Proper regulation of Indian hedgehog (Ihh) signaling is vital for chondrocyte proliferation and differentiation in the growth plate. Its dysregulation causes skeletal dysplasia, osteoarthritis or cartilaginous neoplasia. Here, we show that Suppressor of fused (Sufu) and Kif7 are essential regulators of Ihh signaling. While Sufu acts as a negative regulator of Gli transcription factors, Kif7 functions both positively and negatively in chondrocytes. Kif7 plays a role in the turnover of Sufu and the exclusion of Sufu-Gli complexes from the primary cilium. Importantly, halving the dose of Sufu restores normal hedgehog pathway activity and chondrocyte development in Kif7-null mice, demonstrating that the positive role of Kif7 is to restrict the inhibitory activity of Sufu. Furthermore, Kif7 also inhibits Gli transcriptional activity in the chondrocytes when Sufu function is absent. Therefore, Kif7 regulates the activity of Gli transcription factors through both Sufu-dependent and -independent mechanisms.

Background: Hh and Pthlh signaling pathways play important roles in regulating growth plate chondrocyte differentiation. Results: Pthlh increases Sufu protein levels, and Sufu is required for the effect of Pthlh on chondrocyte differentiation. Conclusion: Pthlh regulates chondrocyte differentiation and Gli activity in a Sufu-dependent manner. Significance: The interaction between Pthlh, PKA, Sufu, and Gli transcriptional activities explains how Pthlh regulates chondrocyte differentiation.