related gene families are very large and function without the need for a Smoothened homolog, suggesting a distinct pattern of selection for cholesterol modification and sensing mechanisms in [[Body cavity|coelomate]] and pseudo-coelomate lineages<ref name="Novatchkova2005" />.

[[Lancelet]]s, which are primitive [[chordate]]s, possess only one homologue of Drosophila ''Hh'' (figure 7). Vertebrates, on the other hand, have several hedgehog ligands that fall within three subgroups - ''desert'', ''indian'' and ''sonic'', each represented by a single mammalian gene. This is probably a consequence of the two genome duplications that occurred early in the vertebrate evolutionary history.<ref>{{cite online journal| author=Wada H, and Makabe K | title=Genome duplications of early vertebrates as a possible chronicle of the evolutionary history of the neural crest | journal=International Journal of Biological Science | volume=2 | year=2006 | issue=3 | pages=133-141 | id=16763673}}</ref> Two such events would have produced four homologous genes, one of which must have been lost. Desert hedgehogs are the most closely related to Drosophila ''Hh''. Additional gene duplications occurred within some species<ref name="Ingham2001" /> such as the [[zebrafish]] ''Danio rerio'', which has an additional ''tiggywinkle hedgehog'' gene in the ''sonic'' group. Various vertebrate lineages have adapted hedgehogs to unique developmental processes. For example, a homologue of the ''X.laevis'' ''banded hedgehog'' is involved in regeneration of the [[salamander]] limb.<ref>{{cite online journal| author=Stark DR, Gates PB, Brockes JP and Ferretti P | year=1998 | title=Hedgehog family member is expressed throughout regenerating and developing limbs | journal=Dev Dyn | volume=212 | issue=3 | pages=352-363}} {{Entrez Pubmed|9671939}}</ref>

''shh'' has undergone accelerated evolution in the primate lineage leading to humans.<ref>{{cite online journal| author=Dorus S, Anderson JR, Vallender EJ, Gilbert SL, Zhang L, Chemnick LG, Ryder OA, Li W and Lahn BT | year=2006 | title=Sonic Hedgehog, a key development gene, experienced intensified molecular evolution in primates | journal=Human Molecular Genetics | volume=15 | issue=13 | pages=2031-2037 | id=16687440}}</ref> Dorus ''et al'' hypothesise that this allowed for more complex regulation of the protein and may have played a role in the increase in volume and complexity of the human brain.

The frizzled family of [[Wnt signaling pathway|WNT]] receptors have some sequence similarity to Smoothened <ref name="Graul2001">Graul RC and Sadee W (2001) "Evolutionary relationships among G protein-coupled receptors using a clustered database approach" in ''AAPS PharmSci'' Volume 3 article E12.{{Entrez Pubmed|11741263}}</ref>. However, G proteins have been difficult to link to the function Smoothened. Smoothened seems to be a functionally divergent member of the G protein coupled receptor super family. Other similarities between the WNT and Hh signaling pathways have been reviewed<ref name="Nusse2003">Nusse R (2003) "Wnts and Hedgehogs: lipid-modified proteins and similarities in signaling mechanisms at the cell surface" in ''Development'' Volume 130, pages 5297-5305. {{Entrez Pubmed|14530294}}</ref>. Nusse observed that, "a signaling system based on lipid-modified proteins and specific membrane translocators is ancient, and may have been the founder of the Wnt and Hh signaling systems".

It has been suggested that invertebrate and vertebrate signaling downstream from Smoothened has diverged significantly<ref name="Varjosalo2006">Varjosalo M, Li SP and Taipale J (2006) "Divergence of hedgehog signal transduction mechanism between Drosophila and mammals" in ''Developmental cell'' Volume 10, pages 177-186.{{Entrez Pubmed|16459297}}.</ref>. The role of Suppressor of Fused (SUFU) has been enhanced in vertebrates compared to ''Drosophila'' where its role is relatively minor. Costal-2 is particularly important in ''Drosophila''. The protein kinase Fused is a regulator of SUFU in ''Drosophila'', but may not play a role in the Hh pathway of vertebrates<ref name="Chen2006">Chen MH, Gao N, Kawakami T and Chuang PT (2005) "Mice deficient in the fused homolog do not exhibit phenotypes indicative of perturbed hedgehog signaling during embryonic development" in ''Molecular Cell Biology'' Volume 25, pages 7042-7053.{{Entrez Pubmed|16055716}}.</ref>. In vertebrates, Hh signalling has been heavily coupled to [[cilia]] <ref name="Huangfu2006">Huangfuand D and Anderson KV (2006) "Signaling from Smo to Ci/Gli: conservation and divergence of Hedgehog pathways from Drosophila to vertebrates" in ''Development'' Volume 133, pages 3-14. {{Entrez Pubmed|16339192}}.</ref>

==Research==
Research is currently being done by Robbie Bayly at the [[University of Texas]] at Austin to further understand the effects of the hedgehog signaling pathway on embryonic chicken brain matter.

[http://www.hhmi.org/jessell/jessell_isi/ Thomas Jessel]  --Hedgehog pathway and Cell Differentiation

[http://www.hhmi.org/research/investigators/scottm.html Matthew Scott]  --Cholesterol, Embryonic development, and Disease

[http://cancer.stanford.edu/features/faculty/beachy.html Philip Beachy Profile Stanford]  --Hedgehog Signaling in Development and Disease

[http://chen.stanford.edu/research.html James Chen]  --Small molecule modulation of Hedgehog signaling

==See also==
*[[Sonic hedgehog]], best studied ligand of the vertebrate pathway
*[[Smoothened]], the conserved [[GPCR]] component of the pathway
*[[Cyclopamine]], a small molecule inhibitor of Hh signaling

==External links==
* http://hedgehog.sfsu.edu (Hedgehog Pathway Database)

==References==
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{{reflist|2}}
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{{Cell signaling}}
{{Signaling proteins}}

[[Category:Cell signaling]]
[[Category:Hedgehog signaling pathway]]

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