Wnt/Wingless signaling through beta-catenin requires the function of both LRP/Arrow and frizzled classes of receptors.
BACKGROUND
Wnt/Wingless (Wg) signals are transduced by seven-transmembrane Frizzleds (Fzs) and the single-transmembrane LDL-receptor-related proteins 5 or 6 (LRP5/6) or Arrow. The aminotermini of LRP and Fz were reported to associate only in the presence of Wnt, implying that Wnt ligands form a trimeric complex with two different receptors. However, it was recently reported that LRPs activate the Wnt/beta-catenin pathway by binding to Axin in a Dishevelled--independent manner, while Fzs transduce Wnt signals through Dishevelled to stabilize beta-catenin. Thus, it is possible that Wnt proteins form separate complexes with Fzs and LRPs, transducing Wnt signals separately, but converging downstream in the Wnt/beta-catenin pathway. The question then arises whether both receptors are absolutely required to transduce Wnt signals.


RESULTS
We have established a sensitive luciferase reporter assay in Drosophila S2 cells to determine the level of Wg--stimulated signaling. We demonstrate here that Wg can synergize with DFz2 and function cooperatively with LRP to activate the beta-catenin/Armadillo signaling pathway. Double-strand RNA interference that disrupts the synthesis of either receptor type dramatically impairs Wg signaling activity. Importantly, the pronounced synergistic effect of adding Wg and DFz2 is dependent on Arrow and Dishevelled. The synergy requires the cysteine-rich extracellular domain of DFz2, but not its carboxyterminus. Finally, mammalian LRP6 and its activated forms, which lack most of the extracellular domain of the protein, can activate the Wg signaling pathway and cooperate with Wg and DFz2 in S2 cells. We also show that the aminoterminus of LRP/Arr is required for the synergy between Wg and DFz2.


CONCLUSIONS
Our study indicates that Wg signal transduction in S2 cells depends on the function of both LRPs and DFz2, and the results are consistent with the proposal that Wnt/Wg signals through the aminoterminal domains of its dual receptors, activating target genes through Dishevelled.
Schweizer
Liang
L
Cell Biology Program, Sloan-Kettering Institute for Cancer Research, 1275 York Avenue, New York, NY 10021, USA. schweizl@mskcc.org
Varmus
Harold
H
D000818:Animals; D015152:Blotting, Northern; D002460:Cell Line; D003598:Cytoskeletal Proteins / Q000235:genetics / Q000378:metabolism*; D004330:Drosophila / Q000166:cytology / Q000235:genetics / Q000378:metabolism; D029721:Drosophila Proteins / Q000235:genetics / Q000378:metabolism*; D051157:Frizzled Receptors; D026503:Low Density Lipoprotein Receptor-Related Protein-1 / Q000235:genetics / Q000378:metabolism*; D008156:Luciferases / Q000235:genetics / Q000378:metabolism; D009154:Mutation; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism*; D034622:RNA Interference; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D043562:Receptors, G-Protein-Coupled; D017981:Receptors, Neurotransmitter / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction / Q000502:physiology; D015534:Trans-Activators / Q000235:genetics / Q000378:metabolism*; D015533:Transcriptional Activation / Q000235:genetics; D014162:Transfection; D051155:Wnt1 Protein; D051176:beta Catenin
D003598:Cytoskeletal Proteins; D029721:Drosophila Proteins; D051157:Frizzled Receptors; D026503:Low Density Lipoprotein Receptor-Related Protein-1; D011518:Proto-Oncogene Proteins; D012333:RNA, Messenger; D043562:Receptors, G-Protein-Coupled; D017981:Receptors, Neurotransmitter; D011993:Recombinant Fusion Proteins; D015534:Trans-Activators; D051155:Wnt1 Protein; D051176:beta Catenin; C101703:fz2 protein, Drosophila; C074622:wg protein, Drosophila; D008156:Luciferases