Metazoans use Hedgehog signaling to direct many stages of embryonic development, and deregulation of this pathway is implicated in many types of cancer. I investigated several steps of Hedgehog pathway transduction that were poorly understood in mechanistic terms. The mature Hedgehog ligand is produced by a self-proteolysis reaction that covalently attaches a cholesterol molecule to the signaling half of the protein. I showed that the catalytic cysteine forms a disulfide bridge that is essential for the folding and function of the C-terminal tail of Hedgehog, and identified two protein disulfide isomerases that remodel this bridge to free the catalytic thiol group after folding is complete. Using pulse chase assays to follow Hedgehog processing, I demonstrated that the self-proteolysis reaction takes place in the endoplasmic reticulum, that the cleaved C-terminal tail of Hedgehog is degraded before moving to the Golgi, and that Hedgehog mutants defective in processing get degraded in their entirety by the same route. Lipidated Hedgehog ligand requires the transmembrane protein Dispatched for secretion. I devised a system to test Dispatched function in cultured cells, and showed that some inactive Dispatched mutants fail to bind Hedgehog, while others bind more tightly than the wild type protein. Scube2 was implicated as a Hedgehog pathway component in zebrafish genetic studies. I showed that Scube2 is a secreted protein that binds Hedgehog via its cholesterol adduct and solubilizes it in aqueous media. Dispatched and Scube2 bind Hedgehog on opposing faces, and they function synergistically to release it from the membrane. Vertebrate Hedgehog signaling relies on intraflagellar transport through an antenna-like organelle called the primary cilium. The Hedgehog receptor Patched and transducer protein Smoothened localize to primary cilia in a mutually exclusive pattern, depending on Hedgehog ligand presence. I showed that cytoplasmic components of the pathway Suppressor of Fused (SuFu, a pathway inhibitor) and Glioma-associated oncogene transcription factors (the Gli family, the effectors of the pathway) localize to primary cilia and accumulate there when Smoothened is activated. SuFu and Gli form a complex that dissociates when the pathway is turned on, and this dissociation depends on trafficking through the cilium.

Hedgehog signaling is essential during embryogenesis and in the maintenance of adult

The Hedgehog (Hh) signaling pathway has received attention regarding its important role in embryonic development, however the mechanism by which pathway regulators, such as Suppressor of fused (Sufu), Kinesin family member 7 (Kif7), and primary cilium, mediate Hh signaling transduction is not entirely understood. The work presented here examines the roles of Sufu and Kif7 in regulating Hh signaling in growth plate chondrocytes, as well as how they mediate parathyroid hormone-like hormone (Pthlh) signaling during chondrocyte development. I show here that Sufu and Kif7 are essential regulators of Indian hedgehog (Ihh) signaling. While Sufu negatively regulates Gli transcription factors, Kif7 functions both positively and negatively in chondrocytes. Kif7 plays a role in Sufu protein degradation and the exclusion of Sufu-Gli complexes from the primary cilium. Importantly, halving the dosage of Sufu restores normal Hh pathway activity and chondrocyte development in Kif7-null mice, demonstrating that the positive role of Kif7 is to restrict the inhibitory function of Sufu. Furthermore, Kif7 exerts inhibitory function on Gli transcriptional activity in chondrocytes when Sufu function is absent. Therefore, Kif7 regulates the activity of Gli transcription factors through both Sufu-dependent and Sufu-independent mechanisms. I show that Sufu is crucial for mediating the negative effect of Pthlh on Gli transcriptional activity and chondrocyte hypertrophic differentiation, whereas Kif7 and primary cilium are dispensable in this process. Although primary cilium is required for Hh ligand-mediated activation of Gli transcription, Pthlh negatively controls Gli transcriptional activity in a cilia-independent manner. The results of this work provide insight into how Hh signaling is regulated by Sufu and Kif7 in the context of primary cilium, but also suggest Sufu serves as an important link between Ihh and Pthlh signaling during growth plate chondrocyte development.

The Hedgehog (Hh) signaling pathway has received attention regarding its important role in embryonic development, however the mechanism by which pathway regulators, such as Suppressor of fused (Sufu), Kinesin family member 7 (Kif7), and primary cilium, mediate Hh signaling transduction is not entirely understood. The work presented here examines the roles of Sufu and Kif7 in regulating Hh signaling in growth plate chondrocytes, as well as how they mediate parathyroid hormone-like hormone (Pthlh) signaling during chondrocyte development. I show here that Sufu and Kif7 are essential regulators of Indian hedgehog (Ihh) signaling. While Sufu negatively regulates Gli transcription factors, Kif7 functions both positively and negatively in chondrocytes. Kif7 plays a role in Sufu protein degradation and the exclusion of Sufu-Gli complexes from the primary cilium. Importantly, halving the dosage of Sufu restores normal Hh pathway activity and chondrocyte development in Kif7-null mice, demonstrating that the positive role of Kif7 is to restrict the inhibitory function of Sufu. Furthermore, Kif7 exerts inhibitory function on Gli transcriptional activity in chondrocytes when Sufu function is absent. Therefore, Kif7 regulates the activity of Gli transcription factors through both Sufu-dependent and Sufu-independent mechanisms. I show that Sufu is crucial for mediating the negative effect of Pthlh on Gli transcriptional activity and chondrocyte hypertrophic differentiation, whereas Kif7 and primary cilium are dispensable in this process. Although primary cilium is required for Hh ligand-mediated activation of Gli transcription, Pthlh negatively controls Gli transcriptional activity in a cilia-independent manner. The results of this work provide insight into how Hh signaling is regulated by Sufu and Kif7 in the context of primary cilium, but also suggest Sufu serves as an important link between Ihh and Pthlh signaling during growth plate chondrocyte development.

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During embryogenesis, airway epithelial cells possess primary cilia, and HH signaling guides lung development. As epithelial cells mature, they produce hundreds of motile cilia and continue to produce the sonic hedgehog (SHH) ligand, which is found apically in the thin layer of liquid covering airways. However, whether ciliated airway cells express apical HH signaling components and what their function might be have remained unknown. Here we show that motile cilia are enriched for HH signaling proteins, including patched 1 and smoothened. These cilia are also enriched for proteins affecting cAMP-dependent signaling, including Gαi and adenylyl cyclase 5/6. Surprisingly, SHH in differentiated airway epithelia did not elicit the canonical SHH signaling pathway that regulates transcription during development. But instead, activating HH signaling decreases intracellular levels of cAMP, which reduces ciliary beat frequency and airway surface liquid pH, similar to changes that have been observed in the airway of people with chronic obstructive pulmonary disease (COPD). Furthermore, we observed that significant increase of SHH ligand expression in differentiated airway epithelia with COPD, suggesting a potential role of SHH signaling in the pathogenesis of airway disease. Collectively, our study indicates that airway cilia detect apical SHH to mediate airway physiology through noncanonical HH signaling. SHH may dampen defenses at the contact point between the environment and the lung, perhaps counterbalancing processes that stimulate airway defenses. This may suggest a potential role of SHH signaling in the pathogenesis of airway disease, such as COPD.

Small molecules play important roles in therapeutics and drug discovery. Significant progress has been made by the chemical biology community to discover small-molecule probes to explore biological processes and to treat disease. This thesis describes both the discovery of novel probes for the Hedgehog (Hh) pathway and the application of small molecules in identifying cancer dependencies. / Chemistry and Chemical Biology

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Our laboratory has identified two secreted proteins, Wnt7a and Sonic hedgehog (Shh), that regulate satellite cell (SC) fate, during muscle differentiation. While Wnt7a stimulates symmetric SC division through the planar cell polarity (PCP) pathway, Shh activates Myf5 expression in the committed SC following asymmetric division through cilia-mediated Hedgehog (Hh) signaling. Crosstalk between these pathways has been well characterized during development, and is likely to be conserved in muscle regeneration. Indeed, accumulating evidence suggests the PCP pathway influences primary cilia formation, an organelle required for proper Hh signal transduction. Here we show that Wnt7a treatment in primary myoblasts increases the presence of primary cilia. Additionally, using myofiber culture, we demonstrate that Wnt7a increases myogenin (MyoG) expression. Removal of primary cilia through a small interfering RNA (siRNA) targeted towards IFT88 impedes Wnt7a mediated MyoG expression, suggesting crosstalk between the PCP and Hh pathways facilitates muscle differentiation. Furthermore, through siRNA knockdown we have identified the downstream PCP effectors, Inturned and Fuzzy as the main candidates responsible for this crosstalk. Knockdown of either Inturned or Fuzzy impedes Wnt7a-mediated MyoG expression. Taken together our data demonstrates crosstalk between the PCP pathway and Hh signaling regulates the differentiation of SCs.