Amer1/WTX couples Wnt-induced formation of PtdIns(4,5)P₂to LRP6 phosphorylation

Abstract

Phosphorylation of the Wnt receptor low-density lipoprotein receptor-related protein 6 (LRP6) by glycogen synthase kinase 3b (GSK3b) and casein kinase 1c (CK1c) is a key step in Wnt/b-catenin signalling, which requires Wnt-induced formation of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P 2 ). Here, we show that adenomatous polyposis coli membrane recruitment 1 (Amer1) (also called WTX), a membrane associated PtdIns(4,5)P 2 -binding protein, is essential for the activation of Wnt signalling at the LRP6 receptor level. Knockdown of Amer1 reduces Wnt-induced LRP6 phosphorylation, Axin translocation to the plasma membrane and formation of LRP6 signalosomes. Overexpression of Amer1 promotes LRP6 phosphorylation, which requires interaction of Amer1 with PtdIns(4,5)P 2 . Amer1 translocates to the plasma membrane in a PtdIns(4,5)P 2 -dependent manner after Wnt treatment and is required for LRP6 phosphorylation stimulated by application of PtdIns(4,5)P 2 . Amer1 binds CK1c, recruits Axin and GSK3b to the plasma membrane and promotes complex formation between Axin and LRP6. Fusion of Amer1 to the cytoplasmic domain of LRP6 induces LRP6 phosphorylation and stimulates robust Wnt/b-catenin signalling. We propose a mechanism for Wnt receptor activation by which generation of PtdIns (4,5)P 2 leads to recruitment of Amer1 to the plasma membrane, which acts as a scaffold protein to stimulate phosphorylation of LRP6.

Amer2 Protein Is a Novel Negative Regulator of Wnt/β-Catenin Signaling Involved in Neuroectodermal Patterning

“…Amer2 shares sequence similarities with Amer1 in the APCbinding and membrane association motifs (3,20). Our data show that these motifs are functional, leading to the recruitment of APC to the plasma membrane by Amer2.…”

“…1E). We have shown previously that Amer1 associates with the plasma membrane by binding to phosphatidylinositol 4,5-bisphosphate, the predominant phospholipid at the plasma membrane (19), via these lysine motifs (3,20). Indeed, recombinant Amer2(2-230) bound to membrane-spotted phosphatidylinositol phosphates, including phosphatidylinositol 4,5-bisphosphate, suggesting that it interacts with the plasma membrane in a similar fashion as Amer1 (Fig.…”

The -Catenin Destruction Complex

“…Many of the proteins that are necessary for b-catenin destruction, including GSK-3, CK1, Axin, and APC, also have roles in stabilizing b-catenin on Wnt stimulation (Liu et al 2002;Zeng et al 2005Zeng et al , 2008Takacs et al 2008;Tanneberger et al 2011), thus confounding genetic analyses and cell culture overexpression studies in difficult-to-predict ways. Biochemical studies of purified protein complexes, as well as experiments that carefully control expression levels, will be important in resolving many of the mysteries surrounding the destruction complex.…”

“…The recently described WTX/AMER1 protein binds to the APC arm domain and to PIP 2 , and appears to be important for targeting the complex to the plasma membrane, although this protein appears to be specific to vertebrates (Grohmann et al 2007;Tanneberger et al 2011). In addition, the carboxy-terminal-binding protein (CtBP) binds to the 15-mer repeats of APC and promotes polymerization of truncated APC to form cytoplasmic puncta (Hamada and Bienz 2004;Schneikert et al 2011), but the relationship between these puncta and the Wnt-stimulated assemblies is unclear.…”

Frizzled and LRP5/6 Receptors for Wnt/ -Catenin Signaling

“…WTX binds to Axin and promotes b-catenin degradation in absence of Wnt (Major et al 2007). However, upon Wnt CK1 Axin DAX P P P P P P P P P P P P P P P Wnt Wnt Wnt DIX Dish eve lled G S K 3 D AX DIX P P P P P P P P P P P P P P P P P P P P Tanneberger et al 2011). WTX, which is vertebrate specific, thus has dichotomic roles in Wnt signaling, analogous to those for Axin and GSK3.…”