The Sonic Hedgehog signaling pathway is central in vertebrate development and in several human disease states. Our current understanding of the pathway outlines a straight-forward mechanism whereby the Shh ligand is perceived by the nine-pass transmembrane receptor Ptch1. In its active state Ptch1 represses Smo, a GPCR like protein, which acts as the ultimate on and off switch of the Shh pathway. When Shh binds Ptch1, Ptch1 becomes inactivated and Smo becomes activated, subsequently leading to the downstream activation of the Shh response.
Despite this framework our understanding of how Ptch1-Shh binding leads to Smo activation remains rudimentary. To understand this crucial step of the pathway one must consider that Ptch1 exists within the Shh receptosome. The Shh receptosome, consists of several membrane bound and transmembrane proteins, which bind Shh. Delineating the function of these proteins is important for understanding how receptosome-Shh interactions lead to Shh pathway activation. Boc, Cdo and Gas1 are secondary receptors of Shh that are known agonists of the pathway. Furthermore, all three proteins are suspected to interact with Ptch1 and there collective presence is necessary for activation of the Shh pathway. Ptch2, a paralogue of Ptch1, is dispensable for development, but has been documented to act as a repressor of the Shh pathway.
We sought to further characterize the function of Ptch2, Boc, Cdo, and Gas1 in the context of Ptch1. Through in vitro experiments we have determined that Shh binding to Ptch1; or Boc, Cdo and Gas1 alone is insufficient to potentiate positive Shh signaling, however, Shh binding to Ptch1 alone is sufficient to cause Smo localization to the primary cilia, an event associated with active Shh signaling. Additionally, we have found that Ptch2 functions as a repressor of the Shh pathway in the absence of Ptch1, further suggesting that Ptch2 and Ptch1 share overlapping functions. Moreover, the distinct possibility remains that like other proteins of the Resistance Nodulation Family, Ptch1 and Ptch2 may interact. Thus, the simple model of Ptch1 mediated Shh reception needs to be revised to include the collective activity of Ptch2, Boc, Cdo and Gas1.