Ptc1 and Ptc2 transcripts provide distinct readouts of hedgehog signaling activity during chick embryogenesis

Hedgehog (Hh) signaling in vertebrates controls patterning and differentiat ion of a broad range of tissues during development. The Hh receptor Patched (Ptc) is a critical regulator of signaling, maintaining active repression of the pathway in the absence of stimulation, limiting excess diffusion of ligand, and providing an efficient negative feedback mechanism for fine-tun ing the responsiveness of receiving cells. Two distinct Ptc genes have been isolated from several vertebrates. Here, we describe the cloning of a seco nd Ptc gene from chick (Ptc2). We show that Ptc1 and Ptc2 are both upregula ted at sites of active Hh signaling but that the expression patterns of the se genes only partially overlap, thus providing distinct readouts of Hh pat hway stimulation. We also show that chick Ptc2 is expressed in the posterio r apical ectodermal ridge (AER) of the limb bud in a pattern similar to Fgf 4 and that the induction of Ptc2 within the AER, like that of Fgf4, is medi ated via antagonism of BMP signaling. The differential responsiveness of ce lls to Hh pathway stimulation (as marked by the differential induction of P tc genes) suggests heterogeneity in the mechanisms by which Hh signals are transduced within different populations of receiving cells. (C) 2001 Academ ic Press.