Regulators of Shh Signaling in Limb Patterning and Development

The embryonic limb is one of the best understood examples of pattern formation in vertebrates. Patterning, along its anterior-posterior (AP) and proximo-distal (PD) axes, has been studied almost exclusively within the framework of positional information - the idea that the fate of individual cells is determined by their positional value in an embryonic field. This value is conferred through signaling molecules, called morphogens, in a concentration dependent manner. This idea is best illustrated in the chick wing whose AP pattern is imposed by an organizing center called the "zone of polarizing activity" (ZPA), through secretion of the morphogen Sonic hedgehog (Shh). Studies over the past 20 years show that Shh regulates patterning and growth through regulation of Gli-family transcription factors. The mouse has been an excellent genetic model for the study of Shh-mediated limb patterning and signal transduction, as well as the pathway's role in development and disease. However, these studies have also revealed critical differences between the mouse and chick models resulting in a need to re-evaluate positional information as a universal framework for vertebrate development. In this thesis, I propose the two-population model as a complimentary framework that allows us to reconcile the discrepancies of existing models. Using genetic models in the mouse, and pharmacological manipulation in the chick, I demonstrate that specific changes in Gli activity produce a defined spectrum of limb malformations and I utilize this spectrum as a tool to study a previously uncharacterized component of the Shh signal cascade, the receptor Ptch2, demonstrating that it is a critical, context-dependent regulator of the Shh signaling receptor Smoothened. This body of work illustrates that the vertebrate limb remains a prescient model for the study of signal transduction, development and disease and opens the door for the study of universal mechanisms of patterning in vertebrate organisms.Ph.D.2016-11-30 00:00:0