Constructing a dynamic model of hedgehog signaling

The evolutionarily conserved Hedgehog (Hh) signaling pathway is required for proper embryonic patterning and development. Deregulated Hh signaling has been implicated in various human carcinomas and developmental defects, thus elucidation of the underlying signaling mechanisms is of great importance. My work has centered on investigating how downstream Hh signaling effectors localize and interact to regulate pathway activity via the transcription factor Cubitus interruptus (Ci), with the goal of building a new model of Hh signaling. I approached this problem by testing current models of signaling and comparing the conflicting aspects. Current models of Hh signaling can be categorized by the ratio of signaling components that each requires. Thus, to distinguish between models, I quantitated the steady-state molar ratio of five core components. The quantitation revealed vast differences in the relative levels of components and allowed clear distinctions to be made between models of signaling. I also examined the function Costal-2 (Cos2), a core component required for Hh signaling and essential to understanding the regulatory mechanisms within the pathway. Cos2's functional role has not been well understood, but here I provide the first evidence that Cos2 is a kinesin-like protein and its intrinsic motility is required for its function. This motility appears to be Hh regulated, as well as ATP- and microtubule-dependent. Moreover, Cos2 recruits and transports other Hh signaling components. I demonstrate that Cos2 motility likely plays an important role in regulating the activity of Ci. Together, these results provide further insight into Hh pathway operation by suggesting new possible mechanisms for signaling and providing the first step in critically reevaluating current views of Hh signaling. Here, I present a new model of signaling that incorporates the element of Cos2 motility and accounts for the relative amounts of the signaling components, in an effort to demonstrate that signaling events are not static, but dynamically regulated.