Wdpcp affects skeletogenesis via the hedgehog pathway

A forward genetics screen in mice identified a mutation in the gene Wdpcp that leads to severe skeletal dysmorphogenesis with disrupted formation of the primary cilium. The primary cilium is a transiently formed organelle that facilitates the processing of hedgehog transcription factors Gli2 and Gli3. Conditional deletion of Wdpcp in the limb bud mesenchyme recapitulates the appendicular skeletal phenotype of the constitutive Wdpcp loss of function mutant. Loss of Wdpcp disrupts formation of the repressor form of Gli3, leading to hyper activation of hedgehog pathway. This hyper activation delays chondrogenesis via increased expression of bone morphogenetic protein (BMP) inhibitor Grem1, suppressing activation of BMP pathway necessary for chondrogenesis. Suppression of hedgehog signaling and Grem1 expression and concomitant rescue of chondrogenesis is demonstrated with treatment of in vitro limb bud micromass cultures with direct hedgehog transcription factor Gli inhibitor GANT61, but not hedgehog signal transducer Smoothened inhibitor. This indicates that Wdpcp functions below Smoothened in the hedgehog pathway and directly implicates hedgehog signaling in the observed chondrogenic delay. In order to determine the effects of Wdpcp loss independent of this observed chondrogenic delay, we generated chondrocyte specific deletion mouse model of Wdpcp. Loss of Wdpcp and formation of the primary cilium in chondrocytes disrupts formation of trabecular bone, and mouse embryonic fibroblasts (MEFs) lacking Wdpcp have diminished in vitro osteogenesis. Activation of hedgehog pathway in MEFs lacking Wdpcp is rescued with lentiviral infection with a form of Gli2 mimicking amino terminal phosphorylation. We also demonstrate that overactivation of PKA in MEFs can restore disrupted formation of the repressor form of Gli3. These findings define an important role for Wdpcp and primary cilia in skeletal development and in coordinating the post-translational modification of Gli transcription factors necessary for normal hedgehog signaling.