Mechanistic Diversification of the Hedgehog Signaling Pathway: Insights into Left-Right Asymmetry and Transduction by Motile Cilia in the Sea Urchin

摘要

A relatively small number of developmental signaling pathways govern the early patterning of most metazoa. Comparing the function and transduction of these pathways in diverse taxa offers unique insight into their evolution. One pathway in particular, Hedgehog (Hh) signaling, is essential for embryonic patterning in many species throughout the animal kingdom. In this dissertation we examine the role of Hh signaling in left-right symmetry breaking and its mechanism of signal transduction in the sea urchin. By studying Hh in the sea urchin, a basal deuterostome, we are able to gain unique insight into the evolution of this pathway. In this dissertation we present evidence that in the sea urchin, Hh signaling is upstream of right-sided nodal expression, defining an evolutionary conserved symmetry-breaking pathway. We also present results that provide insight into the mechanism of Hh signal transduction showing that Hh signaling is dependent on motile cilia in the sea urchin.;Studies of left-right asymmetry in the sea urchin have revealed a conserved symmetry-breaking pathway involving Nodal and BMP. A role for Hh, known to regulate Nodal expression in vertebrates, has yet to be defined. In chapter 2, we present data that resolve a role for Hh signaling in establishing left-right asymmetry in the sea urchin. We found that inhibition of Hh signaling results in significant down regulation of Nodal and it's targets. Curiously, we found normal asymmetric gene expression of the left-side marker SoxE in these embryos, indicating that this regulation occurs after the initial asymmetry is established. We conclude that Hh plays a role in maintaining and reinforcing asymmetric right-sided nodal expression. These results further refine the pathway of symmetry breaking in the sea urchin and define a previously unresolved function of Hh signaling and symmetry breaking in this process.;The mechanism of intracellular Hh transduction has evolved between protostome and vertebrate models, with the later relying on the presence of specialized primary cilia for effective signaling. In Chapter 3, we provide evidence that in sea urchin, Hh signaling requires motile cilia for signal transduction. We found that Hh receiving cells exhibit motile cilia. Furthermore preventing cilia assembly causes Hh phenotypes and decreased expression of Hh target gene expression. Finally, we found that the Hh effector Smo is trafficked into the cilia of Hh receiving cells. This lead us to a model of Hh signal transduction in the sea urchin that is dependent upon motile cilia. This is the first evidence of a motile cilium transducing a developmental pathway in any model system and provides insight into the evolution of the Hh pathway, and the divergent mechanisms of signal transduction observed across phyla.