Hedgehog signaling: Mechanisms and evolution

The Hedgehog (Hh) family of secreted proteins plays essential roles in the development of a wide variety of animal species and underlies multiple human birth defects and cancers. To ensure the proper range of signaling, the Hh proteins are modified with lipids, assembled into water-soluble multimers, and interact with multiple cell surface proteins. In the target cells, a largely conserved intracellular signal transduction pathway, from the cell surface receptor Patched to the Glioma-associated oncogene homolog (Gli) family of transcription factors, mediates the transcriptional responses from fruit flies to mammals. A significant divergence between vertebrates and insects is the vertebrate-specific requirement of cilia for Hh signal transduction and Gli protein activation. Finally, transcription-independent cellular responses to Hh have been described in certain developmental processes. With clinical trial underway to treat Hhrelated diseases, more work is urgently needed to reach a more comprehensive understanding of the molecular mechanisms underlying the regulation of Hh signaling in development and diseases.