Dual Phosphorylation of suppressor of fused (Sufu) by PKA and GSK3beta regulates its stability and localization in the primary cilium

Suppressor of fused (Sufu) is an essential negative regulator of the sonic hedgehog (Shh) pathway, but little is known about how Sufu itself is normally regulated. Here, we report that Sufu is phosphorylated at Ser-342 and Ser-346 by GSK3β and cAMP-dependent protein kinase A (PKA), respectively, and phosphorylation at this dual site stabilizes Sufu against Shh signaling-induced degradation. We further show that localization of Sufu in the primary cilium is induced by Shh signaling and is required for the turnover of both phosphorylated and total Sufu. Perturbing Sufu phosphorylation with PKA inhibitors or replacing Ser-346 with alanine reduced the stay and replacing Ser-342 and Ser-346 with aspartic acid prolonged the stay of Sufu in the cilia. Finally, ciliary localization of Gli2/3 also required Smo and was similarly influenced by perturbations of PKA activity or mutations at the dual Sufu phosphorylation site. Thus, Shh likely induced trafficking of phospho-Sufu into the primary cilium in a complex with Gli2/3, and dephosphorylation triggered a retrograde export, allowing Sufu to be degraded by the ubiquitin-proteasome system.     

Ftm is a novel basal body protein of cilia involved in Shh signalling

In this study we show in mice that Ftm (Rpgrip1l) is located at the ciliary basal body. Our data reveal that Ftm is necessary for developmental processes such as the establishment of left-right asymmetry and patterning of the neural tube and the limbs. The loss of Ftm affects the ratio of Gli3 activator to Gli3 repressor, suggesting an involvement of Ftm in Shh signalling. As Ftm is not essential for cilia assembly but for full Shh response, Ftm can be considered as a novel component for cilium-related Hh signalling. Furthermore, the absence of Ftm in arthropods underlines the divergence between vertebrate and Drosophila Hh pathways.     

Defective ciliogenesis, embryonic lethality and severe impairment of the Sonic Hedgehog pathway caused by inactivation of the mouse complex A intraflagellar transport gene Ift122/Wdr10, partially overlapping with the DNA repair gene Med1/Mbd4

Primary cilia are assembled and maintained by evolutionarily conserved intraflagellar transport (IFT) proteins that are involved in the coordinated movement of macromolecular cargo from the basal body to the cilium tip and back. The IFT machinery is organized in two structural complexes named complex A and complex B. Recently, inactivation in the mouse germline of Ift genes belonging to complex B revealed a requirement of ciliogenesis, or proteins involved in ciliogenesis, for Sonic Hedgehog (Shh) signaling in mammals. Here we report on a complex A mutant mouse, defective for the Ift122 gene. Ift122-null embryos show multiple developmental defects (exencephaly, situs viscerum inversus, delay in turning, hemorrhage and defects in limb development) that result in lethality. In the node, primary cilia were absent or malformed in homozygous mutant and heterozygous embryos, respectively. Impairment of the Shh pathway was apparent in both neural tube patterning (expansion of motoneurons and rostro-caudal level-dependent contraction or expansion of the dorso-lateral interneurons), and limb patterning (ectrosyndactyly). These phenotypes are distinct from both complex B IFT mutant embryos and embryos defective for the ciliary protein hennin/Arl13b, and suggest reduced levels of both Gli2/Gli3 activator and Gli3 repressor functions. We conclude that complex A and complex B factors play similar but distinct roles in ciliogenesis and Shh/Gli3 signaling.