Regulation of Graded Intracellular Gli Activities in Hedgehog Dependent Ventral Spinal Cord Ppatterning

The Hedgehog (Hh) signaling pathway is essential to guide tissue patterning and cell fate specification during development. Congenital malformations arise from disruptions to Hh signaling, whereas aberrant pathway activation is implicated in cancer formation. The outcome of Hh signaling is mediated by Gli transcription factors. In the spinal cord, a balance of Gli activators and repressors orchestrates the dorsoventral spatial expression of cell fate determinants in naïve progenitors. How a gradient of Hh signaling governs Gli to generate distinct intracellular Gli activities for the specification of diverse ventral cell fates remains unclear. Proteolytic processing, proteasomal degradation, sequestration and subcellular distribution, mediated by a number of post-translational modifications including phosphorylation and ubiquitination, all play a role to regulate Gli. Using genetic and biochemical approaches, my studies have revealed that Kinesin family protein 7 (Kif7), Suppressor of fused (Sufu) and Speckle-type POZ protein (Spop) cooperatively and antagonistically regulate intracellular Gli activities. I demonstrated that the distinct and overlapping negative functions of Kif7 and Sufu restrict Hh pathway activation and prevent ectopic specification of ventral progenitor cell fates. I established a positive role for Kif7 to promote robust Hh signaling by antagonizing the formation of inhibitory Sufu-Gli complexes, a crucial step toward Gli transcriptional activation. My work also uncovered an essential, positive role for Sufu to promote maximal Hh signaling, demonstrating that sequestration of Gli by Sufu serves dual purposes. I showed that Sufu is required for Gli stabilization, to offset Spop-mediated Gli ubiquitination and proteasomal degradation. Moreover, I ascertained the biological function of mammalian Spop to down-regulate Hh pathway activity and prevent ectopic pathway activation in the ventral spinal cord. Altogether, these findings elucidate how Kif7, Sufu and Spop translate graded Shh signals into distinct intracellular Gli activities, providing significant insight to our understanding of Hh signaling.Ph