Hedgehog Signalling Pathway

The Hedgehog signaling pathway is a signaling pathway that transmits information to embryonic cells required for proper cell differentiation. Different parts of the embryo have different concentrations of hedgehog signaling proteins.

The pathway takes its name from its polypeptide ligand, an intercellular signaling molecule called Hedgehog (Hh) found in fruit flies of the genus Drosophila; fruit fly larva lacking the Hh gene are said to resemble hedgehogs. Hh is one of Drosophila's segment polarity gene products, involved in establishing the basis of the fly body plan. Larvae without Hh, are short and spiny, resembling the hedgehog animal.

Mammals have three Hedgehog homologues, Desert (DHH), Indian (IHH), and Sonic (SHH), of which Sonic is the best studied. The pathway is equally important during vertebrate embryonic development and is therefore of interest in evolutionary developmental biology.

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 microtubules . The SCF complex targets the 155 kDa full length Ci protein for proteosome-dependent cleavage, which generates a 75 kDa fragment (CiR). CiR builds up in the cell and diffuses into the nucleus, where it acts as a co-repressor for Hedgehog (Hh) target genes.The steps leading to Ci protein proteolysis include phosphorylation of Ci protein by several protein kinases; PKA, GSK3β and CK1 .The Drosophila protein Slimb is part of an SCF complex that targets proteins for ubiquitylation. Slimb binds to phosphorylated Ci protein.

In the absence of Hh , a cell-surface transmembrane protein called Patched (PTCH) acts to prevent high expression and activity of a 7 membrane spanning receptor called Smoothened (SMO). Patched has sequence similarity to known membrane transport proteins. When extracellular Hh is present , 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. 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.