Journal of Biology Biomed Central

Journal of Biology BioMed Central Research news Agonizing Hedgehog Jonathan B Weitzman Published: 6 November 2002 Journal of Biology 2002, 1:7 The electronic version of this article is the complete one and can be found online at http://jbiol.com/content/1/2/7 © 2002 BioMed Central Ltd ISSN 1475-4924 An approach using ‘chemical genetics’ has identified small-molecule agonists of the Hedgehog signaling pathway that may lead the way to drugs for chronic degenerative diseases. It is a rare treat when a drug discovery segmentation, and it was subsequently events in a range of developing tissues program teaches us something about found to control patterning of struc- (see the ‘Background’ box) [2,3]. the biology of the process that it tures such as the eye and the abdominal Recently, signaling by Hh has been attempts to modulate. The paper by cuticle. In mammals there are three hh shown to be important for patterning Maria Frank-Kamenetsky and col- homologs, called Sonic Hedgehog, of the cerebellum, where it promotes leagues in this issue of the Journal of Indian Hedgehog and Desert Hedgehog the proliferation of granule neuron Biology [1] presents a compelling (Shh, Ihh and Dhh, respectively), which precursors. A link between Hh proteins example of how the search for thera- have been implicated in patterning and stem-cell proliferation has raised peutics can provide powerful experi- mental tools and insights into fundamental biology, blurring the dis- tinction between applied and basic The bottom line research. By characterizing a small group of chemically similar agonists of • Frank-Kamenetsky and colleagues designed a cell-based, high-throughput the Hedgehog signaling pathway, assay to screen 140,000 compounds to find modulators of the Hedgehog Frank-Kamenetsky et al. have been able signaling pathway. to propose a new model for how the Smoothened component of the • They identified a small group of related synthetic non-peptidyl molecules Hedgehog-receptor complex works, that can act as agonists of Hedgehog signals at nanomolar concentra- and to hint at the existence of natural- tions, having previously identified small-molecule Hedgehog antagonists. ligand agonists of the Hedgehog signaling pathway (see ‘The bottom • A range of in vitro and in vivo assays were used to show that the line’ box for a summary of their work). agonists can be used as drugs to overcome Hedgehog-signaling defects and to promote cell proliferation. Hedgehog history Signaling by the Hedgehog (Hh) • The action of the agonist compounds is independent of the Hedgehog family of secreted proteins plays a ligand and the inhibitory receptor Patched. Further characterization central role in regulating cell differenti- revealed that the agonists bind directly to the receptor Smoothened. ation and tissue patterning during development [2]. The hedgehog gene • These data give new insights into the nature of Hedgehog-Smoothened (hh) was first identified by virtue of its signaling and raise the possibility of analogous endogenous modulators role in the specification of positional of Hh signaling. identity during Drosophila embryonic Journal of Biology 2002, 1:7 7.2 Journal of Biology 2002, Volume 1, Issue 2, Article 7 Weitzman http://jbiol.com/content/1/2/7 the enticing possibility that modulating a proto-oncogene and activates signal- (Johns Hopkins University School of Hh signaling might be relevant for the ing in response to Hh ligand. The Smo Medicine, Maryland, USA). “The previ- clinical management of certain degener- protein is a seven-transmembrane ous models are not tenable,” and he ative diseases. Indeed, it was recently receptor that resembles conventional G- suggests that alternative models of demonstrated that Shh might be effec- protein-coupled receptors (Figure 1a). receptor function must be considered. tive in treating peripheral nerve damage It appears that Ptc inhibits Smo, or degenerative brain disorders such as although the precise mechanisms are The power of chemical Parkinson’s disease [4,5]. Perhaps not unclear. Hh stimulation relieves Smo genetics surprisingly given its role in develop- from inhibition by Ptc, leading to the Frank-Kamenetsky et al. [1] chose to ment, misregulated Hh signaling has generation of an intracellular signal use chemical genetics in an attempt to also been implicated in cancer [3]. that culminates in a nuclear transcrip- identify compounds that could inter- Specifically, medulloblastoma and basal tional response (Figure 1b). When Ptc fere with the inhibition of Smo by Ptc, cell carcinoma (BCC) are associated is removed the pathway is constitu- or could activate Smo independent of with inappropriate activation of Hh sig- tively ‘on’, independent of the Hh Ptc, or that might act downstream of naling [6,7]. These observations moti- ligand, whereas certain mutations in Smo. In addition to making effective vated Frank-Kamenetsky and colleagues Smo can activate Hh signaling, bypass- drug candidates, these molecules might to search for small-molecule modula- ing Ptc regulation. A heteromeric also help to illuminate the complex tors of the Hh pathway, with the hope receptor model has been proposed, in mechanisms underlying signaling by that antagonists and agonists might be which Hh interacts directly with Ptc Hh, Ptc, and Smo (see the ‘Behind the used as drugs to treat proliferative or and thereby affects the interaction of scenes’ box for more of the background degenerative diseases, and that small Ptc with Smo [8]. “But things look to the work). molecules might prove more amenable much more complicated than we had The high-throughput screen was to pharmacological delivery than the earlier thought,” says Philip Beachy elegantly simple. First, Frank-Kamenetsky Hh-family proteins themselves. Sending the signal Hedgehog signaling challenges the Background way we normally think about signal transduction pathways. Biology is full • Mammals have three Hedgehog (Hh) proteins (Sonic Hedgehog, of examples of extracellular ligands Indian Hedgehog and Desert Hedgehog) that are processed to that bind to specific cell-surface recep- generate functional extracellular ligands. Two receptors are involved in tors, initiating a cascade of biochemi- Hh signaling: Patched (Ptc) is a negative regulator of the Hh-triggered cal events (often involving protein signaling pathway and Smoothened (Smo) is a positive activator kinases) that leads to the activation of (Hh, Ptc and Smo were all originally named for the cuticular pheno- a transcription factor and the induc- types of mutant Drosophila larvae). When Hh ligands bind to Ptc they tion of a set of effector genes. But relieve the negative inhibition and Smoothened initiates a signaling Hedgehog signaling is not so simple. cascade that results in the activation of nuclear transcription factors of Even the ligand is complicated: the the Gli family that regulate effector gene transcription (see Figure 1). Hh proteins undergo unusual process- ing and cleavage to generate an extra- • Hh signaling has been implicated in a wide range of developmental cellular cholesterol-linked peptide processes. Small-molecule modulators of Hh signaling are that serves as the signaling ligand [2]. potential candidates as therapeutic drugs to treat human diseases – And the receptor component is far molecules that mimic Hh signaling (agonists) might be used to treat from understood. degenerative disorders such as Parkinson’s disease, whereas The cellular response to Hh is con- blockers (antagonists) could be used as drugs against certain types trolled by two transmembrane pro- of cancer. teins, Patched (Ptc) and Smoothened (Smo). The Ptc protein weaves across • Chemical genetics uses synthetic small molecules to dissect cellular the cell membrane twelve times and functions such as signal transduction pathways. By analogy with loss- resembles some transmembrane chan- and gain-of-function mutations in genetics, the functional interactions nels. It acts as a negative regulator of between chemical inducers and inhibitors is used to define the the Hh signal and has been defined as hierarchical relationship between protein components of the pathway. a tumor-suppressor. In contrast, Smo is Journal of Biology 2002, 1:7 http://jbiol.com/content/1/2/7 Journal of Biology 2002, Volume 1, Issue 2, Article 7 Weitzman 7.3 (a) (b) researchers then turned to an in vivo Hedgehog ligands model, feeding the compounds to pregnant mice and following the Dhh Shh Ihh effects on the phenotypes of embryos lacking Shh or Smo. The treated embryos displayed activated Hh signaling, demonstrating that the compounds were not toxic and could cross the placental barrier. The devel- Membrane opmental defects of Shh-/- embryos were rescued by treatment with the Cytoplasm agonist but the compound had no Patched Smoothened effect on Hh signaling in the absence of Smo. Nucleus “Once we knew that the agonists were targeting Smo, we wanted to Gli investigate whether they bound to Smo directly and how they activated Effector gene Hh signaling,” says Porter. The cell line that was created for the screen served Figure 1 as a useful tool to test the effects of The Hedgehog signaling pathway. (a) In the absence of Hedgehog ligands, Patched protein inhibits known antagonists on the function of the activity of Smoothened, which resembles G-protein-coupled receptors. (b) On activation by a the agonist. An anti-Hh blocking anti- ligand of the Hedgehog family, the inhibition of Smoothened by Patched is relieved and Smoothened body had no effect, so the agonist is freed to trigger an intracellular signaling cascade; this ultimately leads to transcriptional regulation by transcription factors of the Gli family. must work downstream of the Hh-Ptc interaction. But the agonist was blocked by antagonists that work at the level of Smo or further down- et al. identified a cell line that responds effective at eliciting a cellular response, stream. “We have similar conclusions,” well to Hh stimulation. The introduc- affecting cells when applied in the says Beachy, whose group used photo- tion of a reporter gene (encoding the nanomolar range.

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