The third intracellular loop of the rat gonadotropin-releasing hormone receptor couples the receptor to Gs- and G(q/11)-mediated signal transduction pathways: evidence from loop fragment transfection in GGH3 cells.

The GnRH receptor (GnRH-R) belongs to the rhodopsin/beta-adrenergic family of G protein-coupled receptors. The intracellular domains of these receptors, particularly the regions closest to the plasma membrane in intracellular loops 2 (2i) and 3 (3i) as well as some regions located in the membrane-proximal end of the COOH-terminus, are frequently important sites for G protein coupling and specificity determination. Although studies in mouse and human GnRH-R have identified loop 2i as a critical determinant for coupling the receptor to the G(q/11)-mediated signal transduction pathway, given the functional similarity among the members of this particular G protein-coupled receptor subfamily and the fact that the GnRH-R lacks the typical intracellular COOH-terminal domain of its superfamily (a potential site for G protein coupling), we investigated the possibility that loop 3i of this receptor also participates in GnRH-R coupling to G proteins. GGH(3)1' cells, a pituitary-derived cell line that expresses a functional rat GnRH-R coupled to both Gs and G(q/11) proteins, were transiently transfected with a plasmid DNA containing a complementary DNA (cDNA) coding for the entire loop 3i of the GnRH-R as well as with other expression plasmids containing cDNAs encoding loop 3i of other Gs-, G(i/o)-, or G(q/11)-coupled receptors. The effects of coexpression of these loops with the wild-type GnRH-R on inositol phosphate (IP) production, cAMP accumulation, and PRL release were then examined. Transfection of GGH(3)1' cells with the cDNA for loop 3i of the rat GnRH-R (efficiency, 35-45%) maximally inhibited buserelin-stimulated IP turnover by 20% as well as cAMP accumulation and PRL secretion by 30%. This attenuation in cellular responses to a GnRH agonist was statistically significant (P < 0.05) compared with the responses exhibited by GGH(3)1' cells transfected with a control plasmid and stimulated with the same GnRH agonist. Transfection of minigenes coding for loop 3i of the M1Ach-muscarinic and the alpha1B-adrenergic (G(q/11)-coupled) receptors resulted in 25-55% inhibition of maximal GnRH-evoked IP turnover. Paradoxically, loop 3i from the M1Ach-muscarinic receptor also maximally inhibited GnRH agonist-stimulated cAMP accumulation and PRL release by 40% (both effects mediated through activation of the Gs protein). Transfection of loop 3i from the D1A -dopamine receptor (coupled to the Gs protein) produced a selective attenuation (40%) in Gs-mediated cellular responses. In contrast, receptor/G protein coupling appeared unaffected by expression of loop 3i domains derived from two receptors coupled to G(i/o) proteins (M2Ach-muscarinic and alpha2A-adrenergic receptors). These data indicate that the third intracellular loop of the rat GnRH-R is involved in receptor G(q/11) protein coupling and/or selectivity, and in the GGH(3)1' cell line, this loop is also involved in signal transduction mediated through the Gs protein pathway.