Abstract
Signal transduction via the Frizzled family of seven-transmembrane receptors controls important developmental processes. Aberrant signaling caused by altered Frizzled receptor activity or by mutations in downstream signaling components has been implicated in several adult pathologies. A diverse array of intracellular signaling pathways has been suggested to transduce the signals exerted in cells when secreted ligands of the Wnt family bind to Frizzled receptors. Studies with a chimeric receptor composed of Frizzled-2 and the beta2-adrenergic receptor have suggested that the binding of Wnt-5a to Frizzled-2 results in the activation of G proteins of the Galpha(i) family, the mobilization of calcium from intracellular stores and the induction of gene transcription through nuclear factor of activated T cells. In this report, we demonstrate by using beta-lactamase reporter gene technology that full-length, wild-type human Frizzled-2 does not couple to calcium-mediated signaling in HEK293 cells following stimulation with purified recombinant mouse Wnt-5a. In contrast, when stimulated with recombinant mouse Wnt-3a, Frizzled-2 activates the canonical Wnt/Frizzled signaling pathway, involving the transcriptional modulator beta-catenin. Our report underlines the importance of using cell lines stably overexpressing wild-type Frizzled receptors and the use of purified ligands when studying receptor pharmacology. This approach has allowed us to measure the half-maximal concentration for activation of human Frizzled-2 (1.5+/-0.4 nM; avg.+/-SD) and human Frizzled-1 (1.3+/-0.5 nM) following stimulation by Wnt-3a. Our results suggest that there is receptor redundancy with regard to Wnt-3a reception. In addition, we introduce beta-lactamase reporter gene technology as an alternative to luciferase-based reporters to measure Frizzled receptor modulation for the discovery of Frizzled receptor-interacting drugs.