Gonadotropin-Releasing Hormone (GnRH) Receptor Structure and GnRH Binding

Abstract

Gonadotropin-releasing hormone (GnRH) regulates reproduction. The human GnRH receptor lacks a cytoplasmic carboxy-terminal tail but has amino acid sequence motifs characteristic of rhodopsin-like, class A, G protein-coupled receptors (GPCRs). This review will consider how recent descriptions of X-ray crystallographic structures of GPCRs in inactive and active conformations may contribute to understanding GnRH receptor structure, mechanism of activation and ligand binding. The structures confirmed that ligands bind to variable extracellular surfaces, whereas the seven membrane-spanning α-helices convey the activation signal to the cytoplasmic receptor surface, which binds and activates heterotrimeric G proteins. Forty non-covalent interactions that bridge topologically equivalent residues in different transmembrane (TM) helices are conserved in class A GPCR structures, regardless of activation state. Conformation-independent interhelical contacts account for a conserved receptor protein structure and their importance in the GnRH receptor structure is supported by decreased expression of receptors with mutations of residues in the network. Many of the GnRH receptor mutations associated with congenital hypogonadotropic hypogonadism, including the Glu2.53(90) Lys mutation, involve amino acids that constitute the conserved network. Half of the ~250 intramolecular interactions in GPCRs differ between inactive and active structures. Conformation-specific interhelical contacts depend on amino acids changing partners during activation. Conserved inactive conformation-specific contacts prevent receptor activation by stabilizing proximity of TM helices 3 and 6 and a closed G protein-binding site. Mutations of GnRH receptor residues involved in these interactions, such as Arg3.50(139) of the DRY/S motif or Tyr7.53(323) of the N/DPxxY motif, increase or decrease receptor expression and efficiency of receptor coupling to G protein signaling, consistent with the native residues stabilizing the inactive GnRH receptor structure. Active conformation-specific interhelical contacts stabilize an open G protein-binding site. Progress in defining the GnRH-binding site has recently slowed, with evidence that Tyr6.58(290) contacts Tyr5 of GnRH, whereas other residues affect recognition of Trp3 and Gly10NH2. The surprisingly consistent observations that GnRH receptor mutations that disrupt GnRH binding have less effect on “conformationally constrained” GnRH peptides may now be explained by crystal structures of agonist-bound peptide receptors. Analysis of GPCR structures provides insight into GnRH receptor function.

Genetics of ncHH: from a peculiar inheritance of a novel GNRHR mutation to a comprehensive review of the literature

“…As is the case for disease‐causing mutations of other GPCRs (Tao & Conn, ), most of the ncHH‐associated GNRHR mutations that have been characterized in vitro decrease cell surface expression of the mutant GnRH‐R protein (Table ). In many cases, the mutated residues in the GnRH‐R are equivalent to residues that form conserved intramolecular (interhelical) interactions that define and stabilize the conserved GPCR structural fold (Table ) (Venkatakrishnan et al ., ; Cvicek et al ., ; Flanagan & Manilall, ), suggesting that disruption of their role in receptor folding may cause misfolding that decreases protein expression. Of the four class IV mutations of GNRHR , three involve residues that have previously been implicated in conserved GPCR activation pathways (Table ), suggesting that the mutations disrupt intramolecular interactions that mediate activation of the GnRH‐R (Flanagan & Manilall, ).…”

“…In many cases, the mutated residues in the GnRH‐R are equivalent to residues that form conserved intramolecular (interhelical) interactions that define and stabilize the conserved GPCR structural fold (Table ) (Venkatakrishnan et al ., ; Cvicek et al ., ; Flanagan & Manilall, ), suggesting that disruption of their role in receptor folding may cause misfolding that decreases protein expression. Of the four class IV mutations of GNRHR , three involve residues that have previously been implicated in conserved GPCR activation pathways (Table ), suggesting that the mutations disrupt intramolecular interactions that mediate activation of the GnRH‐R (Flanagan & Manilall, ). The prevalence of mutations that disrupt intramolecular interactions that are conserved in GPCRs suggests that an in silico tool based on GPCR structures may better predict the effects of disease‐associated mutations in GNRHR and other GPCR genes.…”

“…Of the class III GNRHR mutations, four, including the p.Gly99Glu mutation, are located at the TMD2‐ECL1 interface of the GnRH‐R protein, where artificially created GnRH‐R mutants have identified three additional residues that are also important for ligand binding (Flanagan & Manilall, ). Amino acid residues in the TMD2‐ECL1 region of the GnRH‐R may directly contact GnRH, as has been proposed for the Asp98 and Asn102 residues (Flanagan & Manilall, ), or they may form intramolecular interactions that stabilize the shape of the ligand binding pocket. Mutations of these residues may disrupt a direct contact with the ligand, disrupt intramolecular interactions that configure the ligand‐binding pocket or sterically inhibit direct contact of the ligand with one or more other residues.…”

Preclinical evaluation of new GnRH‐I receptor radionuclide therapy with 177Lu‐peptide tracer

“…Many reports are available on the developing radiolabelled GnRH analogues. 125 I‐labelled GnRH was developed for receptor affinity studies in mutated GnRH receptors, permeability, protein binding, and the detection of GnRH receptors . Barda et al reported the design and synthesis of a GnRH analogue for tumor imaging as a suitable probe labeled with 99m Tc .…”

Gq-Induced Apoptosis is Mediated by AKT Inhibition That Leads to PKC-Induced JNK Activation

“…Aside of these GqPCR-induced effects, these central receptors were also implicated in growth arrest or apoptosis in few pathological systems [15][16][17]. For instance, cardiac hypertrophy seems to be mediated by auto/paracrine mediators acting through GqPCRs [18,19], and gonadotropin-releasing hormone (GnRH [20]) was shown to induce apoptosis of prostate cancer cells through its GqPCR [21]. Surprisingly, overexpression of constitutively active Gq induces apoptosis of COS7, CHO [22], and HeLa [23] cells as well.…”