Smurfs Regulate Sonic Hedgehog Signaling Transduction

Sonic Hedgehog(Shh) pathway is one of the most fundamental signaltransduction pathways in vertebrate embryonic development, being responsible forpatterning the neural tube, axial skeleton, limbs, lungs, skin, hair and teeth.Disruption of Shh signaling was found to be the underlying cause of numerousdevelopmental disorders and that perturbations of Shh signaling play a causative orfacilitating role in many human cancers. Given these important physiological roles, itwas no surprise that mechanism of Shh signaling transduction was the emphasis andhotspot of developmental and oncologic study.In vertebrates, Shh signaling transduction initiates from the binding of Shhligand to the12thtransmembrane receptor, Patched1(Ptch1), which releases itsrepression on7thtransmembrane receptor, Smoothened (Smo), and activates theexpression of the target genes. However, the mechanism in which Smo modulates theabundance and activity of transcription factor Gli remains mysterious.Cell surface reception of Shh concentration gradient that conveys the instructionof patterning is crucial during mammalian development. Studies have shown thatwhen Shh ligand is secreted and transported to the vicinity of receiving cells, themovement that Ptch exiting from and Smo importing into the primary cilium plays animportant role in discerning the minute difference of the signal. During the endocyticpathway from plasma membrane to endosomes, receptors can be sorted to thelysosomes, or back to the plasma membrane. A major sorting signal is ubiquitination.and many HECT-domain E3ligases have been implicated in the ubiquitin control ofendocytosis But the full view of the ubiquitin ligases involved in the regulation of receptormovement or Shh signal transduction is remaining unsettled. In order to find thereceptor trafficking related ubiquitin ligases systematically, we first screened theHECT E3ligases by the use of a set of siRNA based system. After the luciferasereporter and cell immunofluorescence image screening, we found that Smurf1,Smurf2and Wwp1can regulate the ciliary location of Ptch1and Gli3, andup-regulate the Gli1activity. Our data also suggest that Ptch1is the substrate ofSmurf1and Smurf2, and its degradation is modulated by both lysosome andproteasome. Thus, we provide a novel target for the research of Shh mechanism.