Simultaneous and independent visualization of the gonadotropin-releasing hormone receptor and its ligand: evidence for independent processing and recycling in living cells.

The first step in GnRH signaling is binding by the peptide to its plasma membrane receptor (GnRHR). The receptor is a member of the seven transmembrane G protein-coupled class but lacks the characteristic C-terminal cytoplasmic tail, making it among the smallest receptors in this superfamily. It has been known since 1980 that agonist occupancy of the GnRHR results in patching, capping, and internalization, although it has not been possible to localize the unoccupied GnRHR, because elaboration of receptor antisera has not been easy to achieve. The recent production of a green fluorescent protein (GFP) conjugate of the GnRHR ("rGnRHR-C-tail-GFP") that is expressed in cells, targeted to the plasma membrane, binds GnRH analogs and couples to G proteins has made it possible to monitor movement of the unoccupied receptor by confocal microscopy. In the present study, we used this probe, along with Texas Red conjugates of a GnRH agonist, to examine simultaneous processing of the receptor and its ligands. The preparation of the GFP GnRHR chimera has been described. A Texas Red conjugate was made from the GnRH agonist D-Lys6-Pro9-des-Gly10EA-GnRH by standard procedures. Bioactivity of this conjugate was confirmed. Confocal fluorescence images of living GGH3 cells showed that the agonist binds the GFP-GnRH receptor construct on the cell membrane and causes the internalization of vesicles delimited by a membrane. Shortly after internalization, the agonist separates from receptor inside the vesicle, although it is still enclosed in membranes containing free receptor. As the vesicles approach the perinuclear space, the separation between receptor and agonist is more pronounced. Free receptor appears at the cell membrane after the internalization of agonist has been completed. The protein synthesis inhibitor, cycloheximide (1 mM) did not inhibit this process, suggesting that the free receptor results from the recycling of previously internalized vesicles rather than from newly synthesized receptor. These studies show visual evidence for recycling of the GnRH receptor in cultured cells.