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.

Cell biology of polycystin-2

“…In this case, Ca 2+ binding proteins/effectors should be present in the cilium. This suggestion is supported by the presence of Ca 2+ -binding proteins in the flagellum [65] and more recent studies showing that Hedgehog [66][67][68] and platelet-derived growth factor receptor α (PDGFR α) signaling [69] occurs in the cilium, indicating that the cilium may actually function as a platform for signal transduction. Interestingly, PKD2 has been shown to be activated by EGF in the plasma membrane [36] and to co-localize in the primary cilium with the EGFR and phosphatidylinositol-4, 5-bisphosphate (PIP 2 ) [36].…”

Planar cell polarity effector gene Fuzzy regulates cilia formation and Hedgehog signal transduction in mouse

“…Recent studies indicate that cilia are essential for intracellular transduction of Hedgehog (Hh) signals (Huangfu et al, 2003;Haycraft et al, 2005;Huangfu and Anderson, 2005;Liu et al, 2005;May et al, 2005). The Hh family of secreted proteins (Shh, Dhh, and Ihh) regulates development of multiple organ systems in mammals through a morphogen-like mechanism (Jiang and Hui, 2008).…”

“…Paradoxically, the central nervous system (CNS) is partially dorsalized suggesting a decrease in Hh pathway activity, whereas in the limbs, many digits are observed (polydactyly), suggesting an increased Hh pathway activity. Detailed genetic and biochemical studies indicate that such paradoxical phenotypes result from both a reduction in the proteolytic processing of Gli3 and a failure to activate the Gli activator functions (Haycraft et al, 2005;Huangfu and Anderson, 2005;Liu et al, 2005;May et al, 2005;Jia et al, 2009). Ciliogenesis, or cilia formation, has been best studied in Chlamydomonas and C. elegans (Gerdes et al, 2009).…”

Targeting the CB2 cannabinoid receptor in osteoporosis

“…For the target cell, hedgehog signaling can be facilitated by the presence of heparin proteoglycans and lower affinity hedgehog coreceptor proteins that include CDON and BOC [49]. Further downstream, integration of vertebrate hedgehog signaling into the primary cilium means that signal processing requires the activities of numerous ciliar transport proteins to shuttle Gli proteins into and out of the cilium [41,50,51]. Genetic ablation of individual ciliar transport proteins in mice confers phenotypes that are reiterative of mutations in the primary hedgehog regulatory genes.…”