| 5-HT2A serotonin receptors are important for a variety of functions including vascular smooth muscle contraction, platelet aggregation, and the modulation of perception, cognition, and emotion. As with other GPCRs, elucidating the mechanisms of signal transduction and regulation for 5-HT2A receptors is likely to be great relevance for the rational design of novel medications. In particular, one point at which the regulation of GPCR signaling converges is the endocytic pathway. While the role of arrestins on endocytosis of 5-HT2A receptors are cell type specific and, therefore, the choice of endocytotic pathway is likely to play a major role in various functions involving the 5-HT2A receptors. In a search for 5-HT2A receptor interacting proteins, we discovered that caveolin-1 (Cav-1), a scaffolding protein enriched in caveolae, directly associates with 5-HT2A receptors in a number of cell types including C6 glioma cells, transfected HEK-293 cells and rat brain synaptic membrane preparations. To address the functional significance of this interaction we performed RNA interference (RNAi)-mediated knock-down of Cav-1 in C6 glioma cells---a cell type which endogenously expresses both 5-HT2A receptors and Cav-1. We discovered that the in vitro knockdown of Cav-1 in C6 glioma cells nearly abolished 5-HT2A receptor mediated signal transduction as measured by calcium flux assays. RNAi-mediated knock-down of Cav-1 also greatly attenuated endogenous Galphaq-coupled P2Y-purinergic receptor mediated signaling without altering the signaling of PAR-1 thrombin receptors. Cav-1 appeared to modulate 5-HT2A signaling by facilitating the interaction of 5-HT2A receptors with Galphaq. These studies provide compelling evidence for a prominent role of Cav-1 in regulating the functional activity of not only 5-HT2A serotonin receptors but also selected Galphaq coupled receptors. |
