Developmental Signals - Sonic hedgehog

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Introduction

Sonic.jpg

(SHH, Shh) A secreted growth factor that binds patched (ptc) receptor on cell membrane. This protein has many different functions in different tissues of the developing embryo, the main role would be of establishing "pattern". SHH name derives from homology to the original fruitfly (drosophila) "hedgehog" mutation, where these flies have hairs located in regions (ventral) which are normally hair-free, and therefore have a disrupted body pattern.

Sonic Hedgehog was isolated by homology to the Hedgehog (Hh) gene. A single Hh homolog is present in Drosophila, but three Hh homologs, Sonic Hh, Indian Hh, and Desert Hh, are present in mammals. SHH is a true signaling switch used in differentiating subpopulations of cells throughout the embryo. Depending on where the signal is being secreted, how far away the responsive cell population is and how SHH is proteolytically cleaved, will determine SHH function. SHH binds to the membrane receptors Patched (ptc) and BOC/CDON.

Nomenclature note - upper case SHH in humans, lower case shh used for other species.


Factor Links: AMH | hCG | BMP | sonic hedgehog | bHLH | HOX | FGF | FOX | Hippo | LIM | Nanog | NGF | Nodal | Notch | PAX | retinoic acid | SIX | Slit2/Robo1 | SOX | TBX | TGF-beta | VEGF | WNT | Category:Molecular

| Category:Sonic Hedgehog

Some Recent Findings

Mesenchymal cells of the developing limb bud possess long and highly dynamic cytoplasmic extensions.[1]
  • GATA6 Is a Crucial Regulator of Shh in the Limb Bud[2] "In the limb bud, patterning along the anterior-posterior (A-P) axis is controlled by Sonic Hedgehog (Shh), a signaling molecule secreted by the "Zone of Polarizing Activity", an organizer tissue located in the posterior margin of the limb bud. We have found that the transcription factors GATA4 and GATA6, which are key regulators of cell identity, are expressed in an anterior to posterior gradient in the early limb bud, raising the possibility that GATA transcription factors may play an additional role in patterning this tissue. While both GATA4 and GATA6 are expressed in an A-P gradient in the forelimb buds, the hindlimb buds principally express GATA6 in an A-P gradient." Limb Development
  • Dynamic interpretation of hedgehog signaling in the Drosophila wing disc [3]
  • Patched 1 is a crucial determinant of asymmetry and digit number in the vertebrate limb[4]
  • Uncoupling Sonic hedgehog control of pattern and expansion of the developing limb bud[5] "One of the first changes we noted was that the dorsoventral polarity of the forebrain was disturbed, which manifested as a loss of Shh in the ventral telencephalon, a reduction in expression of the ventral markers Nkx2.1 and Dlx2, and a concomitant expansion of the dorsal marker Pax6. In addition to changes in the forebrain neuroectoderm, we observed altered gene expression patterns in the facial ectoderm. For example, Shh was not induced in the frontonasal ectoderm, and Ptc and Gli1 were reduced in both the ectoderm and adjacent mesenchyme."
  • THM1 negatively modulates mouse sonic hedgehog signal transduction and affects retrograde intraflagellar transport in cilia.[6]
  • Triphalangeal thumb-polysyndactyly syndrome and syndactyly type IV are caused by genomic duplications involving the long-range, limb-specific SHH enhancer.[7]
  • Notochord-derived Shh concentrates in close association with the apically positioned basal body in neural target cells and forms a dynamic gradient during neural patterning. [8]

Hedgehog Family

Hedgehog signaling pathway[9]

Vertebrates have a number of signaling proteins and receptors that belong to the hedgehog (Hh) family.

Signaling proteins

  1. Desert hedgehog (Dhh)
  2. Indian hedgehog (Ihh)
  3. Sonic hedgehog (Shh)

Receptors

  • Patched homologs - Patched-1 (Ptc1) and Patched-2 (Ptc2)
  • Cubitus interruptus (Ci) homologs - Gli1, Gli2 and Gli3

Functions

Developmental patterning signal.

Neural

Shh frog notochord
  • SHH is secreted by the notochord, ventralizes the neural tube, inducing the floor plate and motor neurons.
  • Regulation of patched by sonic hedgehog in the developing neural tube.[10] "The pattern of PTC expression suggests that Sonic hedgehog may play an inductive role in more dorsal regions of the neural tube than have been previously demonstrated. Examination of the pattern of PTC expression also suggests that PTC may act in a negative feedback loop to attenuate hedgehog signaling."
Neural tube - SHH model.jpg
Neural tube - SHH model[11]

Neural Crest

  • Neural crest cell survival in the foregut endoderm of jaw development[12]
  • Sonic hedgehog is required for cardiac outflow tract and neural crest cell development.[13]

Hearing

During mouse cochlea development the adjacent auditory (spiral) ganglion acts as a source of Sonic hedgehog. Without SHH the cochlear duct is shortened, sensory hair cell precursors prematurely leave the mitotic cell cycle, and hair cell differentiation closely follows cell cycle exit in a similar apical-to-basal direction.[14]

Somite

A study has shown an essential role for Shh signalling in the control of laminin-111 synthesis and in the initiation of basement membrane assembly in the myotome.[15]

Limb

Mouse hindlimb budsZPA Shh expression.[16]
  • SHH is secreted by the zone of polarizing activity (ZPA) organizing limb axis formation.
  • Extended exposure to Sonic hedgehog is required for patterning the posterior digits of the vertebrate limb.[17]
Limb patterning factors 03.jpg Limb patterning factors 07.jpg
Shh mouse forelimb E11.5[18] Shh mouse hindlimb E12.5[18]
Mouse Limb Images: Tbx3 and Tbx2 forelimb E10 | Alx3 and Gli3 forelimb E10 | Fgf and Hox forelimb E10.5 | Bmp4 forelimb E11.5 | Bmp4 hindlimb E11.5 | Shh forelimb E11.5 | Fgf8 hindlimb E11.5 | Sox9 forelimb E12.5 | Msx2 forelimb E12.5 | Shh hindlimb E12.5
Links: Fgf | Hox | Shh | Sox | Limb Development | Mouse Development
Limb bud geometry and patterning.jpg
Geometry and patterning of the developing limb bud[19]

Model GATA6 hindlimb.jpg

Model for GATA6 action Patterning developing Hindlimb Buds[2]

Face

SHH is secreted by the forebrain neuroectoderm and the facial ectoderm for upper face and nasofrontal bud development. PMID: 15979605

Other

SHH has still others roles in organ development in lung, pancreas.

Signaling Pathway

SHH signaling model

The secreted protein binds to the cell surface membrane protein the patched receptor (ptc).

Canonical Pathway

Series of repressive interactions which culminates in GLI-mediated transcriptional regulation of a variety of cellular processes.

Non-Canonical Pathway

Suggested that there are at least 3 other potential pathways[20]:

  1. Involves hedgehog pathway components but which is independent of GLI-mediated transcription.
  2. Direct interaction of hedgehog signaling components with components of other molecular pathways.
  3. "Non-contiguous" or "atypical" interaction of core hedgehog pathway components with one another.

SHH Receptors

Patched Receptor

In the fly, D. melanogaster, patched (ptc) is a protein ligand receptor involved in the smo receptor signalling pathway which is a component of the integral plasma membrane protein. There are homologues in Homo sapiens , Mus , Caenorhabditis elegans , Drosophila sp. , Mus musculus and Saccharomyces cerevisiae.

D. melanogaster expression in the embryo (Malpighian tubule , analia , embryonic/larval hindgut , head and 2 other listed tissues). Protein interacts genetically with fu , rho , hh , ci , gsb , B , kn , N , l(1)sc , smo , Su(fu) and vn . There are 62 recorded mutant alleles , of which at least 16 are available from the public stock centers. Amorphic mutations have been isolated which affect the anterior wing , the costal cell , the wing vein and 8 other listed tissues and are embryonic lethal, visible and tissue polarity. ptc is discussed in 325 published references , dated between 1948 and 1999. These include at least 58 studies of mutant phenotypes , one study of wild-type function and 4 molecular studies . Among findings on ptc mutants, ptc mutant analysis and stage-specific laser inactivation of ptc protein indicates that ptc activity is functionally redeployed after the segmentation phenocritical period to discriminate between neural and epithelial cell fates. (Some text modified from Flybase entry for Hedgehog Gene and refers to the fruitfly hh gene)

BOC/CDON Receptors

Two recently identified related receptors for SHH, Boc and Cdon are cell surface receptors of the immunoglobulin (Ig)/fibronectin type III that interact with each other and are coexpressed in development.

  • Brother of Cdon (BOC) OMIM 608708
  • Cell adhesion molecule-regulated/downregulated by oncogenes (CDON) OMIM 608707

Megalin

In development, it has been shown that sonic hedgehog can also bind megalin (lipoprotein receptor-related protein-2, LRP2, gp330) a transmembrane protein which acts as an endocytic receptor on the apical surface of polarised epithelial cells. It requires interaction with another protein, cubulin, for the endocytosis of ligands.

Hedgehog-Interacting Protein

(Hhip) A surface receptor antagonist that is equipotent against all three mammalian Hh homologs. The structure of human HHIP is comprised of two EGF domains and a six-bladed beta-propeller domain.


Abnormalities

Holoprosencephaly

Defects in the SHH protein or its signalling pathway are a cause of Holoprosencephaly (International Classification of Diseases - Q04 Other congenital malformations of brain - Q04.2 Holoprosencephaly).

Below are shown images of human fetal holoprosencephaly and associated cyclopia.[21]

Human holoprosencephaly cyclopia dissection.jpg Proboscis histology.jpg
Human holoprosencephaly cyclopean dissection Proboscis histology
Links: Human holoprosencephaly cyclopia dissection | Proboscis histology | Holoprosencephaly | Neural Abnormalities | Sonic hedgehog


References

  1. Timothy A. Sanders, Esther Llagostera, Maria Barna. Specialized filopodia direct long-range transport of SHH during vertebrate tissue patterning. Nature Apr 28, 2013.
  2. 2.0 2.1 <pubmed>24415953</pubmed>
  3. <pubmed>19787036</pubmed> | PLOS
  4. <pubmed>19783740</pubmed>
  5. <pubmed>18410737</pubmed>
  6. <pubmed>18327258</pubmed>
  7. <pubmed>18417549</pubmed>
  8. <pubmed>18272593</pubmed>
  9. <pubmed>19040769</pubmed>| PMC2614485 | Genome Biology
  10. <pubmed>8790332</pubmed>
  11. <pubmed>20532235</pubmed>
  12. <pubmed>16868080</pubmed>
  13. <pubmed>15936751</pubmed>
  14. <pubmed>23918393</pubmed>| Proc Natl Acad Sci U S A.
  15. <pubmed>19783738</pubmed>
  16. <pubmed>20386744</pubmed>| PMC2851570 | PLoS Genet.
  17. <pubmed>17610861</pubmed> | Dev Biol
  18. 18.0 18.1 <pubmed>17194222</pubmed>| PMC1713256 | PLoS Genet.
  19. <pubmed>20644713</pubmed>| PMC2903596 | PLoS
  20. <pubmed>19399989</pubmed>
  21. <pubmed>19563629</pubmed>| PMC2709107


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Cite this page: Hill, M.A. (2019, September 19) Embryology Developmental Signals - Sonic hedgehog. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Developmental_Signals_-_Sonic_hedgehog

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© Dr Mark Hill 2019, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G