The conserved transmembrane RING finger protein PLR-1 downregulates Wnt signaling by reducing Frizzled, Ror and Ryk cell-surface levels in C. elegans

• Published in: Development (Cambridge) Vol. 141; no. 3; pp. 617 - 628
• Main Authors: Moffat, Laura L, Robinson, Ryan E, Bakoulis, Anastasia, Clark, Scott G
• Format: Journal Article
• Language: English
• Published: England Company of Biologists 01.02.2014
• Subjects: Animals; Caenorhabditis elegans - cytology; Caenorhabditis elegans - metabolism; Caenorhabditis elegans Proteins - chemistry; Caenorhabditis elegans Proteins - metabolism; Cell Membrane - metabolism; Cell Movement; Cell Polarity; Conserved Sequence; Cytosol - metabolism; Down-Regulation; Endosomes - metabolism; Green Fluorescent Proteins - metabolism; Lysine - metabolism; Membrane Proteins - metabolism; Mutation - genetics; Neurons - cytology; Neurons - metabolism; Protein Structure, Secondary; Receptor Protein-Tyrosine Kinases - metabolism; Receptor Tyrosine Kinase-like Orphan Receptors - metabolism; RING Finger Domains; Structure-Activity Relationship; Ubiquitin-Protein Ligases - chemistry; Ubiquitin-Protein Ligases - metabolism; Wnt Signaling Pathway; Membrane protein trafficking; C. elegans; Wnt signaling; Neuronal polarity
• DOI: 10.1242/dev.101600
• ISSN: 0950-1991

Wnts control a wide range of essential developmental processes, including cell fate specification, axon guidance and anteroposterior neuronal polarization. We identified a conserved transmembrane RING finger protein, PLR-1, that governs the response to Wnts by lowering cell-surface levels of the Frizzled family of Wnt receptors in Caenorhabditis elegans. Loss of PLR-1 activity in the neuron AVG causes its anteroposterior polarity to be symmetric or reversed because signaling by the Wnts CWN-1 and CWN-2 are inappropriately activated, whereas ectopic PLR-1 expression blocks Wnt signaling and target gene expression. Frizzleds are enriched at the cell surface; however, when PLR-1 and Frizzled are co-expressed, Frizzled is not detected at the surface but instead is colocalized with PLR-1 in endosomes. The Frizzled cysteine-rich domain (CRD) and invariant second intracellular loop lysine are crucial for PLR-1 downregulation. The PLR-1 RING finger and protease-associated (PA) domain are essential for activity. In a Frizzled-dependent manner, PLR-1 reduces surface levels of the Wnt receptors CAM-1/Ror and LIN-18/Ryk. PLR-1 is a homolog of the mammalian transmembrane E3 ubiquitin ligases RNF43 and ZNRF3, which control Frizzled surface levels in an R-spondin-sensitive manner. We propose that PLR-1 downregulates Wnt receptor surface levels via lysine ubiquitylation of Frizzled to coordinate spatial and temporal responses to Wnts during neuronal development.