Primary Cilia are WNT Transducing Organelles whose Biogenesis is Regulated by a WNTPP1 axis

WNT signalling is of paramount importance in development, stem cell maintenance, and disease. WNT ligands typically signal via receptor activation at the plasma membrane to induce {beta}-catenin-dependent gene activation. Here we show that in primary cilia, WNT receptors relay a WNT/GSK3 signal that {beta}-catenin-independently promotes ciliogenesis. Innovations supporting this conclusion are monitoring acute WNT co-receptor activation (phospho-LRP6) and identifying and mutating the LRP6 ciliary targeting sequence. Ciliary WNT signalling inhibits protein phosphatase 1 (PP1) activity, a negative regulator of ciliogenesis, by decommissioning GSK3-mediated phosphorylation of the PP1 regulatory inhibitor subunit PPP1R2. Accordingly, deficiency of WNT/GSK3 signalling by depletion of cyclin Y and cyclin-Y-like protein 1 induces widespread primary cilia defects in mouse embryonic neuronal precursors, kidney proximal tubules, and adult mice preadipocytes. We conclude that primary cilia are WNT PP1 signalling organelles. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=127 SRC="FIGDIR/small/519813v1_ufig1.gif" ALT="Figure 1"> View larger version (37K): org.highwire.dtl.DTLVardef@4e8f4dorg.highwire.dtl.DTLVardef@1a246f6org.highwire.dtl.DTLVardef@e0df10org.highwire.dtl.DTLVardef@154a139_HPS_FORMAT_FIGEXP M_FIG A Localized WNT PP1 Signalling Axis Promotes Ciliogenesis The WNT co-receptor LRP6 localizes to the ciliary membrane, where it is phospho-primed via a CCNY/L1-dependent CDK (not shown). WNT signalling inhibits GSK3 (not shown) and leads to inhibition of Protein phosphatase 1, a negative regulator of ciliogenesis. Right, CCNY/L1 deficiency disrupts the WNT PP1 signalling axis, leading to ciliary defects. C_FIG