Frizzled-7 turnover at the plasma membrane is regulated by cell density and the Ca2+-dependent protease calpain-1

Frizzled are seven-transmembrane domain G-protein coupled receptors involved in cell polarity and Wnt signaling. The mechanisms regulating their turnover at the plasma membrane remain unclear. We have identified a regulated C-terminus cleavage of Frizzled-7 in endothelial cells using ectopic expression of N- and C-termini-tagged Frizzled-7 proteins. This specific cleavage produced a 10 kDa C-terminus fragment that remained associated with intracellular vesicles and was localized within the 3rd intracytoplasmic loop using N-terminal sequencing and targeted mutagenesis. Frizzled-7 mutated forms displaying reduced C-terminus cleavage were also defective for dvl2 translocation at the plasma membrane. PMA, an activator of PKC and endocytosis, but not Wnt13A and Wnt5A, increased the appearance of Frizzled-7 C-terminus-containing vesicles and Frizzled-7 cleavage. Concanavalin-A, an inhibitor of receptor internalization decreased both constitutive and PMA-induced Frizzled-7 cleavage, while inhibition of the endocytic pathway with Δ95-295-Eps15 dominant-negative prevented only PMA-induced Frizzled-7 cleavage. Frizzled-7 C-terminus cleavage was increased with cell density and by the Ca2+ ionophore ionomycin and was decreased by specific calpain inhibitors, by the expression of DN-calpain-1 and the down-regulation of calpain-1 levels by siRNAs. Altogether, our findings pinpoint calpain-1 as a regulator of Frizzled-7 turnover at the plasma membrane and reveal a link between Frizzled-7 cleavage and its activity.