Hedgehog signaling from the primary cilium to the nucleus: an emerging picture of ciliary localization, trafficking and transduction

Abstract

The unexpected connection between cilia and signaling is one of the most exciting developments in cell biology in the past decade. In particular, the Hedgehog (Hh) signaling pathway relies on the primary cilium to regulate tissue patterning and homeostasis in vertebrates. A central question is how ciliary localization and trafficking of Hh pathway components lead to pathway activation and regulation. In this review, we discuss recent studies that reveal the roles of ciliary regulators, components and structures in controlling the movement and signaling of Hh players. These findings significantly increase our mechanistic understanding of how the primary cilium facilitates Hh signal transduction and form the basis for further investigations to define the function of cilia in other signaling processes.

Figure 1. The mammalian Hh signaling pathway

A. In the absence of the Hh ligand, the Hh receptor Patched (Ptch1) inhibits the accumulation of the signal transducer, Smoothened (Smo), on the ciliary membrane. As a result, at the base of the primary cilium, PKA and Kif7 promote proteolytic processing of the transcription factor Gli3 bythe proteasome into a repressor form (GliR) that suppresses Hh target gene expression in the nucleus. In addition, Sufu stabilizes the Gli proteins and inhibits the transcriptional activity of Gli2, while PKA prohibits the accumulation of full-length Gli2 (GliFL) in the cilium. All of these events ensure silencing of the Hh pathway without the ligand. B. The Hh ligand binds to its receptor Ptch1 and co-receptors Boc/Cdon. Ptch1 is internalized with Hh, relieving the inhibition on Smo. Smo accumulates in the ciliary membrane through both lateral transport and the secretory pathways. Phosphorylation of Smo, for instance, at the EvC zone in osteoblasts leads to its dimerization and activation. This in turn abrogates PKA function and promotes the movement of Sufu-Gli2/3 complexes and Kif7 to the ciliary tip and perhaps dissociation of Gli2/3 from Sufu in this process. Kif7 also facilitates the trafficking of Gli2/3 into the cilium (for example, in chondrocytes). Accumulation of Gli2/3 at the ciliary tip is associated with the production of Gli activators (GliA), which are derived from the full-length Gli proteins.Accumulation of GliA to the nucleus enables activation of Hh target genes such as Ptch1, Gli1 and Hhip1.

Figure 2. Control of trafficking of Hh components on the primary cilium

A. The primary cilium is anchored at the plasma membrane through transitional fibers emanating from the basal body. The ciliary membrane maintains its unique membrane composition through multiple mechanisms including a diffusion barrier surrounding the ciliary base. Ciliary proteins pass through the ciliary gate to enter and exit the cilium. The ciliary gate is mostly made up of the transition zone. This specialized region is characterized by Y-shaped links that demarcate the ciliary necklace and connect the axoneme to the ciliary membrane. Septin ring and a nuclear pore-like structure also contribute to the ciliary diffusion barrier at its base. The transition zone contains several protein complex networks that function in ciliary assembly and sorting. For instance, transition zone proteins Tectonic 1 and 2 participate in targeting Smo and Arl13b to the primary cilium in some cell types. Septin appears to confer important barrier properties at the ciliary base and is known to control Smo trafficking. An intraflagellar transport (IFT) system is responsible for ciliary construction and maintenance. It consists of the IFT particles (A and B complexes) that are moved along the axoneme through their interactions with the anterograde (kinesin II) and retrograde (dynein 2) motors. IFT is known to be involved in Hh signal transduction. Several components of the IFT complexes have been shown to influence the distribution of Hh components on the cilium and affect Hh signaling in a complex manner. For instance, disruption of IFT139 (THM1) in IFT A complex or IFT122/25 in IFT B complex both leads to accumulation of Hh components on the cilium but Hh signaling is enhanced in Thm1/Ift122 mutants while it is reduced in Ift25 mutants. The coat-like BBSome complex is involved in the trafficking of molecules between the plasma and ciliary membrane and may contribute to the dynamic ciliary distribution of Smo and Ptch1. Taken together, these studies highlight the fact that multiple mechanisms regulate ciliary protein distribution and understanding how they interact to control Hh component distribution and signaling on the cilium is a key unresolved issue. B. Summary of changes in ciliary distribution of Hh pathway components and Hh pathway activity in various mutants that affect ciliary structure and function.