Abstract
GnRH binds to a specific G protein-coupled receptor in the pituitary to regulate synthesis and secretion of gonadotropins. Using RT-PCR and human pituitary poly(A)+ RNA as a template, the full-length GnRH receptor (wild type) and a second truncated cDNA characterized by a 128-bp deletion between nucleotide positions 522 and 651 were cloned. The deletion causes a frame shift in the open reading frame, thus generating new coding sequence for further 75 amino acids. The truncated cDNA arises from alternative splicing by accepting a cryptic splicing acceptor site in exon 2. Distinct translation products of approximately 45–50 and 42 kDa were immunoprecipitated from COS-7 cells transfected with cDNA coding for wild type GnRH receptor and the truncated splice variant, respectively. Immunocytochemical and enzyme-linked immunosorbent assay studies revealed a membranous expression pattern for both receptor isoforms. Expression of the splice variant, however, occurred at a significantly lower cell surface receptor density. In terms of ligand binding and phospholipase C activation, the wild type receptor showed characteristics of a typical GnRH receptor, whereas the splice variant was incapable of ligand binding and signal transduction. Coexpression of wild type and truncated proteins in transiently or stably transfected cells, however, resulted in impaired signaling via the wild type receptor by reducing maximal agonist-induced inositol phosphate accumulation. The inhibitory effect depended on the amount of splice variant cDNA cotransfected and was specific for the GnRH receptor because signaling via other Gq/11-coupled receptors, such as the thromboxane A2, M5 muscarinic, and V1 vasopressin receptors, was not affected. Immunological studies revealed that coexpression of the wild type receptor and the truncated splice variant resulted in impaired insertion of the wild type receptor into the plasma membrane. Thus, expression of truncated receptor proteins may highlight a novel principle of specific functional inhibition of G protein-coupled receptors.
Changes to Gonadotropin-Releasing Hormone (GnRH) Receptor Extracellular Loops Differentially Affect GnRH Analog Binding and Activation: Evidence for Distinct Ligand-Stabilized Receptor Conformations