Cyclic GMP Phosphodiesterase in Signaling

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Regulation of intracellular levels of cyclic nucleotides is a major paradigm in cell signaling and, in particular, signaling by G-protein-coupled receptors (GPCRs). Recenlty, we have shown that two members of the family 5 of 7-transmembrane segmented receptors that includes Frizzleds are GPCRs with respect to several downstream signaling pathways. Two observations provide the basis for the specific aims in this proposal: suppression of the heterotrimeric G-protein subunits Go_t2/o in mouse F9 teratocarcinoma stem cells and treatment with inhibitors of cyclic GMP PDE, such as IBMX, zaprinast, and dipyridamole, block the ability of the GPCR rat Fz2 to signal at the level of calcium transients and cyclic GMP. We have created a chimeric receptor composed of the transmembrane, ligand-binding domain and the exofacial segments of the well-known GPCR 132-adrenergic receptor to which the three intracellular loops (iloops 1-3) and the C- terminal, cytoplasmic tail of Rfz2 have been spliced. This novel chimera binds _-adrenergic agonists and antagonists, signaling to calcium mobilization and reduction in intracellular concentrations of cyclic GMP, but not t0 G_.s and adenylylcyclase, like the wild-type 132-adrenergic receptor. We have validated the functional capability of the construct and propose two specific aims to elucidate biochemicaHy a new role for Go_t2/o and cyclic GMP PDE in signaling: to test for the direct interaction between Fz2 and heterotrimeric G-proteins expressed in Sf9 cells biochemically by baculovirus-induced expression of these components and by BRET in F9 cells; and, to establish the molecular identity of the PDE(s) responsible for this signaling by expression, purification, and reconstitution of the triad of receptor/G-protein/PDE in vitro and complementary studies in F9 cells using antisense suppression/rescue as well as BRET analysis of protein-protein interactions. We shall employ a novel drug-induced secretion system in these cells to validate (with native ligand and receptor) the observations exploited by use of the chimera. These studies are highly relevant to signaling as well as to elucidation of basis for human diseases in which alterations in signaling pathways translate into aberrant biology.