Aim: The localization and function of several G protein-coupled receptors, including beta-adrenergic receptors and NK 1 receptors, are regulated via lipid rafts in the plasma membrane. These domains are enriched in cholesterol, gangliosides and sphingolipids, and play an important role in regulating signal transduction in most cell types. Serotonin (5-hydroxytryptamine, 5-HT), acting via 14 different receptors, regulates as diverse effects as mood, metabolism and smooth muscle contraction. 5-HT(7) receptors are involved in the regulation of depression, circadian rhythms, thermoregulation and vasodilatation. Ligand binding and signalling via the 5-HT(7) receptor are regulated by membranous cholesterol. Here we investigated the role of sphingomyelin and gangliosides on binding of 5-HT to 5-HT(7) receptors to further examine the role of lipid raft constituents on 5-HT(7) receptor function.
Methods: HeLa cells stably transfected with the human 5-HT(7) receptor were treated with Fumonisin B(1) or (+/-)-threo-1-Phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) to reduce sphingomyelin or ganglioside levels, respectively. The effects of these treatments were investigated by the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl-tetrazolium bromide (MTT) viability assay, cholesterol analysis and [(3)H]5-HT binding studies on intact cells.
Results: Treatments with 20 mum Fumonisin B(1) for 24 h or with 10 mum PDMP for 48 h had no effects of total levels if 5-HT(7) receptors, but caused significant decreases in maximum [(3)H]5-HT binding to 5-HT(7) receptors. The effects were cholesterol-independent as levels of cholesterol remained unaffected by either treatment.
Conclusion: These data demonstrate a role for sphingomyelin and gangliosides in regulating binding of [(3)H]5-HT to 5-HT(7) receptors. These observations further strengthen that actions of 5-HT via 5-HT(7) receptors are dependent upon lipid raft integrity.