| During animal development, the Hedgehog (Hh) signal transduction pathway plays a key role in multiple processes including cell fate determination, tissue patterning and morphogenesis. In humans, aberrant Hh signaling has been linked to several genetic disorders and cancers. Hh signaling is ultimately mediated through the activities of the bi-functional Gli/Ci transcription factors. Previously, the mechanism of mammalian Hh signal transduction was extrapolated largely from observations in Drosophila. In this paradigm Suppressor of fused (Su(fu)), a novel protein, inhibits pathway activity by preventing the nuclear accumulation of Ci activator, but is dispensable for development. We have previously cloned mouse Su(fu) and demonstrated it plays a conserved role to inhibit Hh signaling in vitro. Furthermore, mutations in human Su(fu) were found associated with medulloblastoma, implicating Su(fu) as a tumor suppressor. In this thesis, I employ biochemical, cell biological and genetic approaches to investigate the mechanisms through which Su(fu) inhibits activity of the mammalian Gli genes (Glil-3). I find Su(fu) controls the subcellular localization of Gli1 and Gli2 through distinct mechanisms. Moreover, Su(fu) can inhibit Gli activity independent of sequestration, suggesting Su(fu) regulates Gli activity through both nuclear and cytoplasmic means. Loss-of-function analysis in mice revealed Su(fu) is indispensable for embryogenesis, acting as a critical rate-limiting factor to control pathway activity. My genetic and biochemical analyses demonstrated a role for Su(fu) to promote efficient processing of Gli3 into a transcriptional repressor. That Su(fu) promotes Gli repressor formation was unexpected as no similar role has been identified in flies. Significantly, preliminary analyses suggest that Su(fu) controls Gli3 processing through multiple mechanisms. Most notably, Su(fu) appears to facilitate efficient cAMP-dependent protein kinase-mediated phosphorylation of Gli3. Together this work uncovers important mechanistic insight into the regulation of mammalian Hh signaling and places Su(fu) in a central role as an inhibitory component acting through several mechanisms. |
