Hedgehog (Hh) proteins regulate important developmental processes, including cell proliferation and differentiation. Although Patched acts as the main Hh receptor in Drosophila, Hh signaling absolutely requires the additional Hh-binding proteins Ihog and Boi. Here we show that, unexpectedly, cerebellar granule neuron progenitors (CGNPs) lacking Boc and Cdon, the vertebrate orthologs of Ihog and Boi, still proliferate in response to Hh. This is because in their absence, Gas1, an Hh-binding protein not present in Drosophila, mediates Hh signaling. Consistently, only CGNPs lacking all three molecules-Boc, Cdon, and Gas1-have a complete loss of Hh-dependent proliferation. In a complementary manner, we find that a mutated Hh ligand that binds Patched1 but not Boc, Cdon, or Gas1 cannot activate Hh signaling. Together, this demonstrates an absolute requirement for Boc, Cdon, and Gas1 in Hh signaling and reveals a distinct requirement for ligand-binding components that distinguishes the vertebrate and invertebrate Hh receptor systems.
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