Surface retention of an inactivating lutropin receptor mutant in exoloop 3

Mutations of gonadotropins and gonadotropin receptors: elucidating the physiology and pathophysiology of pituitary-gonadal function

The recent unraveling of structures of genes for the gonadotropin subunits and gonadotropin receptors has provided reproductive endocrinologists with new tools to study normal and pathological functions of the hypothalamic-pituitary-gonadal axis. Rare inactivating mutations that produce distinctive phenotypes of isolated LH or FSH deficiency have been discovered in gonadotropin subunit genes. In addition, there is a common polymorphism in the LHbeta subunit gene with possible clinical significance as a contributing factor to pathologies of LH-dependent gonadal functions. Both activating and inactivating mutations have been detected in the gonadotropin receptor genes, a larger number in the LH receptor gene, but so far only a few in the gene for the FSH receptor. These mutations corroborate and extend our knowledge of clinical consequences of gonadotropin resistance and inappropriate gonadotropin action. The information obtained from human mutations has been complemented by animal models with disrupted or inappropriately activated gonadotropin ligand or receptor genes. These clinical and experimental genetic disease models form a powerful tool for exploring the physiology and pathophysiology of gonadotropin function and provide an excellent example of the power of molecular biological approaches in the study of pathogenesis of diseases.

Identification of ionizable amino acid residues on the extracellular domain of the lutropin receptor involved in ligand binding

The LH receptor (LHR) is a G protein-coupled receptor characterized by a relatively large N-terminal extracellular domain responsible for high affinity ligand binding. Based on a model proposed for a major portion of the extracellular domain that contains a number of leucine-rich repeats, nine ionizable amino acid residues (Glu57, Glu80, Lys158, Glu181, Lys183, Glu184, Glu188, Lys190, and Asp206) were selected for charge reversal mutagenesis based on their locations in the proposed model and their potential to serve as ligand contact sites. Mutant LHR complementary DNAs were transiently transfected into COS-7 cells, and the expressed receptors were characterized by Western blot analysis, competitive ligand (hCG) binding, and ligand-mediated cAMP production. The most interesting mutants were K158E, K183E, E184K, and D206K, which were present on the plasma membrane fraction, as judged by Western blots, but were incapable of binding hCG and, of course, were deficient in hCG-mediated cAMP production. Other replacements at these positions, K158R,Q,G; K183R,Q,G; E184N; and D206E,Q, led to cell surface binding and signaling. The mutants E57K, E189K, and K190E behaved similarly to wild-type LHR; E80K was trapped intracellularly, but bound ligand in solubilized cells; and E181K was not expressed or was rapidly degraded. These results, based on 18 point mutants of LHR, indicate that Lys158, Lys183, Glu184, and Asp206 are involved, either directly or indirectly, in gonadotropin binding and support the general nature of the proposed model.