Amino Acid-stimulated Ca2+ Oscillations Produced by the Ca2+-sensing Receptor Are Mediated by a Phospholipase C/Inositol 1,4,5-Trisphosphate-independent Pathway That Requires G12, Rho, Filamin-A, and the Actin Cytoskeleton*

The G protein-coupled Ca2+-sensing receptor (CaR) is an allosteric protein that responds to two different agonists, Ca2+ and aromatic amino acids, with the production of sinusoidal or transient oscillations in intracellular Ca2+ concentration ([Ca2+]i). Here, we examined whether these differing patterns of [Ca2+]i oscillations produced by the CaR are mediated by separate signal transduction pathways. Using real time imaging of changes in phosphatidylinositol 4,5-biphosphate hydrolysis and generation of inositol 1,4,5-trisphosphate in single cells, we found that stimulation of CaR by an increase in the extracellular Ca2+ concentration ([Ca2+]o) leads to periodic synthesis of inositol 1,4,5-trisphosphate, whereas l-phenylalanine stimulation of the CaR does not induce any detectable change in the level this second messenger. Furthermore, we identified a novel pathway that mediates transient [Ca2+]i oscillations produced by the CaR in response to l-phenylalanine, which requires the organization of the actin cytoskeleton and involves the small GTPase Rho, heterotrimeric proteins of the G12 subfamily, the C-terminal region of the CaR, and the scaffolding protein filamin-A. Our model envisages that Ca2+ or amino acids stabilize unique CaR conformations that favor coupling to different G proteins and subsequent activation of distinct downstream signaling pathways.