Primary Cilia Are Not Required for Normal Canonical Wnt Signaling in the Mouse Embryo

Abstract

BackgroundSonic hedgehog (Shh) signaling in the mouse requires the microtubule-based organelle, the primary cilium. The primary cilium is assembled and maintained through the process of intraflagellar transport (IFT) and the response to Shh is blocked in mouse mutants that lack proteins required for IFT. Although the phenotypes of mouse IFT mutants do not overlap with phenotypes of known Wnt pathway mutants, recent studies report data suggesting that the primary cilium modulates responses to Wnt signals.Methodology/Principal FindingsWe therefore carried out a systematic analysis of canonical Wnt signaling in mutant embryos and cells that lack primary cilia because of loss of the anterograde IFT kinesin-II motor (Kif3a) or IFT complex B proteins (Ift172 or Ift88). We also analyzed mutant embryos with abnormal primary cilia due to defects in retrograde IFT (Dync2h1). The mouse IFT mutants express the canonical Wnt target Axin2 and activate a transgenic canonical Wnt reporter, BAT-gal, in the normal spatial pattern and to the same quantitative level as wild type littermates. Similarly, mouse embryonic fibroblasts (MEFs) derived from IFT mutants respond normally to added Wnt3a. The switch from canonical to non-canonical Wnt also appears normal in IFT mutant MEFs, as both wild-type and mutant cells do not activate the canonical Wnt reporter in the presence of both Wnt3a and Wnt5a.ConclusionsWe conclude that loss of primary cilia or defects in retrograde IFT do not affect the response of the midgestation embryo or embryo-derived fibroblasts to Wnt ligands.

Intraflagellar Transport Proteins Are Essential for Cilia Formation and for Planar Cell Polarity

“…4,16 -21 However, in contrast to the well-established role of cilia in the hedgehog pathway, 2,3,22 the role of cilia in the Wnt pathways is unclear. 4,15,23,24 In this study, we demonstrate that, in the kidney duct of ift57 hi3417 and ift172 hi2211 mutants, the organization of basal bodies is impaired, a phenotype consistent with compromised PCP signaling. We further show that knockdown of prickle 1 (pk1), a core PCP player, leads to disorganization of basal bodies and kidney cyst formation.…”

Cilia and coordination of signaling networks during heart development

“…[198][199][200] Moreover, in the mouse inner ear, establishment of PCP was shown to depend on the formation of functional primary cilia. 201 However, other studies in mice and zebrafish reveal no obvious canonical or non-canonical Wnt phenotypes in cilia mutants, 202,203 emphasizing the importance of further studies in this area.…”

Context-Dependent Regulation of Wnt Signaling through the Primary Cilium

“…To establish this thesis, mutants lacking components of the IFT B complex (ift172 and ift88), anterograde (kif3a) and retrograde (Dync2h1) motors were crossed to a Wnt reporter line, BATGal. 47 Examination of reporter activity in all four mutant mouse lines at embryonic day nine (E9) reveal similar b-galactosidase expression levels relative to wild-type littermates. Furthermore, a transcriptional target of the canonical Wnt pathway, Axin2, was expressed at similar levels in mutant and control mouse lines, suggesting that loss of IFT, and therefore cilia, does not perturb canonical Wnt signaling.…”