Hedgehog (Hh) signaling is highly conserved across chordates of different taxonomical classification. It is essential for development of the embryo. The Hedehog pathway was first described and characterized in the fruit fly, Drosophila melanogaster. In mammals, Hh signaling regulates cell-fate, tissue polarity, and patterning during early embryogenesis and the morphogenesis of specific organs and tissues. It is subsequently silenced in most adult tissues but can be reactivated following injury to promote repair and regeneration.
Different Hh ligands, including homologs sonic hedgehog (SHH), indian hedgehog (IHH), and desert hedgehog (DHH) are produced as precursors that undergo autocatalytic cleavage, C-terminal cholesterol attachment, and N-terminal palmitoylation prior to secretion. Release and extracellular accumulation of the mature ligands is regulated by the homologs of the Drosophila dispatched (Disp) protein. Binding of secreted Hh ligands to the receptor homolog (PTCH1, PTCH2) triggers the stimulation of the signaling network: the repressive effect of PTCH on the transmembrane receptor SMO is relieved which leads to the activation of glioma-associated oncogene (GLI) transcription factors.
A key specialized structure in this process is the microtubule-based primary cilium. In the absence of the Hh signal unprocessed, non-activating GLI proteins as well as their regulator SUFU are concentrated in the distal tip of the primary cilium. Upon binding of the Hh ligand, PTCH relocates to the cell surface, thus rendering translocation of SMO to the primary cilium and the down-regulation of SUFU possible. Several structurally essential primary cilium proteins have also regulatory effects on the Hh signaling cascade.
Aberrant Hh/GLI regulation leads to major tissular disorders and the development of a wide variety of aggressive cancers. The Hh/GLI cascade has also been linked to the regulation of stemness genes and the survival of cancer stem cells.
This gene encodes a protein that is instrumental in patterning the early embryo. It has been implicated as the key inductive signal in patterning of the ventral neural tube, the anterior-posterior limb axis, and the ventral somites. Of three human proteins showing sequence and functional similarity to the sonic hedgehog protein of Drosophila, this protein is the most similar. The protein is...
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Skinny hedgehog' (SKI1) encodes an enzyme that acts within the secretory pathway to catalyze amino-terminal palmitoylation of 'hedgehog' (see MIM 600725).[supplied by OMIM, Jul 2002].
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This gene encodes a member of the patched gene family. The encoded protein is the receptor for sonic hedgehog, a secreted molecule implicated in the formation of embryonic structures and in tumorigenesis, as well as the desert hedgehog and indian hedgehog proteins. This gene functions as a tumor suppressor. Mutations of this gene have been associated with basal cell nevus syndrome, esophageal...
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This gene encodes a member of the Kruppel family of zinc finger proteins. The encoded transcription factor is activated by the sonic hedgehog signal transduction cascade and regulates stem cell proliferation. The activity and nuclear localization of this protein is negatively regulated by p53 in an inhibitory loop. Multiple transcript variants encoding different isoforms have been found for...
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Acts as a permissive factor that is required for the proper regulation of Hedgehog (Hh) target genes in response to Hh signals. Acts downstream of the Smoothened protein to modulate Gli activity in the somites of the developing embryo.
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The pattern of cellular proliferation and differentiation that leads to normal development of embryonic structures often depends upon the localized production of secreted protein signals. Cells surrounding the source of a particular signal respond in a graded manner according to the effective concentration of the signal, and this response produces the pattern of cell types constituting the...
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