PDZ-dependent activation of nitric-oxide syntheses by the serotonin 2B receptor

Taking advantage of three cellular systems, we established that 5-HT2B receptors are coupled with NO signaling pathways. In the 1C11 serotonergic cell line and Mastomys natalensis carcinoid cells, which naturally express the 5-HT2B receptor, as well as in transfected LMTK- fibroblasts, stimulation of the 5-HT2B receptor triggers intracellular cGMP production through dual activation of constitutive nitric-oxide synthase (cNOS) and inducible NOS (iNOS). The group I PDZ motif at the C terminus of the 5-HT2B receptor is required for recruitment of the cNOS and iNOS transduction pathways. Indeed, the 5-HT2B receptor-mediated NO coupling is abolished not only upon introduction of a competitor C-terminal 5-HT2B peptide in the three cell types but also in LMTK- fibroblasts expressing a receptor C-terminally truncated or harboring a point mutation within the PDZ domain. The occurrence of a direct functional coupling between the receptor and cNOS activity is supported by highly significant correlations between the binding constants of drugs on the receptor and their effects on cNOS activity. The 5-HT2B/iNOS coupling mechanisms appear more complex because neutralization of endogenous G alpha(13) by specific antibodies cancels the cellular iNOS response while not interfering with cNOS activities. These findings may shed light on physiological links between the 5-HT2B receptor and NO and constitute the first demonstration that PDZ inter actions participate in downstream transductional pathways of a G protein-coupled receptor.