Desensitization and Internalization of Human and XenopusGonadotropin-Releasing Hormone Receptors Expressed in αT4 Pituitary Cells Using Recombinant AdenovirusThe work was supported in part by the Wellcome Trust (054949), the Medical Research Council (G78/6046), the South African Research Council, and National Research Foundation

• Published in: Endocrinology (Philadelphia) Vol. 141; no. 12; pp. 4564 - 4575
• Main Authors: Hislop, James N, Madziva, Michael T, Everest, Helen M, Harding, Tom, Uney, James B, Willars, Gary B, Millar, Robert P, Troskie, Brigitte E, Davidson, James S, McArdle, Craig A
• Format: Journal Article
• Language: English
• Published: Washington Oxford University Press 01-12-2000
• Subjects: Adenoviruses; Adenylate cyclase; Cellular structure; Comparative studies; Density; Desensitization; Gonadotropin-releasing hormone; Inositol phosphate; Inositol phosphates; Inositols; Internalization; Ligands; Mammals; Medical research; Pharmacology; Physiology; Pituitary; Pituitary (anterior); Progenitor cells; Receptor density; Receptors; Transfection; Vertebrates
• ISSN: 0013-7227; 1945-7170
• DOI: 10.1210/endo.141.12.7813

Nonmammalian vertebrates express at least two forms of GnRH and distinct forms of GnRH receptor (GnRH-R) have coevolved with their ligands. Mammalian and nonmammalian GnRH-R have key structural differences (notably the lack of C-terminal tails in mammalian GnRH-R) and comparative studies are beginning to reveal their functional relevance. However, cellular context and receptor density influence G protein-coupled receptor function and may be important variables in such work using heterologous expression systems. Here we report a comparative study using αT4 cells (gonadotrope progenitors that lack endogenous GnRH-R) transfected with a mammalian (human) or nonmammalian (Xenopus laevis type I) GnRH-R. Because conventional transfection strategies proved inefficient, recombinant adenovirus expressing these receptors were constructed, enabling controlled and efficient GnRH-R expression. When expressed in αT4 cells at physiological density, these GnRH-Rs retain the pharmacology of their endogenous counterparts (as judged by ligand specificity in radioligand binding and inositol phosphate accumulation assays) but do not activate adenylyl cyclase and are not constitutively active. Moreover, the Xenopus GnRH-R rapidly desensitizes and internalizes in these cells, whereas the human GnRH-R does not, and the internalization rates are not dependent upon receptor number. These data extend studies in COS, HEK, and GH3 cells showing that other GnRH-R with C-terminal tails desensitize and internalize rapidly, whereas tail-less mammalian GnRH-R do not. Retention of these distinctions at physiological receptor density in gonadotrope lineage cells, supports the argument that the evolution of nondesensitizing mammalian GnRH-Rs is functionally relevant and related to the development of mammalian reproductive strategies.