Abstract
Wnt ligands conduct their functions in canonical Wnt signaling by binding to two receptors, the single transmembrane low density lipoprotein receptor-related proteins 5 and 6 (LRP5/6) and seven transmembrane (7TM) Frizzled receptors. Subsequently, phosphorylation of serine/threonine residues within five repeating signature PPPSP motifs on LRP6 is responsible for LRP6 activation. GSK3beta, a cytosolic kinase for phosphorylation of a downstream effector beta-catenin, was proposed to participate in such LRP6 phosphorylation. Here, we report a new class of membrane-associated kinases for LRP6 phosphorylation. We found that G protein-coupled receptor kinases 5 and 6 (GRK5/6), traditionally known to phosphorylate and desensitize 7TM G protein-coupled receptors, directly phosphorylate the PPPSP motifs on single transmembrane LRP6 and regulate Wnt/LRP6 signaling. GRK5/6-induced LRP6 activation is inhibited by the LRP6 antagonist Dickkopf. Depletion of GRK5 markedly reduces Wnt3A-stimulated LRP6 phosphorylation in cells. In zebrafish, functional knock-down of GRK5 results in reduced Wnt signaling, analogous to LRP6 knock-down, as assessed by decreased abundance of beta-catenin and lowered expression of the Wnt target genes cdx4, vent, and axin2. Expression of GRK5 rescues the diminished beta-catenin and axin2 response caused by GRK5 depletion. Thus, our findings identify GRK5/6 as novel kinases for the single transmembrane receptor LRP6 during Wnt signaling.
Publication types
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Research Support, N.I.H., Extramural
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Research Support, Non-U.S. Gov't
MeSH terms
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Amino Acid Sequence
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Animals
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Axin Protein
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Cells, Cultured
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Fibroblasts / cytology
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Fibroblasts / physiology
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Frizzled Receptors / genetics
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Frizzled Receptors / metabolism
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G-Protein-Coupled Receptor Kinase 5 / genetics
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G-Protein-Coupled Receptor Kinase 5 / metabolism*
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G-Protein-Coupled Receptor Kinases / genetics
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G-Protein-Coupled Receptor Kinases / metabolism*
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HeLa Cells
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Homeodomain Proteins / genetics
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Homeodomain Proteins / metabolism
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Humans
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Low Density Lipoprotein Receptor-Related Protein-6
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Mice
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Mice, Knockout
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Molecular Sequence Data
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Oligonucleotides, Antisense / genetics
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Oligonucleotides, Antisense / metabolism
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Phosphorylation
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Receptors, LDL / genetics
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Receptors, LDL / metabolism*
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Repressor Proteins / genetics
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Repressor Proteins / metabolism
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Signal Transduction / physiology
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Wnt Proteins / genetics
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Wnt Proteins / metabolism*
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Zebrafish / embryology
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Zebrafish / genetics
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Zebrafish / metabolism
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Zebrafish Proteins / genetics
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Zebrafish Proteins / metabolism
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beta Catenin / genetics
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beta Catenin / metabolism
Substances
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Axin Protein
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Frizzled Receptors
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Homeodomain Proteins
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LRP6 protein, human
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Low Density Lipoprotein Receptor-Related Protein-6
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Lrp6 protein, mouse
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Oligonucleotides, Antisense
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Receptors, LDL
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Repressor Proteins
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Wnt Proteins
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Zebrafish Proteins
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beta Catenin
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G-Protein-Coupled Receptor Kinase 5
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G-Protein-Coupled Receptor Kinases
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G-protein-coupled receptor kinase 6
Associated data
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GENBANK/GQ903573
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GENBANK/GQ903574