A conserved Asn in TM7 of the thyrotropin receptor is a common requirement for activation by both mutations and its natural agonist

Novel presentation of the c.1856A > G (p.Asp619Gly) TSHR gene-activating variant: relapsing hyperthyroidism in three subsequent generations manifesting in early childhood and an in vitro functional study

BACKGROUND

Familial non-autoimmune hyperthyroidism is a rare disease caused by germline activating variants in the thyroid-stimulating hormone receptor (TSHR) gene. The c.1856A > G (p.Asp619Gly) pathogenic variant has been described in cases of toxic adenoma but never before, to our knowledge, in a case of familial non-autoimmune hyperthyroidism.

PATIENT FINDINGS

A 3-year-old boy was admitted for acute gastroenteritis presenting with goiter and tall stature. Laboratory findings revealed peripheral hyperthyroidism and negativity for thyroid autoantibodies. Antithyroid drug treatment was effective, but relapses occurred shortly after attempts to decrease the drug dose. As the boy's father and paternal grandmother also experienced relapsing hyperthyroidism manifesting in early childhood, genetic testing of TSHR was indicated. The c.1856A > G (p.Asp619Gly) pathogenic variant was found in all three affected family members. Functional in vitro characterization of the variant verified that it enhances constitutional activation of the receptor, leading to increased production of cyclic adenosine monophosphate. Total thyroidectomy was indicated in the boy due to an unsatisfactory prognosis. Due to persistent positive thyroglobulin serum concentration, a diagnostic radioiodine scan was performed approximately 2 years later. Residual thyroid tissue was revealed; therefore, radioiodine ablative therapy was performed. Despite adequate thyroxine substitution over a long period of follow-up, TSH remained suppressed.

CONCLUSIONS

Unlike Graves' disease, familial non-autoimmune hyperthyroidism cases present with antithyroid drug-dependence. Not ultrasound but positive thyroglobulin serum concentration indicated residual thyroid tissue. Early detection of residual thyroid tissue and radioiodine ablation prevented the subject from experiencing relapsing hyperthyroidism and undergoing unnecessary repeated surgery. Life-long hormone substitution should be adjusted to free thyroxine rather than TSH serum concentrations.

Structural-Functional Features of the Thyrotropin Receptor: A Class A G-Protein-Coupled Receptor at Work

The thyroid-stimulating hormone receptor (TSHR) is a member of the glycoprotein hormone receptors, a sub-group of class A G-protein-coupled receptors (GPCRs). TSHR and its endogenous ligand thyrotropin (TSH) are of essential importance for growth and function of the thyroid gland and proper function of the TSH/TSHR system is pivotal for production and release of thyroid hormones. This receptor is also important with respect to pathophysiology, such as autoimmune (including ophthalmopathy) or non-autoimmune thyroid dysfunctions and cancer development. Pharmacological interventions directly targeting the TSHR should provide benefits to disease treatment compared to currently available therapies of dysfunctions associated with the TSHR or the thyroid gland. Upon TSHR activation, the molecular events conveying conformational changes from the extra- to the intracellular side of the cell across the membrane comprise reception, conversion, and amplification of the signal. These steps are highly dependent on structural features of this receptor and its intermolecular interaction partners, e.g., TSH, antibodies, small molecules, G-proteins, or arrestin. For better understanding of signal transduction, pathogenic mechanisms such as autoantibody action and mutational modifications or for developing new pharmacological strategies, it is essential to combine available structural data with functional information to generate homology models of the entire receptor. Although so far these insights are fragmental, in the past few decades essential contributions have been made to investigate in-depth the involved determinants, such as by structure determination via X-ray crystallography. This review summarizes available knowledge (as of December 2016) concerning the TSHR protein structure, associated functional aspects, and based on these insights we suggest several receptor complex models. Moreover, distinct TSHR properties will be highlighted in comparison to other class A GPCRs to understand the molecular activation mechanisms of this receptor comprehensively. Finally, limitations of current knowledge and lack of information are discussed highlighting the need for intensified efforts toward TSHR structure elucidation.

Constitutive activity in gonadotropin receptors

Constitutively active mutants (CAMs) of gonadotropin receptors are, in general, rare conditions. Luteinizing hormone-choriogonadotropin receptor (LHCGR) CAMs provoke the dramatic phenotype of familial gonadotropin-independent isosexual male-limited precocious puberty, whereas in females, there is not yet any identified phenotype. Only one isolated follicle-stimulating hormone receptor (FSHR) CAM (Asp567Gly) has so far been detected in a single male patient, besides other FSHR weak CAMs linked to pregnancy-associated ovarian hyperstimulation syndrome or to impaired desensitization and internalization. Several animal models have been developed for studying enhanced gonadotropin action; in addition to unraveling valuable new information about the possible phenotypes of isolated FSHR and LHCGR CAMs in women, the information obtained from these mouse models has served multiple translational goals, including the development of new diagnostic and therapeutic targets as well as the prediction of phenotypes for mutations not yet identified in humans. Mutagenesis and computational studies have shed important information on the physiopathogenic mechanisms leading to constitutive activity of gonadotropin receptors; a common feature in these receptor CAMs is the release of stabilizing interhelical interactions between transmembrane domains (TMDs) 3 and 6 leading to an increase, with respect to the wild-type receptor, in the solvent accessibility at the cytosolic extension of TMDs 3, 5, and 6, which involves the highly conserved Glu/Asp-Arg-Tyr/Trp sequence. In this chapter, we summarize the structural features, functional consequences, and mechanisms that lead to constitutive activation of gonadotropin receptor CAMs and provide information on pharmacological approaches that might potentially modulate gonadotropin receptor CAM function.

Constitutive activities in the thyrotropin receptor: regulation and significance

The thyroid-stimulating hormone receptor (TSHR, or thyrotropin receptor) is a family A G protein-coupled receptor. It not only binds thyroid-stimulating hormone (TSH, or thyrotropin) but also interacts with autoantibodies under pathological conditions. The TSHR and TSH are essential for thyroid growth and function and thus for all thyroid hormone-associated physiological superordinated processes, including metabolism and development of the central nervous system. In vitro studies have found that the TSHR permanently stimulates ligand-independent (constitutive) activation of Gs, which ultimately leads to intracellular cAMP accumulation. Furthermore, a vast variety of constitutively activating mutations of TSHR-at more than 50 different amino acid positions-have been reported to enhance basal signaling. These lead in vivo to a "gain-of-function" phenotype of nonautoimmune hyperthyroidism or toxic adenomas. Moreover, many naturally occurring inactivating mutations are known to cause a "loss-of-function" phenotype, resulting in resistance to thyroid hormone or hyperthyrotropinemia. Several of these mutations are also characterized by impaired basal signaling, and these are designated here as "constitutively inactivating mutations" (CIMs). More than 30 amino acid positions with CIMs have been identified so far. Moreover, the permanent TSHR signaling capacity can also be blocked by inverse agonistic antibodies or small drug-like molecules, which both have a potential for clinical usage. In this chapter, information on constitutive activity in the TSHR is described, including up- and downregulation, linked protein conformations, physiological and pathophysiological conditions, and related intracellular signaling.

Novel insights on thyroid-stimulating hormone receptor signal transduction

The TSH receptor (TSHR) is a member of the glycoprotein hormone receptors, a subfamily of family A G protein-coupled receptors. The TSHR is of great importance for the growth and function of the thyroid gland. The TSHR and its endogenous ligand TSH are pivotal proteins with respect to a variety of physiological functions and malfunctions. The molecular events of TSHR regulation can be summarized as a process of signal transduction, including signal reception, conversion, and amplification. The steps during signal transduction from the extra- to the intracellular sites of the cell are not yet comprehensively understood. However, essential new insights have been achieved in recent years on the interrelated mechanisms at the extracellular region, the transmembrane domain, and intracellular components. This review contains a critical summary of available knowledge of the molecular mechanisms of signal transduction at the TSHR, for example, the key amino acids involved in hormone binding or in the structural conformational changes that lead to G protein activation or signaling regulation. Aspects of TSHR oligomerization, signaling promiscuity, signaling selectivity, phenotypes of genetic variations, and potential extrathyroidal receptor activity are also considered, because these are relevant to an understanding of the overall function of the TSHR, including physiological, pathophysiological, and pharmacological perspectives. Directions for future research are discussed.

Molecular sampling of the allosteric binding pocket of the TSH receptor provides discriminative pharmacophores for antagonist and agonists

The TSHR (thyrotropin receptor) is activated endogenously by the large hormone thyrotropin and activated pathologically by auto-antibodies. Both activate and bind at the extracellular domain. Recently, SMLs (small-molecule ligands) have been identified, which bind in an allosteric binding pocket within the transmembrane domain. Modelling driven site-directed mutagenesis of amino acids lining this pocket led to the delineation of activation and inactivation sensitive residues. Modified residues showing CAMs (constitutively activating mutations) indicate signalling-sensitive positions and mark potential trigger points for agonists. Silencing mutations lead to an impairment of basal activity and mark contact points for antagonists. Mapping these residues on to a structural model of TSHR indicates locations where an SML may switch the receptor to an inactive or active conformation. In the present article, we report the effects of SMLs on these signalling-sensitive amino acids at the TSHR. Surprisingly, the antagonistic effect of SML compound 52 was reversed to an agonistic effect, when tested at the CAM Y667A. Switching agonism to antagonism and the reverse by changing either SMLs or residues covering the binding pocket provides detailed knowledge about discriminative pharmacophores. It prepares the basis for rational optimization of new high-affinity antagonists to interfere with the pathogenic activation of the TSHR.

ER-bound steps in the biosynthesis of G protein-coupled receptors

The polypeptide of a G protein-coupled receptor is inserted into the membrane of the endoplasmic reticulum while being translated and this process by itself may be sufficient to establish the proper receptor fold. X-ray structures reveal a common polypeptide topology with little variation in the alignment and orientation of the seven transmembrane segments, the proximal carboxyl terminus (C-tail) and parts of the extracellular loops. These define a structural core the stability of which probably represents a major criterion for the receptor to pass endoplasmic reticulum (ER) quality control; point mutations affecting the structure of the core have an extraordinary chance of causing receptor retention. In contrast, cytoplasmic loops 2 and 3 and the distal C-tail are poorly ordered at least in the absence of an interaction partner. Similarly, the amino terminal tail of rhodopsin-related receptors (but not of receptor subtypes where ligand binding requires a stable fold of the N-tail) is unlikely to establish a stable fold. These segments can cause ER retention when mutated to inappropriately expose hydrophobic peptide patches; to prevent protein aggregation chaperone molecules attach to them thus initiating selection for ER-associated degradation. It is less clear however if there are additional mechanisms to specifically survey the transmembrane core at the level of the lipid bilayer or if insufficient packing is detected due to misalignment of the cytoplasmic or extracellular face of the receptor.

From molecular details of the interplay between transmembrane helices of the thyrotropin receptor to general aspects of signal transduction in family a G-protein-coupled receptors (GPCRs)

Transmembrane helices (TMHs) 5 and 6 are known to be important for signal transduction by G-protein-coupled receptors (GPCRs). Our aim was to characterize the interface between TMH5 and TMH6 of the thyrotropin receptor (TSHR) to gain molecular insights into aspects of signal transduction and regulation. A proline at TMH5 position 5.50 is highly conserved in family A GPCRs and causes a twist in the helix structure. Mutation of the TSHR-specific alanine (Ala-593⁵·⁵⁰) at this position to proline resulted in a 20-fold reduction of cell surface expression. This indicates that TMH5 in the TSHR might have a conformation different from most other family A GPCRs by forming a regular α-helix. Furthermore, linking our own and previous data from directed mutagenesis with structural information led to suggestions of distinct pairs of interacting residues between TMH5 and TMH6 that are responsible for stabilizing either the basal or the active state. Our insights suggest that the inactive state conformation is constrained by a core set of polar interactions among TMHs 2, 3, 6, and 7 and in contrast that the active state conformation is stabilized mainly by non-polar interactions between TMHs 5 and 6. Our findings might be relevant for all family A GPCRs as supported by a statistical analysis of residue properties between the TMHs of a vast number of GPCR sequences.

G protein-coupled receptors: mutations and endocrine diseases

Over the past 20 years, naturally occurring mutations that affect G protein-coupled receptors (GPCRs) have been identified, mainly in patients with endocrine diseases. The study of loss-of-function or gain-of-function mutations has contributed to our understanding of the pathophysiology of several diseases with classic hypophenotypes or hyperphenotypes of the target endocrine organs, respectively. Simultaneously, study of the mutant receptors ex vivo was instrumental in delineating the relationships between the structure and function of these important physiological and pharmacological molecules. Now that access to the crystallographic structure of a few GPCRs is available, the mechanics of these receptors can be studied at the atomic level. Progress in the fields of cell biology, molecular pharmacology and proteomics has also widened our view of GPCR functions. Initially considered simply as guanine nucleotide exchange factors capable of activating G protein-dependent regulatory cascades, GPCRs are now known to display several additional characteristics, each susceptible to alterations by disease-causing mutations. These characteristics include functionally important basal activity of the receptor; differential activation of various G proteins; differential activation of G protein-dependent and independent effects (biased agonism); interaction with proteins that modify receptor function; dimerization-dependent effects; and interaction with allosteric modulators. This Review attempts to illustrate how natural mutations of GPCR could contribute to our understanding of these novel facets of GPCR biology.

Lack of in vitro constitutive activity for four previously reported TSH receptor mutations identified in patients with nonautoimmune hyperthyroidism and hot thyroid carcinomas

OBJECTIVE

Constitutively activating mutations (CAMs) of the TSHR are the major cause for nonautoimmune hyperthyroidism. Re-examination of constitutive activity previously determined in CHO cell lines recently demonstrated the caveats for the in vitro determination of constitutive TSHR activity, which leads to false positive conclusions regarding the molecular origin of hyperthyroidism or hot thyroid carcinomas.

DESIGN

Mutations L677V and T620I identified in hot thyroid carcinomas were previously characterized in CHO and in 3T3-Vill cell lines, respectively, stably expressing the mutant without determination of TSHR expression. F666L identified in a patient with hot thyroid nodules, I691F in a family with nonautoimmune hyperthyroidism and F631I identified in a hot thyroid carcinoma were not characterized for their in vitro function. Therefore, we decided to (re)evaluate the in vitro function of these five TSHR variants by determination of cell surface expression, and intracellular cAMP and inositol phosphate levels and performed additionally linear regression analyses to determine basal activity independently from the mutant's cell surface expression in COS-7 and HEK(GT) cells.

RESULTS AND CONCLUSIONS

Only one (F631I) of the five investigated TSHR variants displayed constitutive activity for G(α) s signalling and showed correlation with the clinical phenotype. The previous false classification of T620I and L677V as CAMs is most likely related to the fact that both mutations were characterized in cell lines stably expressing the mutated receptor construct without assessing the respective receptor number per cell. Other molecular aetiologies for the nonautoimmune hyperthyroidism and/or hot thyroid carcinomas in these three patients and one family should be elucidated.

Lack of consistent association of thyrotropin receptor mutations in vitro activity with the clinical course of patients with sporadic non-autoimmune hyperthyroidism

BACKGROUND

Up to date, 14 patients with sporadic non-autoimmune hyperthyroidism (SNAH) caused by sporadic germline mutations in the TSH receptor (TSHR) gene have been reported. Despite considerable differences in the activity of hyperthyroidism, all SNAH case reports concluded that the demonstrated constitutive activity explains the phenotype.

AIM

Recently, linear regression analysis (LRA) of constitutive activity as a function of TSHR expression determined by 125I-bTSH binding or fluorescence activated cell sorting analysis was described as a more reliable way of characterizing the in vitro activity (IVA) of a constitutively activating TSHR mutation. Therefore, we analyzed a possible genotype-phenotype correlation in a systematic review of the case reports and investigated the TSHR mutation's LRA in selected cases.

MATERIAL AND METHODS

We determined the LRA for all sporadic germline mutations which had not previously been reported. Moreover, we systematically evaluated all case reports of SNAH for evidence of an association of the clinical course (CC) with the IVA of the mutated TSHR.

RESULTS

The LRA determined were: M453T (5.2+/-0.8), L512Q (4.5+/-0.7), I568T (25.6+/-6.3), F631L (45.9+/-9.4), T632I (14.5+/-2.7), D633Y (16.4+/-6.4). None of the 10 examined clinical signs showed a significant association with the LRA. Moreover, the comparison of the CC of patients harboring the same mutation (S281N, M453T, I568T, S505N) also showed no relation of the clinical activity with a high LRA.

CONCLUSION

Considering the different diagnostic circumstances, therapeutic strategies and the limitations of a systematic analysis of case reports due to the restricted number of case reports and limited follow-up we found no consistent relation of the TSHR mutation's IVA determined by LRA with the CC of patients with SNAH. This may also be due to the action of genetic, epigenetic, and environmental modifiers.

Current standards, variations, and pitfalls for the determination of constitutive TSHR activity in vitro

Constitutively activating mutations of the TSHR are the major cause for nonautoimmune hyperthyroidism, which is based on ligand independent, permanent receptor activation. Several reports have highlighted the difficulties to determine whether a TSHR mutation is constitutively active or not especially for borderline cases with only a slight increase of the basal cAMP activity. Current methods to precisely classify such mutants as constitutively active or not, are limited. In some cases, in vitro characterization of TSHR mutants has led to false positive conclusions regarding constitutive TSHR activity and subsequently the molecular origin of hyperthyroidism. For characterization of constitutive TSHR activity, a particular point to consider is that basal receptor activity tightly correlates with the receptor number expressed on the cell surface. Therefore, a comparison of the receptors basal activity in relation to the wild type is only possible with determination of the receptor cell surface expression. Thus, the experimental approaches to determine constitutive TSHR activity should consider the receptor's cell surface expression. We here provide a description of three methods for the determination of constitutive TSHR activity: (A) the evaluation of constitutive TSHR activity under conditions of equal receptor expression; (B) computation of the specific constitutive activity; and (C) the linear regression analysis (LRA). To date, LRA is the best experimental approach to characterize the mutant's basal activity as a function of TSHR cell surface expression. This approach utilizes a parallel measurement of basal cAMP values and receptor cell surface expression and therefore provides a more reliable decision with respect to the presence or absence of constitutive activity.

Cases of borderline in vitro constitutive thyrotropin receptor activity: how to decide whether a thyrotropin receptor mutation is constitutively active or not?

BACKGROUND

Previous in vitro data for several constitutively activating thyrotropin receptor (TSHR) mutations reported divergent results for the constitutive activity of the same mutations. Moreover, several case reports have highlighted the difficulties in determining whether a TSHR mutation is constitutively active or not. Retrospectively, this has repeatedly been the case for mutants with only a slight increase of basal cAMP activity. We re-examined 10 previously described TSHR germline mutations with minor increases of basal cAMP activity and analyzed the influences of the cell line and vector system on the basal receptor activity.

METHODS

TSHR mutations were characterized by determination of cell surface expression, cAMP accumulation, and linear regression analysis of constitutive activity.

RESULTS

Re-examination of the previously described constitutively active TSHR germline mutations did not show constitutive activity for R310C and N670S as tested in COS-7 cells and confirmed constitutive activity for the other eight mutations. However, mutant N670S showed a slight but significant increase of basal activity measured by linear regression analysis when analyzed in HEK(GT) cells transiently transfected with pcDNA but not with the pSVL vector. This was not the case for R310C.

CONCLUSIONS

Our findings indicate that current methods to precisely classify mutants with only a slight increase of the basal activity as constitutively active are limited. The results concerning the level of the basal activity can be influenced by the vector and/or the cell system. A comprehensive clinical characterization of the respective patients appears as a necessary and promising adjunct for the activity classification of these borderline mutations.

Evidence for cooperative signal triggering at the extracellular loops of the TSH receptor

The mechanisms governing transition of the thyroid stimulating hormone (TSH) receptor (TSHR) from basal to active conformations are poorly understood. Considering that constitutively activating mutations (CAMs) and inactivating mutations in each of the extracellular loops (ECLs) trigger only partial TSHR activation or inactivation, respectively, we hypothesized that full signaling occurs via multiple extracellular signal propagation events. Therefore, individual CAMs in the extracellular region were combined to create double and triple mutants. In support of our hypothesis, combinations of mutants in the ECLs are in some cases additive, while in others they are even synergistic, with triple mutant I486A/I568V/V656F exhibiting a 70-fold increase in TSH-independent signaling. The proximity but likely different spatial orientation of the residues of activating and inactivating mutations in each ECL supports a dual functionality to facilitate signal induction and conduction, respectively. This is the first report for G-protein coupled receptors, suggesting that multiple and cooperative signal propagating events at all three ECLs are required for full receptor activation. Our findings provide new insights concerning molecular signal transmission from extracellular domains toward the transmembrane helix bundle of the glycoprotein hormone receptors.

Constitutively active mutants of the histamine H1 receptor suggest a conserved hydrophobic asparagine-cage that constrains the activation of class A G protein-coupled receptors

The aim of this study was to create and characterize constitutively active mutant (CAM) histamine H(1) receptors (H(1)R) using random mutagenesis methods to further investigate the activation process of the rhodopsin-like family of G protein-coupled receptors (GPCRs). This approach identified position 6.40 in TM 6 as a "hot spot" because mutation of Ile6.40(420) either to Glu, Gly, Ala, Arg, Lys, or Ser resulted in highly active CAM H(1)Rs, for which almost no histamine-induced receptor activation response could be detected. The highly conserved hydrophobic amino acid at position 6.40 defines, in a computational model of the H(1)R, the asparagine cage motif that restrains the side chain of Asn7.49 of the NPxxY motif toward transmembrane domain (TM 6) in the inactive state of the receptor. Mutation of the asparagine cage into Ala or Gly, removing the interfering bulky constraints, increases the constitutive activity of the receptor. The fact that the Ile6.40(420)Arg/Lys/Glu mutant receptors are highly active CAM H(1)Rs leads us to suggest that a positively charged residue, presumably the highly conserved Arg3.50 from the DRY motif, interacts in a direct or an indirect (through other side chains or/and internal water molecules) manner with the acidic Asp2.50..Asn7.49 pair for receptor activation.

International Union of Pharmacology LVIII: update on the P2Y G protein-coupled nucleotide receptors: from molecular mechanisms and pathophysiology to therapy

There have been many advances in our knowledge about different aspects of P2Y receptor signaling since the last review published by our International Union of Pharmacology subcommittee. More receptor subtypes have been cloned and characterized and most orphan receptors de-orphanized, so that it is now possible to provide a basis for a future subdivision of P2Y receptor subtypes. More is known about the functional elements of the P2Y receptor molecules and the signaling pathways involved, including interactions with ion channels. There have been substantial developments in the design of selective agonists and antagonists to some of the P2Y receptor subtypes. There are new findings about the mechanisms underlying nucleotide release and ectoenzymatic nucleotide breakdown. Interactions between P2Y receptors and receptors to other signaling molecules have been explored as well as P2Y-mediated control of gene transcription. The distribution and roles of P2Y receptor subtypes in many different cell types are better understood and P2Y receptor-related compounds are being explored for therapeutic purposes. These and other advances are discussed in the present review.

GRIS: glycoprotein-hormone receptor information system

The glycoprotein-hormone receptor information system (GRIS) presents a comprehensive view on all available molecular data for the lutropin/choriogonadotropin receptor, follitropin receptor, and thyrotropin receptor G protein-coupled receptors. It features a mutation database presently containing 696 point mutations, combined with all sequences and the associated homology models. The mutation information was automatically extracted from the literature and manually augmented with respect to constitutivity, surface expression, sensitivity to hormones, and binding affinity. All information in this integrated system is presented in a G protein-coupled receptor specialist-friendly way. A series of interactive tools such as rotamer analysis, mutation prediction, or cavity visualization aids with the design and interpretation of experiments. A universal residue numbering system has been introduced to ease database searches as well as the use of the information in conjunction with literature data from diverse origins. Users can upload new mutations. GRIS is freely accessible at http://gris.ulb.ac.be/.

Molecular mechanisms of the neural melanocortin receptor dysfunction in severe early onset obesity

The neural melanocortin receptors, melanocortin-3 and -4 receptors (MC3R and MC4R), have been shown to regulate different aspects of energy homeostasis in rodents. Human genetic studies showed that mutations in the MC4R gene are the most common monogenic form of obesity. Functional analyses of the mutant receptors revealed multiple defects. A classification scheme is presented for cataloguing the ever-increasing array of MC4R mutations. Functional analysis of the only inactivating MC3R mutation is also summarized. Insights from the analyses of the naturally occurring mutations in the MC3R and MC4R on the structure and function of these receptors are highlighted.

Linking agonist binding to histamine H1 receptor activation

G protein-coupled receptors (GPCRs) constitute a large and functionally diverse family of transmembrane proteins. They are fundamental in the transfer of extracellular stimuli to intracellular signaling pathways and are among the most targeted proteins in drug discovery. The detailed molecular mechanism for agonist-induced activation of rhodopsin-like GPCRs has not yet been described. Using a combination of site-directed mutagenesis and molecular modeling, we characterized important steps in the activation of the human histamine H1 receptor. Both Ser3.36 and Asn7.45 are important links between histamine binding and previously proposed conformational changes in helices 6 and 7. Ser3.36 acts as a rotamer toggle switch that, upon agonist binding, initiates the activation of the receptor through Asn7.45. The proposed transduction involves specific residues that are conserved among rhodopsin-like GPCRs.

An activation switch in the rhodopsin family of G protein-coupled receptors: the thyrotropin receptor

We aimed at understanding molecular events involved in the activation of a member of the G protein-coupled receptor family, the thyrotropin receptor. We have focused on the transmembrane region and in particular on a network of polar interactions between highly conserved residues. Using molecular dynamics simulations and site-directed mutagenesis techniques we have identified residue Asn-7.49, of the NPxxY motif of TM 7, as a molecular switch in the mechanism of thyrotropin receptor (TSHr) activation. Asn-7.49 appears to adopt two different conformations in the inactive and active states. These two states are characterized by specific interactions between this Asn and polar residues in the transmembrane domain. The inactive gauche+ conformation is maintained by interactions with residues Thr-6.43 and Asp-6.44. Mutation of these residues into Ala increases the constitutive activity of the receptor by factors of approximately 14 and approximately 10 relative to wild type TSHr, respectively. Upon receptor activation Asn-7.49 adopts the trans conformation to interact with Asp-2.50 and a putatively charged residue that remains to be identified. In addition, the conserved Leu-2.46 of the (N/S)LxxxD motif also plays a significant role in restraining the receptor in the inactive state because the L2.46A mutation increases constitutive activity by a factor of approximately 13 relative to wild type TSHr. As residues Leu-2.46, Asp-2.50, and Asn-7.49 are strongly conserved, this molecular mechanism of TSHr activation can be extended to other members of the rhodopsin-like family of G protein-coupled receptors.

Analysis of the thyrotropin receptor-thyrotropin interaction by comparative modeling

We have used the most advanced programs currently available to construct the first three-domain structure of the human thyrotropin receptor (TSHR) using comparative modeling. The model consists of a leucine-rich domain (LRD; amino acids 36-281; porcine ribonuclease inhibitor used as a template for modeling), a cleavage domain (CD; amino acids 282-409; tissue inhibitor of matrix metalloproteinases 2 as template) and transmembrane domain (TMD amino acids 410-699; bovine rhodopsin as template). Models of human, porcine, and bovine TSH were also constructed (human chorionic gonadotropin [hCG] and human follicle stimulating hormone [hFSH] as templates). The LRD has a characteristic horseshoe shape with 10 tandem homologous repeats. The CD consists of beta-barrel and alpha helix structures (OB-like fold) with two disulfide bridges and the structure around these disulfide bridges remains stable after cleavage. The TMD presents the typical seven membrane-spanning helices. The TSH, LRD, CD, and TMD models were brought together in an extensive series of docking experiments. Known features of the TSH-TSHR interaction were used for selection of appropriate complexes that were then validated using a different set of experimental data. A similar approach was used to build a model of a complex between the TSHR and a monoclonal TSHR antibody with weak thyroid stimulating activity. Human thyrotropin (hTSH) alpha chains were found to make contact with many amino acids on the LRD surface and CD surface whereas no interaction between the beta chains and the CD were found. The higher affinity of bovine thyrotropin (bTSH) and porcine thyrotropin (pTSH) (relative to hTSH) for the TSHR is explained well by the models in terms of charge-charge interactions between their alpha chains and the receptor. Experimental observations showing increased sensitivity of the TSHR to hCG after mutation of TSHR Lys209 to Glu are explained well by our model. Furthermore, several mutations in the TMD that are associated with increased TSHR basal activity are predicted from our model to be caused by the formation of new interactions that stabilize the activated form of the TMD.

Modulation of ligand selectivity associated with activation of the transmembrane region of the human follitropin receptor

Recently, three naturally occurring mutations in the serpentine region of the FSH receptor (FSHr) (D567N and T449I/A) have been identified in three families with spontaneous ovarian hyperstimulation syndrome (OHSS). All mutant receptors displayed abnormally high sensitivity to human chorionic gonadotropin and, in addition, D567N and T449A displayed concomitant increase in sensitivity to TSH and detectable constitutive activity. In the present study, we have used a combination of site-directed mutagenesis experiments and molecular modeling to explore the mechanisms responsible for the phenotype of the three OHSS FSHr mutants. Our results suggest that all mutations lead to weakening of interhelical locks between transmembrane helix (TM)-VI and TM-III, or TM-VI and TM-VII, which contributes to maintaining the receptor in the inactive state. They also indicate that broadening of the functional specificity of the mutant FSHr constructs is correlated to their increase in constitutive activity. This relation between basal activity and functional specificity is a characteristic of the FSHr, which is not shared by the other glycoprotein hormone receptors. It leads to the interesting suggestion that different pathways have been followed during primate evolution to avoid promiscuous stimulation of the TSHr and FSHr by human chorionic gonadotropin. In the hFSHr, specificity would be exerted both by the ectodomain and the serpentine portion.