Gli2 and Gli3 Localize to Cilia and Require the Intraflagellar Transport Protein Polaris for Processing and Function

Abstract

Intraflagellar transport (IFT) proteins are essential for cilia assembly and have recently been associated with a number of developmental processes, such as left-right axis specification and limb and neural tube patterning. Genetic studies indicate that IFT proteins are required for Sonic hedgehog (Shh) signaling downstream of the Smoothened and Patched membrane proteins but upstream of the Glioma (Gli) transcription factors. However, the role that IFT proteins play in transduction of Shh signaling and the importance of cilia in this process remain unknown. Here we provide insights into the mechanism by which defects in an IFT protein, Tg737/Polaris, affect Shh signaling in the murine limb bud. Our data show that loss of Tg737 results in altered Gli3 processing that abrogates Gli3-mediated repression of Gli1 transcriptional activity. In contrast to the conclusions drawn from genetic analysis, the activity of Gli1 and truncated forms of Gli3 (Gli3R) are unaffected in Tg737 mutants at the molecular level, indicating that Tg737/Polaris is differentially involved in specific activities of the Gli proteins. Most important, a negative regulator of Shh signaling, Suppressor of fused, and the three full-length Gli transcription factors localize to the distal tip of cilia in addition to the nucleus. Thus, our data support a model where cilia have a direct role in Gli processing and Shh signal transduction.

Bardet-Biedl syndrome: an emerging pathomechanism of intracellular transport

“…Second, in Xenopus embryos, it was shown that PCP components, Dishevelled and Inturned, are required for cilia biogenesis and Hedgehog signaling [105]. Indeed, these findings serve to consolidate previous studies, which found that ciliogenic intraflagellar transport pathway components are required for normal Hedgehog signaling in vertebrates [106][107][108][109][110]. Finally, the observation that key components of PCP, namely Inversin, Vangl2, Dishevelled and Inturned, all localise at or near basal bodies and/or cilia [64,105,111] provides further support for the notion that primary cilia-basal bodycentrosomal components play central roles in conducting PCP signaling.…”

Decoding the Hedgehog signal in animal development

“…Cilia localization of Smo is stimulated by Hh and inhibited by an Hh-pathway-specific inhibitor [174]. In support of the functional significance of cilia localization of Smo, an oncogenic form of Smo is constitutively localized in cilia, whereas a mutant form of Smo lacking a conserved cilia localization motif fails to support Hh signaling [174][175][176]. These results suggest that Hh activates Smo at least in part by promoting its cilia localization.…”

“…IFT proteins are essential for the assembly and maintenance of cilia and flagella [173]; hence, the finding that IFT components are required for Hh signaling implies that cilia mediate Hh signal transduction. Indeed, recent studies revealed that several mammalian Hh signaling components including Smo, Su(fu), and all three Gli proteins are localized to cilia [174][175][176]. In addition, cilia localization of Smo is disrupted in IFT mutants.…”

The perennial organelle: assembly and disassembly of the primary cilium

“…Primary cilia have been long suspected to possess important sensory functionalities, but recent data suggest that they also play important roles in the control of cell proliferation and in several developmental signaling pathways. Indeed, the Hedgehog, Wnt and plateletderived growth factor (PDGF) signaling pathways seem to rely on the highly organized confines of the ciliary membrane to coordinate the initiation and transmission of extracellular signals to the interior of the cell (Morgan et al, 2002;Cano et al, 2004;Haycraft et al, 2005;Schneider et al, 2005;Corbit et al, 2005;Corbit et al, 2008). Although it is now clear that primary cilia sample the extracellular environment and are the sites of organization of multiple signal transduction pathways, basic questions regarding the regulated assembly and disassembly of these organelles remain.…”