Hedgehog feedback loop-de-loops

Hedgehog (Hh) signalling patterns animal tissues, such as the Drosophila wing disc. The binding of Hh ligands to Patched receptors indirectly prevents cleavage of Cubitus interruptus (Ci) into a transcriptional repressor. Thus, Hh promotes full-length (activator) Ci to drive target gene expression, such as patched, which provides a negative feedback loop. Now, Robert Holmgren and colleagues use RNA interference, clonal analyses and genetics to reveal other feedback mechanisms. First, they observe that Engrailed, which marks Hh-secreting cells in the posterior wing disc, has a second role in anterior (Hh-responsive) cells downstream of Hh signalling. In turn, Engrailed represses ci (and thus patched) to ‘tune’ Hh gradients. Next, the researchers employ ci^Ce2 mutants, which encode a truncated Ci that mimics the repressor form, to investigate Hh target gene roadkill. Overexpression of roadkill in wild-type or ci^Ce2/+ mutants increases or decreases Hh activity, respectively, indicating that Roadkill affects competition between Ci activator and repressor forms. Indeed, the additional loss of Ci-binding protein Su(Fu) drastically reduces Hh activity. Finally, whereas full-length Ci and Su(Fu) enter the nucleus together, Ci^Ce2 translocates alone. Together, these data show that Roadkill acts to clear full-length Ci after Hh signalling has receded and Su(Fu) protects Ci from Roadkill-mediated degradation.