Order in the classroom: graded responses to instructive Hh signaling in the CNS

In many animals, the secreted Hedgehog (Hh) signaling proteins play important roles during development and in adults. Studies in both flies and vertebrates indicate that Hh functions as a morphogen to elicit different responses at distinct concentration thresholds. In vertebrates, Gli proteins are the primary transcriptional mediators of Hh target genes. However, the mechanisms that implement specific genetic responses to graded Hh-Gli signaling are only just beginning to be understood. In particular, it is unclear whether target gene responses are determined solely by the ambient levels of pathway activity, or if other pathways or factors function to amplify or attenuate the response to this signal to provide an additional level of context that permits a more fine-tuned outcome. Here, I will review recent evidence suggesting that the response of some Hh-Gli target genes in the CNS is regulated by the activity of another important extracellular signal, the canonical Wnt pathway. The possibility that the Hh and Wnt pathways interact at the transcriptional level has broad significance for understanding normal embryogenesis and diagnosing and treating the numerous developmental disorders and cancers that involve these two pathways. Thus, while Hh-Gli signals provide important information, it is likely that they receive assistance from other "instructors".