Sonic hedgehog protein

Sonic hedgehog protein (SHH) is a major signaling molecule of embryonic development in planulozoan animals, encoded by the SHH gene.

This signaling molecule is key in regulating embryonic morphogenesis in complex animals. SHH controls organogenesis and the organization of the central nervous system, limbs, digits and many other parts of the body. Sonic hedgehog is a morphogen that patterns the developing embryo using a concentration gradient characterized by the French flag model. This model has a non-uniform distribution of SHH molecules which governs different cell fates according to concentration. Mutations in this gene can cause holoprosencephaly, a failure of splitting in the cerebral hemispheres, as demonstrated in an experiment using SHH knock-out mice in which the forebrain midline failed to develop and instead only a single fused telencephalic vesicle resulted.

Sonic hedgehog still plays a role in differentiation, proliferation, and maintenance of adult tissues. Abnormal activation of SHH signaling in adult tissues has been implicated in various types of cancers including breast, skin, brain, liver, gallbladder and many more.

Discovery and naming

The hedgehog gene (hh) was first identified in the fruit fly Drosophila melanogaster in the classic Heidelberg screens of Christiane Nüsslein-Volhard and Eric Wieschaus, as published in 1980. These screens, which led to the researchers winning a Nobel Prize in 1995 along with developmental geneticist Edward B. Lewis, identified genes that control the segmentation pattern of the Drosophila embryos. The hh loss of function mutant phenotype causes the embryos to be covered with denticles, i.e. small pointy projections resembling the spikes of a hedgehog. Investigations aimed at finding a hedgehog equivalent in vertebrates by Philip Ingham, Andrew P. McMahon and Clifford Tabin revealed three homologous genes.

Two of these genes, desert hedgehog and Indian hedgehog, were named for species of hedgehogs, while sonic hedgehog was named after the video game character Sonic the Hedgehog. The gene was named by Robert Riddle, a postdoctoral fellow at the Tabin Lab, after his wife Betsy Wilder came home with a magazine containing an advert for the first game in the series, Sonic the Hedgehog (1991). In the zebrafish, two of the three vertebrate hh genes are duplicated: SHH a and SHH b (formerly described as tiggywinkle hedgehog, named for Mrs. Tiggy-Winkle, a character from Beatrix Potter's books for children) and ihha and ihhb (formerly described as echidna hedgehog, named for the spiny anteater and not specifically for the character Knuckles the Echidna in the Sonic franchise).

Function

Of the hh homologues, SHH has been found to have the most critical roles in development, acting as a morphogen involved in patterning many systems—including the anterior pituitary, pallium of the brain, spinal cord, lungs, teeth and the thalamus by the zona limitans intrathalamica. In vertebrates, the development of limbs and digits depends on the secretion of sonic hedgehog by the zone of polarizing activity, located on the posterior side of the embryonic limb bud. and medulloblastoma, as well as the progression of prostate cancer tumours. For SHH to be expressed in the developing embryo limbs, a morphogen called fibroblast growth factors must be secreted from the apical ectodermal ridge.

Sonic hedgehog has also been shown to act as an axonal guidance cue. It has been demonstrated that SHH attracts commissural axons at the ventral midline of the developing spinal cord. Specifically, SHH attracts retinal ganglion cell (RGC) axons at low concentrations and repels them at higher concentrations. The absence (non-expression) of SHH has been shown to control the growth of nascent hind limbs in cetaceans (whales and dolphins).

The SHH gene is a member of the hedgehog gene family with five variations of DNA sequence alterations or splice variants. SHH is located on chromosome seven and initiates the production of Sonic Hedgehog protein. If the SHH gene is mutated or absent, the protein Sonic Hedgehog cannot do its job properly. Sonic hedgehog contributes to cell growth, cell specification and formation, structuring and organization of the body plan. This protein functions as a vital morphogenic signaling molecule and plays an important role in the formation of many different structures in developing embryos. and facial dysmorphia. The notochord—a structure derived from the axial mesoderm—produces SHH, which travels extracellularly to the ventral region of the neural tube and instructs those cells to form the floor plate. Another view of floor plate induction hypothesizes that some precursor cells located in the notochord are inserted into the neural plate before its formation, later giving rise to the floor plate.

The neural tube itself is the initial groundwork of the vertebrate CNS, and the floor plate is a specialized structure, located at the ventral midpoint of the neural tube. Evidence supporting the notochord as the signaling center comes from studies in which a second notochord is implanted near a neural tube in vivo, leading to the formation of an ectopic floor plate within the neural tube.

Sonic hedgehog is the secreted protein that mediates signaling activities of the notochord and floor plate. Studies involving ectopic expression of SHH in vitro and in vivo result in floor plate induction and differentiation of motor neuron and ventral interneurons. On the other hand, mice mutants for SHH lack ventral spinal cord characteristics. In vitro blocking of SHH signaling using antibodies against it shows similar phenotypes. so that a high concentration of SHH results in a local inhibition of cellular proliferation. This inhibition causes the floor plate to become thin compared to the lateral regions of the neural tube. Lower concentration of SHH results in cellular proliferation and induction of various ventral neural cell types. throughout the ventral neural tube. In vitro studies show that incremental two- and threefold changes in SHH concentration give rise to motor neuron and different interneuronal subtypes as found in the ventral spinal cord. These incremental changes in vitro correspond to the distance of domains from the signaling tissue (notochord and floor plate) which subsequently differentiates into different neuronal subtypes as it occurs in vitro. Graded SHH signaling is suggested to be mediated through the Gli family of proteins, which are vertebrate homologues of the Drosophila zinc-finger-containing transcription factor Cubitus interruptus (Ci). Ci is a crucial mediator of hedgehog (Hh) signaling in Drosophila. In vertebrates, three different Gli proteins are present, viz. Gli1, Gli2 and Gli3, which are expressed in the neural tube. Mice mutants for Gli1 show normal spinal cord development, suggesting that it is dispensable for mediating SHH activity. However, Gli2 mutant mice show abnormalities in the ventral spinal cord, with severe defects in the floor plate and ventral-most interneurons (V3). Gli3 antagonizes SHH function in a dose-dependent manner, promoting dorsal neuronal subtypes. SHH mutant phenotypes can be rescued in a SHH/Gli3 double mutant. Gli proteins have a C-terminal activation domain and an N-terminal repressive domain.

SHH is suggested to promote the activation function of Gli2 and inhibit repressive activity of Gli3. SHH also seems to promote the activation function of Gli3, but this activity is not strong enough.

SHH also induces other proteins with which it interacts, and these interactions can influence the sensitivity of a cell towards SHH. Hedgehog-interacting protein (HHIP) is induced by SHH, which in turn attenuates its signaling activity. Vitronectin is another protein that is induced by SHH; it acts as an obligate co-factor for SHH signaling in the neural tube.

There are five distinct progenitor domains in the ventral neural tube: V3 interneurons, motor neurons (MN), V2, V1, and V0 interneurons (in ventral to dorsal order).

It is important to note that SHH is not the only signaling molecule exerting an effect on the developing neural tube. Many other molecules, pathways and mechanisms are active (e.g., RA, FGF, BMP), and complex interactions between SHH and other molecules are possible. BMPs are suggested to play a critical role in determining the sensitivity of neural cell to SHH signaling. Evidence supporting this comes from studies using BMP inhibitors that ventralize the fate of the neural plate cell for a given SHH concentration. On the other hand, mutation in BMP antagonists (e.g., noggin) produces severe defects in the ventral-most characteristics of the spinal cord, followed by ectopic expression of BMP in the ventral neural tube. Interactions of SHH with Fgf and RA have not yet been studied in molecular detail.

Morphogenetic activity

The concentration- and time-dependent, cell-fate-determining activity of SHH in the ventral neural tube makes it a prime example of a morphogen. In vertebrates, SHH signaling in the ventral portion of the neural tube is most notably responsible for the induction of floor plate cells and motor neurons. Higher concentrations of the SHH ligand are found in the most ventral aspects of the neural tube and notochord, while lower concentrations are found in the more dorsal regions of the neural tube.

It is thought that the SHH gradient works to elicit multiple different cell fates by a concentration- and time-dependent mechanism that induces a variety of transcription factors in the ventral progenitor cells.

Tooth development

SHH plays an important role in organogenesis and, most importantly, craniofacial development. Being that SHH is a signaling molecule, it primarily works by diffusion along a concentration gradient, affecting cells in different manners. In early tooth development, SHH is released from the primary enamel knot—a signaling center—to provide positional information in both a lateral and planar signaling pattern in tooth development and regulation of tooth cusp growth. SHH in particular is needed for growth of epithelial cervical loops, where the outer and inner epitheliums join and form a reservoir for dental stem cells. After the primary enamel knots are apoptosed, the secondary enamel knots are formed. The secondary enamel knots secrete SHH in combination with other signaling molecules to thicken the oral ectoderm and begin patterning the complex shapes of the crown of a tooth during differentiation and mineralization. In a knockout gene model, absence of SHH is indicative of holoprosencephaly. However, SHH activates downstream molecules of Gli2 and Gli3. Mutant Gli2 and Gli3 embryos have abnormal development of incisors that are arrested in early tooth development as well as small molars.

Lung development

Although SHH is most commonly associated with brain and limb digit development, it is also important in lung development. Studies using qPCR and knockouts have demonstrated that SHH contributes to embryonic lung development. The mammalian lung branching occurs in the epithelium of the developing bronchi and lungs. SHH expressed throughout the foregut endoderm (innermost of three germ layers) in the distal epithelium, where the embryonic lungs are developing.

SHH is important for regulating dermal adipogenesis by hair follicle transit-amplifying cells (HF-TACs). Specifically, SHH induces dermal angiogenesis by acting directly on adipocyte precursors and promoting their proliferation through their expression of the peroxisome proliferator-activated receptor γ (Pparg) gene.

Clinical significance

Recent studies have shown that abnormalities in SHH signaling pathways are associated with both neurodegenerative diseases and developmental disorders. Mutations in the SHH gene can result in conditions such as holoprosencephaly, a brain defect caused by failure of the embryonic forebrain to divide into two hemispheres. In adults, SHH signaling may be involved in neuronal damage and cognitive impairment in Alzheimers disease. The cleavage is catalysed by a protease within the C-terminal domain. During the reaction, a cholesterol molecule is added to the C-terminus of SHH-N. Thus, the C-terminal domain acts as an intein and a cholesterol transferase. Another hydrophobic moiety, a palmitate, is added to the alpha-amine of N-terminal cysteine of SHH-N. This modification is required for efficient signaling, resulting in a 30-fold increase in potency over the non-palmitylated form and is carried out by a member of the membrane-bound O-acyltransferase family Protein-cysteine N-palmitoyltransferase HHAT.

Robotnikinin

A potential inhibitor of the Hedgehog signaling pathway has been found and dubbed "Robotnikinin"—after Sonic the Hedgehog's nemesis and the main antagonist of the Sonic the Hedgehog game series, Dr. Ivo Robotnik, better known as Dr. Eggman.

Former controversy surrounding name

The gene has been linked to a condition known as holoprosencephaly, which can result in severe brain, skull and facial defects, causing a few clinicians and scientists to criticize the name on the grounds that it sounds too frivolous. It has been noted that mention of a mutation in a sonic hedgehog gene might not be well received in a discussion of a serious disorder with a patient or their family. In November 2006, The New York Times ran an article entitled "'Sonic Hedgehog' Sounded Funny, at First" by science reporter Nicholas Wade, which discussed the controversy over the naming of this molecule and the efforts to rename it, and which included an interview with Sue Povey, professor of biology at University College London and head of the genome nomenclature committee of the Human Genome Organization. This controversy has largely died down, and the name is now generally seen as a humorous relic of the time before the rise of fast, cheap complete genome sequencing and standardized nomenclature. The problem of the "inappropriateness" of the names of genes such as "Mothers against decapentaplegic," "Lunatic fringe," and "Sonic hedgehog" is largely avoided by using standardized abbreviations when speaking with patients and their families.

Gallery

File:Shh and Gli proteins interactions.svg|alt=Interaction between SHH and Gli proteins which gives rise to different ventral neuronal subtypes. | SHH gradient and Gli activity in the vertebrate neural tube

File:Shh processing.svg|Processing of SHH

File:sonichedgehog.svg|Concentration gradient of SHH

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References

External links

An introductory article on SHH at Davidson College
Rediscovering biology: Unit 7 Genetics of development .. Expert interview transcripts interview with John Incardona PhD .. explanation of the discovery and naming of the sonic hedgehog gene
'Sonic Hedgehog' sounded funny at first .. New York Times November 12, 2006 ..
GeneReviews/NCBI/NIH/UW entry on Anophthalmia / Microphthalmia Overview
SHH – sonic hedgehog US National Library of Medicine