Cloning and functional expression of a thyrotropin receptor from the gonads of a vertebrate (bony fish): potential thyroid-independent role for thyrotropin in reproduction

The thyroid stimulating hormone receptor (TSHR) mediates the pituitary cont rol of the development, growth and function of the thyroid. The expression of the gene encoding this receptor is known only in the thyroid, lymphocyte s, fibroblasts, retro-orbital tissue and fat cells. We have cloned a TSHR f rom the gonads of a non-mammalian vertebrate, a bony fish (striped bass, Mo l one saxatilis) in the course of our search for gonadotropin receptors (fo llicle stimulating hormone receptor, FSHR and luteinizing hormone receptor, LHR). RT-PCR analysis demonstrated that the striped bass TSHR (stbTSHR) tr anscripts were abundant in both the thyroid and gonads and detectable in sk eletal muscle, heart and brain tissues. The stbTSHR cDNA encoded a 779-amin o acid glycoprotein hormone receptor with much higher homology (57-59%) to the mammalian TSH receptors than the mammalian LH receptors (47-49%) and FS H receptors (47%), and salmon and catfish gonadotropin receptors (42-45%). There was a TSHR-specific insertion in the extracellular domain as seen in mammalian receptors. Moreover, PCR analysis of genomic DNA indicated the ab sence of the LHR-specific intron in the striped bass TSHR gene. Recombinant stbTSHR expressed in COS1 cells activated reporter genes (luciferase) driv en by either a cAMP response element or the c-fos promoter in response to b ovine TSH, stbLH or hCG, but not human FSH. In situ hybridization studies r evealed the presence of stbTSHR transcripts in the gametes but not in the f ollicular cells. This pattern of expression is unique and suggests a direct , albeit unknown, role for TSH in gamete physiology. (C) 2000 Elsevier Scie nce Ireland Ltd. All rights reserved.