Hedgehog signaling pathway
Encyclopedia
In a growing embryo, cells develop differently in the head or tail end of the embryo, the left or right, and other positions. They also form segments which develop into different body parts. The hedgehog signaling pathway gives cells information that they need to make the embryo develop properly. Different parts of the embryo have different concentrations of hedgehog signaling proteins. The pathway also has roles in the adult. When the pathway malfunctions, it can result in diseases like basal cell carcinoma
Basal cell carcinoma
Basal-cell carcinoma is the most common type of skin cancer. It rarely metastasizes or kills. However, because it can cause significant destruction and disfigurement by invading surrounding tissues, it is still considered malignant. Statistically, approximately 3 out of 10 Caucasians may develop a...

.

The hedgehog signaling pathway is one of the key regulators of animal development and is present in all bilaterians
Bilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...

 . The pathway takes its name from its polypeptide ligand, an intercellular signaling molecule called Hedgehog (Hh) found in fruit flies of the genus Drosophila
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...

. Hh is one of Drosophila's segment polarity gene products, involved in establishing the basis of the fly body plan
Body plan
A body plan is the blueprint for the way the body of an organism is laid out. An organism's symmetry, its number of body segments and number of limbs are all aspects of its body plan...

. The molecule remains important during later stages of embryogenesis
Embryogenesis
Embryogenesis is the process by which the embryo is formed and develops, until it develops into a fetus.Embryogenesis starts with the fertilization of the ovum by sperm. The fertilized ovum is referred to as a zygote...

 and metamorphosis.

Mammals have three Hedgehog homologues, of which Sonic hedgehog
Sonic hedgehog
Sonic hedgehog homolog is one of three proteins in the mammalian signaling pathway family called hedgehog, the others being desert hedgehog and Indian hedgehog . SHH is the best studied ligand of the hedgehog signaling pathway. It plays a key role in regulating vertebrate organogenesis, such as...

 is the best studied. The pathway is equally important during vertebrate embryonic development. In knockout mice lacking components of the pathway, the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

, skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...

, musculature, gastrointestinal tract
Gastrointestinal tract
The human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus. ....

 and lungs fail to develop correctly. Recent studies point to the role of hedgehog signaling in regulating adult stem cell
Adult stem cell
Adult stem cells are undifferentiated cells, found throughout the body after embryonic development, that multiply by cell division to replenish dying cells and regenerate damaged tissues...

s involved in maintenance and regeneration of adult tissues
Biological tissue
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...

. The pathway has also been implicated in the development of some cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...

s. Drugs that specifically target hedgehog signaling to fight this disease are being actively developed by a number of pharmaceutical companies
Pharmaceutical company
The pharmaceutical industry develops, produces, and markets drugs licensed for use as medications. Pharmaceutical companies are allowed to deal in generic and/or brand medications and medical devices...

.

Discovery

In the 1970s, a fundamental problem in developmental biology
Developmental biology
Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis", which is the process that gives rise to tissues, organs and anatomy.- Related fields of study...

 was to understand how a relatively simple egg can give rise to a complex segmented
Segmentation (biology)
Segmentation in biology refers to either a type of gastrointestinal motility or the division of some animal and plant body plans into a series of repetitive segments. This article will focus on the segmentation of animal body plans, specifically using the examples of the phyla Arthropoda,...

 body plan. In the late 1970s Christiane Nüsslein-Volhard
Christiane Nüsslein-Volhard
Christiane Nüsslein-Volhard is a German biologist who won the Albert Lasker Award for Basic Medical Research in 1991 and the Nobel Prize in Physiology or Medicine in 1995, together with Eric Wieschaus and Edward B...

 and Eric Wieschaus isolated mutations in genes that control development of the segmented anterior-posterior body axis of the fly; their "saturation mutagenesis" technique resulted in the discovery of a group of genes involved in the development of body segmentation. In 1995, they shared the Nobel Prize
Nobel Prize in Physiology or Medicine
The Nobel Prize in Physiology or Medicine administered by the Nobel Foundation, is awarded once a year for outstanding discoveries in the field of life science and medicine. It is one of five Nobel Prizes established in 1895 by Swedish chemist Alfred Nobel, the inventor of dynamite, in his will...

 with Edward B. Lewis
Edward B. Lewis
- External links :* *...

 for their work studying genetic mutations in Drosophila embryogenesis
Drosophila embryogenesis
Drosophila embryogenesis, the process by which Drosophila embryos form, is a favorite model system for geneticists and developmental biologists studying embryogenesis. The small size, short generation time, and large brood size make it ideal for genetic studies. Transparent embryos facilitate...

.

The Drosophila hedgehog (hh) gene was identified as one of several genes important for creating the differences between the anterior and posterior parts of individual body segments. The fly hh gene was independently cloned in 1992 by the labs of Jym Mohler, Philip Beachy, and Thomas B. Kornberg
Thomas B. Kornberg
Thomas Bill Kornberg is an American biochemist who was the first person to purify and characterise DNA polymerase II and DNA polymerase III. He is currently a Professor of biochemistry and biophysics at the University of California, San Francisco working on Drosophila melanogaster development.His...

. Some hedgehog mutants result in abnormally-shaped embryos that are unusually short and stubby compared to wild type
Wild type
Wild type refers to the phenotype of the typical form of a species as it occurs in nature. Originally, the wild type was conceptualized as a product of the standard, "normal" allele at a locus, in contrast to that produced by a non-standard, "mutant" allele...

 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 denticles as well as features on adult appendages such as legs and antennae. 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, the animal
Hedgehog
A hedgehog is any of the spiny mammals of the subfamily Erinaceinae and the order Erinaceomorpha. There are 17 species of hedgehog in five genera, found through parts of Europe, Asia, Africa, and New Zealand . There are no hedgehogs native to Australia, and no living species native to the Americas...

).

Fruit fly

Mechanism

Insect cells express a full size zinc-finger transcription factor
Transcription factor
In molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA...

 Cubitus interruptus (Ci), which forms a complex with the kinesin
Kinesin
A kinesin is a protein belonging to a class of motor proteins found in eukaryotic cells. Kinesins move along microtubule filaments, and are powered by the hydrolysis of ATP . The active movement of kinesins supports several cellular functions including mitosis, meiosis and transport of cellular...

- like protein Costal-2 (Cos2) and is localized in the cytoplasm bound to cellular microtubule
Microtubule
Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...

s (Figure 2). The SCF complex targets the 155 kb full length Ci protein for proteosome- dependent cleavage, which generates a 75 kb fragment (CiR). CiR builds up in the cell and diffuses
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

 into the nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...

, where it acts as a co-repressor
Repressor
In molecular genetics, a repressor is a DNA-binding protein that regulates the expression of one or more genes by binding to the operator and blocking the attachment of RNA polymerase to the promoter, thus preventing transcription of the genes. This blocking of expression is called...

 for Hh target genes. The steps leading to Ci protein proteolysis include phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....

 of Ci protein by several protein kinase
Protein kinase
A protein kinase is a kinase enzyme that modifies other proteins by chemically adding phosphate groups to them . Phosphorylation usually results in a functional change of the target protein by changing enzyme activity, cellular location, or association with other proteins...

s; PKA
CAMP-dependent protein kinase
In cell biology, Protein kinase A refers to a family of enzymes whose activity is dependent on cellular levels of cyclic AMP . PKA is also known as cAMP-dependent protein kinase...

, GSK3β
GSK-3
Glycogen synthase kinase 3 is a serine/threonine protein kinase that mediates the addition of phosphate molecules on certain serine and threonine amino acids in particular cellular substrates...

 and CK1
Casein kinase 1
The Casein kinase 1 family of protein kinases are serine/threonine-selective enzymes that function as regulators of signal transduction pathways in most eukaryotic cell types...

 (Figure 2). The Drosophila protein Slimb is part of an SCF complex that targets proteins for ubiquitylation
Ubiquitin
Ubiquitin is a small regulatory protein that has been found in almost all tissues of eukaryotic organisms. Among other functions, it directs protein recycling.Ubiquitin can be attached to proteins and label them for destruction...

. Slimb binds to phosphorylated Ci protein.

In the absence of Hh (Figure 3), a cell-surface transmembrane protein called Patched
PTCH1
Protein patched homolog 1 is a protein that is the member of the Patched family and in humans is encoded by the PTCH1 gene.- Function :PTCH1 is a member of the patched gene family and is the receptor for sonic hedgehog, a secreted molecule implicated in the formation of embryonic structures and in...

 (PTCH) acts to prevent high expression and activity of a 7 membrane spanning receptor called Smoothened
Smoothened
Smoothened is a G protein-coupled receptor protein encoded by the gene of the hedgehog pathway conserved from flies to humans. It is the molecular target of the teratogen cyclopamine....

 (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. 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. It has been suggested that phosphorylation of Smoothened plays a role in Hh-dependent regulation of Smoothened levels.

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 BMP
Bone morphogenetic protein
Bone morphogenetic proteins are a group of growth factors also known as cytokines and as metabologens . Originally discovered by their ability to induce the formation of bone and cartilage, BMPs are now considered to constitute a group of pivotal morphogenetic signals, orchestrating tissue...

 growth factor family). For other Hh-regulated genes, expression requires not only the loss of CiR but also the positive action of uncleaved Ci to act as a transcriptional activator. 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.

Role

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
Transcription factor
In molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA...

 Engrailed
Engrailed (gene)
engrailed is a homeodomain transcription factor involved in many aspects of multicellular development. First known for its role in arthropod embryological development, working in consort with the Hox genes, engrailed has been found to be important in other areas of development...

 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 Wingless
Wnt signaling pathway
The Wnt signaling pathway is a network of proteins best known for their roles in embryogenesis and cancer, but also involved in normal physiological processes in adult animals.-Discovery:...

 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.

Wingless protein acts as an extracellular signal and patterns the adjacent rows of cells by activating its cell surface receptor Frizzled
Frizzled
Frizzled is a family of G protein-coupled receptor proteins that serve as receptors in the Wnt signaling pathway and other signaling pathways. When activated, Frizzled leads to activation of Dishevelled in the cytosol.-Species distribution:...

. Wingless acts on Engrailed-expressing cells to stabilize the stripes of Engrailed expression. Wingless is a member of the Wnt
Wnt signaling pathway
The Wnt signaling pathway is a network of proteins best known for their roles in embryogenesis and cancer, but also involved in normal physiological processes in adult animals.-Discovery:...

 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

The Wingless protein is called "wingless" because of the phenotype
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

 of some wingless fly mutants. Wingless and Hedgehog functioned together during 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.

Annelids

hedgehog is also involved in segmentation in the annelid worms; because parallel evolution
Parallel evolution
Parallel evolution is the development of a similar trait in related, but distinct, species descending from the same ancestor, but from different clades.-Parallel vs...

 seems unlikely, this suggests a common origin of segmentation between the two phyla. Whilst Hh does not induce the formation of segments, it seems to act to stabilize the segmented fields once they have appeared.

Mechanism

Sonic hedgehog
Sonic hedgehog
Sonic hedgehog homolog is one of three proteins in the mammalian signaling pathway family called hedgehog, the others being desert hedgehog and Indian hedgehog . SHH is the best studied ligand of the hedgehog signaling pathway. It plays a key role in regulating vertebrate organogenesis, such as...

 (SHH) is the best studied ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

 of the vertebrate pathway. Most of what is known about hedgehog signaling has been established by studying SHH. It is translated
Translation (genetics)
In molecular biology and genetics, translation is the third stage of protein biosynthesis . In translation, messenger RNA produced by transcription is decoded by the ribosome to produce a specific amino acid chain, or polypeptide, that will later fold into an active protein...

 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
Cholesterol
Cholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...

 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
Secretion
Secretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...

 and consequent paracrine hedgehog signaling require the participation of Dispatched protein(2).

When SHH reaches its target cell, it binds to the Patched-1
PTCH1
Protein patched homolog 1 is a protein that is the member of the Patched family and in humans is encoded by the PTCH1 gene.- Function :PTCH1 is a member of the patched gene family and is the receptor for sonic hedgehog, a secreted molecule implicated in the formation of embryonic structures and in...

 (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 localization of which is controlled by PTCH. PTCH1 has homology to Niemann-Pick disease, type C1 (NPC1
NPC1
Niemann-Pick disease, type C1 also known as NPC1 is a protein which in humans is encoded by the NPC1 gene.NPC1 was identified as the gene that when mutated, results in Niemann-Pick disease, type C...

) that is known to transport lipophilic molecules across a membrane. PTCH1 has a sterol sensing domain (SSD), which has been shown to be essential for suppression of Smo activity. A current theory suggests that PTCH regulates SMO by removing oxysterols from SMO. PTCH acts like a sterol pump and removes oxysterols that have been created by 7-dehydrocholesterol reductase. 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.
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
Agonist
An agonist is a chemical that binds to a receptor of a cell and triggers a response by that cell. Agonists often mimic the action of a naturally occurring substance...

s and antagonist
Antagonist
An antagonist is a character, group of characters, or institution, that represents the opposition against which the protagonist must contend...

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 activators
Activator (genetics)
An activator is a DNA-binding protein that regulates one or more genes by increasing the rate of transcription. The activator may increase transcription by virtue of a connected domain which assists in the formation of the RNA polymerase holoenzyme, or may operate through a coactivator. A...

 Gli1
Gli1
Gli1 is a protein originally isolated in human glioblastoma.-Overview:The Gli proteins are the effectors of Hedgehog signaling and have been shown to be involved in cell fate determination, proliferation and patterning in many cell types and most organs during embryo development.The Gli...

 and Gli2
Gli2
Zinc finger protein GLI2 also known as GLI family zinc finger 2 is a protein that in humans is encoded by the GLI2 gene. The protein encoded by this gene is a transcription factor....

 and the repressor
Repressor
In molecular genetics, a repressor is a DNA-binding protein that regulates the expression of one or more genes by binding to the operator and blocking the attachment of RNA polymerase to the promoter, thus preventing transcription of the genes. This blocking of expression is called...

 Gli3
Gli3
Zinc finger protein GLI3 is a protein that in humans is encoded by the GLI3 gene.This gene encodes a protein that belongs to the C2H2-type zinc finger proteins subclass of the Gli family. They are characterized as DNA-binding transcription factors and are mediators of Sonic hedgehog signaling...

. 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
Smoothened
Smoothened is a G protein-coupled receptor protein encoded by the gene of the hedgehog pathway conserved from flies to humans. It is the molecular target of the teratogen cyclopamine....

.

In addition to PTCH1, mammals have another hedgehog receptor PTCH2 whose sequence identity with PTCH1 is 54%. 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. 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. Furthermore, overexpression of PTCH2 does not replace mutated PTCH1 in basal cell carcinoma
Basal cell carcinoma
Basal-cell carcinoma is the most common type of skin cancer. It rarely metastasizes or kills. However, because it can cause significant destruction and disfigurement by invading surrounding tissues, it is still considered malignant. Statistically, approximately 3 out of 10 Caucasians may develop a...

.

In invertebrates, just as in Drosophila, the binding of hedgehog to PTCH leads to internalisation and sequestration of the ligand. 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.

Role

Members of the hedgehog family play key roles in a wide variety of developmental processes. One of the best studied examples is the action of Sonic hedgehog
Sonic hedgehog
Sonic hedgehog homolog is one of three proteins in the mammalian signaling pathway family called hedgehog, the others being desert hedgehog and Indian hedgehog . SHH is the best studied ligand of the hedgehog signaling pathway. It plays a key role in regulating vertebrate organogenesis, such as...

 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
Morphogen
A morphogen is a substance governing the pattern of tissue development, and the positions of the various specialized cell types within a tissue...

 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
Sonic hedgehog
Sonic hedgehog homolog is one of three proteins in the mammalian signaling pathway family called hedgehog, the others being desert hedgehog and Indian hedgehog . SHH is the best studied ligand of the hedgehog signaling pathway. It plays a key role in regulating vertebrate organogenesis, such as...

. However, precisely how SHH determines digit identity remained elusive until recently. The current model, proposed by Harfe et al., states that both the concentration and the time of exposure to SHH determines which digit the tissue will develop into in the mouse
Mouse
A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...

 embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

 (figure 6).

Digits V, IV and part of III arise directly from cells that express SHH during embryogenesis
Embryogenesis
Embryogenesis is the process by which the embryo is formed and develops, until it develops into a fetus.Embryogenesis starts with the fertilization of the ovum by sperm. The fertilized ovum is referred to as a zygote...

. 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
Sonic hedgehog
Sonic hedgehog homolog is one of three proteins in the mammalian signaling pathway family called hedgehog, the others being desert hedgehog and Indian hedgehog . SHH is the best studied ligand of the hedgehog signaling pathway. It plays a key role in regulating vertebrate organogenesis, such as...

 has been shown to promote the proliferation of adult stem cell
Adult stem cell
Adult stem cells are undifferentiated cells, found throughout the body after embryonic development, that multiply by cell division to replenish dying cells and regenerate damaged tissues...

s from various tissues, including primitive hematopoietic cells, mammary and neural stem cells. Activation of the hedgehog pathway is required for transition of the hair follicle
Hair follicle
A hair follicle is a skin organ that produces hair. Hair production occurs in phases, including a growth phase , and cessation phase , and a rest phase . Stem cells are principally responsible for the production of hair....

 from the resting to the growth phase. Curis Inc. together with Procter & Gamble
Procter & Gamble
Procter & Gamble is a Fortune 500 American multinational corporation headquartered in downtown Cincinnati, Ohio and manufactures a wide range of consumer goods....

 are developing a hedgehog agonist
Agonist
An agonist is a chemical that binds to a receptor of a cell and triggers a response by that cell. Agonists often mimic the action of a naturally occurring substance...

 to be used as a drug for treatment of hair growth disorders.
This failed due to toxicities found in animal models.

Human disease

Disruption of hedgehog signaling during embryonic development, through either deleterious mutation or consumption of teratogens by the gestating mother, can lead to severe developmental abnormalities. Holoprosencephaly
Holoprosencephaly
Holoprosencephaly is a cephalic disorder in which the prosencephalon fails to develop into two hemispheres. Normally, the forebrain is formed and the face begins to develop in the fifth and sixth weeks of human pregnancy...

, the failure of the embryonic prosencephalon
Prosencephalon
In the anatomy of the brain of vertebrates, the prosencephalon is the rostral-most portion of the brain. The prosencephalon, the mesencephalon , and rhombencephalon are the three primary portions of the brain during early development of the central nervous system...

 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. Cyclopia
Cyclopia
Cyclopia is a rare form of holoprosencephaly and is a congenital disorder characterized by the failure of the embryonic prosencephalon to properly divide the orbits of the eye into two cavities...

, one of the most severe defects of holoprosencephaly
Holoprosencephaly
Holoprosencephaly is a cephalic disorder in which the prosencephalon fails to develop into two hemispheres. Normally, the forebrain is formed and the face begins to develop in the fifth and sixth weeks of human pregnancy...

, results if the pathway inhibitor cyclopamine
Cyclopamine
Cyclopamine is a naturally occurring chemical that belongs to the group of steroidal jerveratrum alkaloids. It is a teratogen isolated from the corn lily that causes usually fatal birth defects. It can prevent the fetal brain from dividing into two lobes and cause the development of a single eye...

 is consumed by gestating mammals.

Activation of the hedgehog pathway has been implicated in the development of cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...

s in various organs, including brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

, lung
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...

, mammary gland
Mammary gland
A mammary gland is an organ in mammals that produces milk to feed young offspring. Mammals get their name from the word "mammary". In ruminants such as cows, goats, and deer, the mammary glands are contained in their udders...

, prostate
Prostate
The prostate is a compound tubuloalveolar exocrine gland of the male reproductive system in most mammals....

 and skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...

. Basal cell carcinoma
Basal cell carcinoma
Basal-cell carcinoma is the most common type of skin cancer. It rarely metastasizes or kills. However, because it can cause significant destruction and disfigurement by invading surrounding tissues, it is still considered malignant. Statistically, approximately 3 out of 10 Caucasians may develop a...

, the most common form of cancerous malignancy, has the closest association with hedgehog signaling. Loss-of-function mutations in Patched
PTCH1
Protein patched homolog 1 is a protein that is the member of the Patched family and in humans is encoded by the PTCH1 gene.- Function :PTCH1 is a member of the patched gene family and is the receptor for sonic hedgehog, a secreted molecule implicated in the formation of embryonic structures and in...

 and activating mutations in Smoothened
Smoothened
Smoothened is a G protein-coupled receptor protein encoded by the gene of the hedgehog pathway conserved from flies to humans. It is the molecular target of the teratogen cyclopamine....

 have been identified in patients with this disease. Abnormal activation of the pathway probably leads to development of disease through transformation of adult stem cell
Adult stem cell
Adult stem cells are undifferentiated cells, found throughout the body after embryonic development, that multiply by cell division to replenish dying cells and regenerate damaged tissues...

s into cancer stem cell
Cancer stem cell
Cancer stem cells are cancer cells that possess characteristics associated with normal stem cells, specifically the ability to give rise to all cell types found in a particular cancer sample. CSCs are therefore tumorigenic , perhaps in contrast to other non-tumorigenic cancer cells...

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.

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. 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
Antagonist
An antagonist is a character, group of characters, or institution, that represents the opposition against which the protagonist must contend...

 and agonist
Agonist
An agonist is a chemical that binds to a receptor of a cell and triggers a response by that cell. Agonists often mimic the action of a naturally occurring substance...

 of SMO have already shown to effect the pathway regulation downstream. The most clinically advanced SMO targeting agents are cyclopamine
Cyclopamine
Cyclopamine is a naturally occurring chemical that belongs to the group of steroidal jerveratrum alkaloids. It is a teratogen isolated from the corn lily that causes usually fatal birth defects. It can prevent the fetal brain from dividing into two lobes and cause the development of a single eye...

-competitive. Itraconazole
Itraconazole
Itraconazole , invented in 1984, is a triazole antifungal agent that is prescribed to patients with fungal infections. The drug may be given orally or intravenously.-Medical uses:...

 (Sporanox) has also been shown to target SMO through a mechanism distinct from cyclopamine
Cyclopamine
Cyclopamine is a naturally occurring chemical that belongs to the group of steroidal jerveratrum alkaloids. It is a teratogen isolated from the corn lily that causes usually fatal birth defects. It can prevent the fetal brain from dividing into two lobes and cause the development of a single eye...

 and vismodegib
Vismodegib
Vismodegib is an investigational drug for the treatment of various types of cancer. It has completed a Phase II clinical trial for metastasizing basal-cell carcinoma and is in Phase II trials for metastatic colorectal cancer, small-cell lung cancer, advanced stomach cancer, pancreatic cancer,...

. PTCH and Gli3 (5E1) 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. Arsenic trioxide
Arsenic trioxide
Arsenic trioxide is the inorganic compound with the formula As2O3. This commercially important oxide of arsenic is the main precursor to other arsenic compounds, including organoarsenic compounds. Approximately 50,000 tonnes are produced annually...

 (Trisenox) has also been shown to inhibit hedgehog signaling by interfering with Gli function and transcription.

Metastasis

Activation of the Hedgehog pathway leads to an increase in Snail protein expression and a decrease in E-cadherin and Tight Junctions
Tight junction
Tight junctions, or zonula occludens, are the closely associated areas of two cells whose membranes join together forming a virtually impermeable barrier to fluid. It is a type of junctional complex present only in vertebrates...

. Hedgehog signaling also appears to be a crucial regulator of angiogenesis and thus metastasis.

Tumor regulation

Activation of the Hedgehog pathway leads to an increase in Angiogenic Factors (angiopoietin-1 and angiopoietin-2), Cyclins (cyclin D1 and B1)), anti-apoptotic genes and a decrease in apoptotic genes (Fas).

Clinical trials


  • Itraconazole
    Itraconazole
    Itraconazole , invented in 1984, is a triazole antifungal agent that is prescribed to patients with fungal infections. The drug may be given orally or intravenously.-Medical uses:...


Evolution

Hedgehog-like genes, 2 Patched homologs and Patched-related genes exist in the worm C. elegans
Caenorhabditis elegans
Caenorhabditis elegans is a free-living, transparent nematode , about 1 mm in length, which lives in temperate soil environments. Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner and it has since been used extensively as a model...

. These genes have been shown to code for proteins that have roles in C. elegans development. 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 coelomate
Body cavity
By the broadest definition, a body cavity is any fluid-filled space in a multicellular organism. However, the term usually refers to the space located between an animal’s outer covering and the outer lining of the gut cavity, where internal organs develop...

 and pseudo-coelomate lineages.

Lancelet
Lancelet
The lancelets , also known as amphioxus, are the modern representatives of the subphylum Cephalochordata, formerly thought to be the sister group of the craniates. They are usually found buried in sand in shallow parts of temperate or tropical seas. In Asia, they are harvested commercially as food...

s, which are primitive chordate
Chordate
Chordates are animals which are either vertebrates or one of several closely related invertebrates. They are united by having, for at least some period of their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail...

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. 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 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
Salamander
Salamander is a common name of approximately 500 species of amphibians. They are typically characterized by a superficially lizard-like appearance, with their slender bodies, short noses, and long tails. All known fossils and extinct species fall under the order Caudata, while sometimes the extant...

 limb.

shh has undergone accelerated evolution in the primate lineage leading to humans. 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
Wnt signaling pathway
The Wnt signaling pathway is a network of proteins best known for their roles in embryogenesis and cancer, but also involved in normal physiological processes in adult animals.-Discovery:...

 receptors have some sequence similarity to Smoothened. 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. 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. 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. In vertebrates, Hh signaling has been heavily coupled to cilia 
The hedgehog protein appears to have evolved in two sections, the N-terminal domain (hedge) and the C-terminal domain (hog), that only later were spliced together into a single transcriptional unit . The hedge domain contains a sequence called Hint (Hedgehog INTein), which is similar in sequence and function to bacterial and fungal inteins
Intein
An intein is a segment of a protein that is able to excise itself and rejoin the remaining portions with a peptide bond. Inteins have also been called "protein introns"....

 . Choanoflagellates
Choanoflagellate
The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals...

 contain a region named hoglet that is similar to the hedgehog C-terminal domain. In addition, molecular phylogenetic analysis revealed that hoglet was more similar to hog than it was to bacterial inteins . Choanoflagellates do not contain any regions similar to the hedge domain, suggesting that hog evolved first . Poriferans have both hedge-like proteins (termed hedgling) and hog-like proteins, but they exist as two completely separate transcriptional units . Cnidarians
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...

 contain the hedgling and hog genes, but also have a complete hedgehog gene, indicating that hedge and hog were spliced into hedgehog after the last common ancestor of poriferans and cnidarians . Bilaterians do not contain hedgling genes, suggesting that these were lost by deletion before this branch split from the other metazoans .

See also

  • Sonic hedgehog
    Sonic hedgehog
    Sonic hedgehog homolog is one of three proteins in the mammalian signaling pathway family called hedgehog, the others being desert hedgehog and Indian hedgehog . SHH is the best studied ligand of the hedgehog signaling pathway. It plays a key role in regulating vertebrate organogenesis, such as...

    , best studied ligand of the vertebrate pathway
  • Smoothened
    Smoothened
    Smoothened is a G protein-coupled receptor protein encoded by the gene of the hedgehog pathway conserved from flies to humans. It is the molecular target of the teratogen cyclopamine....

    , the conserved GPCR component of the pathway
  • Inhibitors of Hh signaling
    • Cyclopamine
      Cyclopamine
      Cyclopamine is a naturally occurring chemical that belongs to the group of steroidal jerveratrum alkaloids. It is a teratogen isolated from the corn lily that causes usually fatal birth defects. It can prevent the fetal brain from dividing into two lobes and cause the development of a single eye...

      , a naturally occurring small molecule
    • Vismodegib
      Vismodegib
      Vismodegib is an investigational drug for the treatment of various types of cancer. It has completed a Phase II clinical trial for metastasizing basal-cell carcinoma and is in Phase II trials for metastatic colorectal cancer, small-cell lung cancer, advanced stomach cancer, pancreatic cancer,...

      , an investigational drug

External links

  • http://hedgehog.sfsu.edu (Hedgehog Pathway Database)
  • Netpath
    Netpath
    NetPath is a manually curated resource of human signal transduction pathways. It is a joint effort between Pandey Lab at the Johns Hopkins University and the Institute of Bioinformatics , Bangalore, India, and is also worked on by other parties....

    - A curated resource of signal transduction pathways in humans
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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