Most of the important components of the Hh signal transduction pathway have been known for some time to be conserved from invertebrates to vertebrates [1]. The function of Hh is to regulate the activity of Gli-family transcriptional regulators (Ci in Drosophila, Gli in vertebrates). In the absence of Hh signaling, Ci/Gli transcriptional regulators undergo proteasome-dependent limited proteolysis to a truncated form in which they act as transcriptional repressors, a process that also requires the scaffolding protein Costal-2/Kinesin family member 7 (Cos-2/Kif7). Hh signaling causes Ci/Gli proteins instead to be converted, by an unknown mechanism, to transcriptional activators that translocate to the nucleus and activate target genes. This modification occurs as a consequence of the activity of the G-protein coupled-like receptor Smoothened (Smo), which in the absence of Hh is repressed by the Hh receptor Patched (Ptc). When secreted Hh binds Patched (Ptc), Ptc-mediated repression of Smo is disrupted and Ci/Gli's are activated, possibly through modulation of Cos-2/Kif7 conformation and localization.
A requirement for the cilium in this pathway was first suggested on the basis of ENU mutagenesis screens in mice, which showed that genes required for ciliogenesis are also required for Hh signal transduction (first reported by Kathryn Anderson's group and reviewed in [2]). Disruption of intraflagellar transport (IFT) genes, which encode components of a macromolecular machine that transports cargo on the ciliary axoneme, or of genes encoding the kinesin and dynein motors that transport the IFT complexes (Figure 1a) perturbs Hh signaling in the developing mouse embryo. In tissues that depend on Gli repressors for patterning, such as the limb bud, target genes are inappropriately activated in these mutants. Conversely, tissues that require high levels of Gli activation for patterning, such as the ventral neural tube, exhibit a loss of target gene expression. This combination of loss-and gain-of-function of Hh target genes is characteristic of most mutations that affect cilium formation or function.
Subsequent investigations with overexpressed and endogenous Hh pathway components showed that many are localized to the primary cilium (reviewed in [2]), and that Hh binding to Ptc appears to remove the Hh-Ptc complex from the cilium, permitting accumulation of Smo, Kif7, Gli2, and Gli3 along the ciliary axoneme or at the tip (Figure 1b). The dynamics of Gli transport into the cilium correlate well with transcriptional activation of Hh targets, leading to the hypothesis that Gli proteins are converted to activated forms in the cilium.