Editors' ChoiceDevelopmental Biology

IFT-A for Wnt signaling

Sci. Signal.  06 Oct 2015:
Vol. 8, Issue 397, pp. ec282
DOI: 10.1126/scisignal.aad5718

Primary cilia are membrane protrusions that function as sensory organelles and are critical for proper morphogenic signaling by the Hedgehog pathway. Cells either have one primary cilium or none. Balmer et al. used a targeted RNA interference screen in Drosophila to investigate whether the proteins involved in the primary cilium functioned as regulators of morphogenic pathways in cells lacking this structure. Knockdown of a subset of the genes (17 out of 63) tested reduced Wnt–β-catenin signaling. In the absence of Wnt, cytosolic β-catenin that is not part of the adhesion complex is degraded after phosphorylation by a “destruction” complex, which includes the scaffold proteins axin and APC and the kinases GSK-3β and CK1. The authors focused on a subset (IFT121, IFT122, IFT140, IFT43) of proteins that are part of a cilia-associated complex called the intraflagellar transport complex A (IFT-A)—knockdown of which selectively reduced Wnt–β-catenin signaling without affecting Hedgehog, Notch, or epidermal growth factor receptor signaling. Epistasis experiments placed this complex downstream of the Wnt receptor complex and upstream of β-catenin: Deficiency in any of the four IFT proteins rescued the phenotypes associated with a constitutively activated receptor mutant but failed to rescue the phenotypes associated with a constitutively active β-catenin mutant. Knockdown of any of the four IFT proteins rescued the viability of flies in which axin, a key component of the β-catenin destruction complex, was knocked down and suppressed the excess activity of the Wnt–β-catenin pathway in these flies. Furthermore, combined knockdown of IFT121 or IFT122 and axin increased the ratio of phosphorylated (inactive) to nonphosphorylated (active) β-catenin, whereas single axin knockdown reduced this ratio. This shift in β-catenin phosphorylation status is consistent with the reduction in excess Wnt–β-catenin signaling in these double-knockdown (axin and IFT-A component) flies. Analysis of flies expressing IFT122 or IFT43 tagged with Myc revealed partial colocalization of these tagged IFT-A components with β-catenin in cytoplasmic punctae. These results not only indicated that IFT-A functions to stabilize cytoplasmic β-catenin and thereby promote Wnt signaling but also show how a targeted screen in Drosophila can reveal ciliary-independent functions of proteins associated with the primary cilium.

S. Balmer, A. Dussert, G. M. Collu, E. Benitez, C. Iomini, M. Mlodzik, Components of Intraflagellar Transport complex A function independently of the cilium to regulate canonical Wnt signaling in Drosophila. Dev. Cell 34, 705–718 (2015). [PubMed]