Regulators of Hedgehog Signaling in Chondrocytes: Sufu, Kif7, and Primary Cilium

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.Ph