Snapshot: Hedgehog Signaling Pathway Miao-Hsueh Chen, Christopher W

Home , GAS1, Ligand (biochemistry), Morphogen, Patched, Sonic hedgehog, SUFU

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Cell SnapShot: Hedgehog Signaling Pathway Miao-Hsueh Chen, Christopher W. Wilson, and Pao-Tien Chuang 130 Cardiovascular Research Institute, University of California, San Francisco, CA 94158, USA n. DOI 10.1016/j.cell.2007.07.017 , July27, 2007©2007Elsevier Inc. See online versionforlegend, abbreviations, andreferences. SnapShot: Hedgehog Signaling Pathway Miao-Hsueh Chen, Christopher W. Wilson, and Pao-Tien Chuang Cardiovascular Research Institute, University of California, San Francisco, CA 94158, USA

The Hedgehog (Hh) signal transduction pathway controls numerous processes during fly and vertebrate embryonic development and adult homeostasis, including tissue/organ patterning, cellular proliferation and differentiation, pathfinding, left/right asymmetry, and stem cell maintenance. Hh signaling is dysregulated in several congenital defects and many types of tumors. Hh is able to signal at short-range but also may act as a long-range morphogen in multiple contexts (this aspect is controlled in part by lipid modification of the Hh molecule). Hh signaling controls the balance of the transcriptional activator and repressor forms of Ci/Gli, the ratios of which are essential for interpreting the level of Hh signal in a morphogenetic field and for generating diverse outputs.

Hh-Producing Cell After translation, the Hh precursor enters the ER/Golgi secretory pathway. Autoproteolysis mediated by the C-terminal of Hh (Hh-C) releases the N-terminal signaling peptide (Hh-N), with cholesterol covalently linked to its C terminus. Addition of a palmitate moiety to the N terminus of the Hh signaling fragment is catalyzed by Skinny hedgehog (Ski/Skn). Lipid modifications are essential for the Hh ligand to induce high-level signaling activity and proper formation of a morphogen gradient. Lipidated Hh is assisted in its release from cells by Dispatched and possibly by the heparan sulfate proteoglycans (HSPGs) Dally and Dally-like (Dlp) and can be detected in dis- tant Hh-responsive cells. Although the mechanism of lipidated Hh-N release into the extracellular matrix is unknown, two soluble forms of lipidated Hh-N have been detected including a multimeric protein complex in both flies and vertebrates and a lipoprotein particle in flies. In flies, Hh in the extracellular matrix is protected from degradation by Shifted (Shf) and is funneled to its destination by Dlp. Specific HSPG core proteins may play a role in vertebrate Hh signaling, but this has not yet been shown.

Hh-Responding Cell Hh elicits transcriptional responses by binding to its trimeric receptor Patched (Ptc/Ptch1). Additional Hh coreceptors (Ihog family members, HSPGs, and Gas1) may participate in induction of both transcription-dependent and -independent responses. Binding of Hh to Ptc alleviates repression of Smoothened (Smo), allowing its translocation from endosomes to the cell surface in flies or to the primary cilium in mice. Inhibition of Ptc results in phosphorylation of the cytosolic C-tail of Smo. Flies and vertebrates may use dif- ferent strategies downstream of Smo to modulate activity of the Ci/Gli transcription factors. In fly, recruitment of the atypical kinesin Costal-2 (Cos-2)/Fused kinase (Fu)/Cubitus interruptus (Ci) complex to the phosphorylated Smo C-tail results in activation of the Ci transcription factor, which then enters the nucleus and activates target gene expression. In vertebrates, generation of activated Gli transcription factors involves the primary cilium, but it is currently unclear if a kinesin-like scaffold is an intermediary between Smo and Gli. Numerous poorly characterized factors have been reported to play roles in modulating Gli activator and repressor activity. Two general mechanisms exist to downregulate Hh signaling after the initial response. Ci/Gli transcription factors are ubiquitinated by Cullin (Cul3) and HIB/SPOP complexes; the expression of Hh-binding proteins, such as Ptc and Hhip1, is upregulated to serve as a sink for extracellular ligand.

Hh-Nonresponding Cell In the absence of Hh ligand, Ptc inhibits Smo and prevents its translocation to the cell surface in flies or to the primary cilium in mice. The nucleocytoplasmic distribution of newly synthesized Ci is controlled by the Ci/Gli-binding protein Sufu. In fly, microtubule-associ- ated Cos-2 serves as a scaffold for protein kinase A (PKA), casein kinase I (CKI), and glycogen synthase kinase 3 (GSK3), which phos- phorylate Ci. The SCF-Slimb-Cul1-Roc1a complex recognizes phosphorylated Ci and targets it for proteasome-dependent limited pro- teolysis, which removes C-terminal transcriptional activation domains. The newly generated Ci repressor translocates to the nucleus where it inhibits Hh target gene activation in conjunction with Sufu. In vertebrates, the primary cilium is involved in efficient generation of Gli repressors, which are formed through a similar phosphorylation-dependent mechanism. Sufu may also mediate interactions between Gli repressors and the HDAC1 repression complex.

Abbreviations Arrb2, arrestin β2; Boc, cdo-binding protein; Boi, brother of ihog; Botv, brother of tout-velu; Cdo, cell adhesion molecule-related/downregu- lated by oncogenes; Ext1/2, exostoses 1/2; Extl3, exostoses-like 3; Gas1, growth arrest specific-1; GRK2, G protein-coupled receptor kinase 2; HDAC1, histone deacetylase 1 complex; Ihog, interference hedgehog; IFT, intraflagellar transport; Igu/DZip1, iguana/DAZ interacting protein 1; MIM, missing in metastasis; Sotv, sister of tout-velu; SPOP, speckle-type POZ protein; Sufu, suppressor of fused; Ttv, tout-velu.

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