Research resource: novel structural insights bridge gaps in glycoprotein hormone receptor analyses

Mutations in G Protein-Coupled Receptors: Mechanisms, Pathophysiology and Potential Therapeutic Approaches

There are approximately 800 annotated G protein-coupled receptor (GPCR) genes, making these membrane receptors members of the most abundant gene family in the human genome. Besides being involved in manifold physiologic functions and serving as important pharmacotherapeutic targets, mutations in 55 GPCR genes cause about 66 inherited monogenic diseases in humans. Alterations of nine GPCR genes are causatively involved in inherited digenic diseases. In addition to classic gain- and loss-of-function variants, other aspects, such as biased signaling, trans-signaling, ectopic expression, allele variants of GPCRs, pseudogenes, gene fusion, and gene dosage, contribute to the repertoire of GPCR dysfunctions. However, the spectrum of alterations and GPCR involvement is probably much larger because an additional 91 GPCR genes contain homozygous or hemizygous loss-of-function mutations in human individuals with currently unidentified phenotypes. This review highlights the complexity of genomic alteration of GPCR genes as well as their functional consequences and discusses derived therapeutic approaches. SIGNIFICANCE STATEMENT: With the advent of new transgenic and sequencing technologies, the number of monogenic diseases related to G protein-coupled receptor (GPCR) mutants has significantly increased, and our understanding of the functional impact of certain kinds of mutations has substantially improved. Besides the classical gain- and loss-of-function alterations, additional aspects, such as biased signaling, trans-signaling, ectopic expression, allele variants of GPCRs, uniparental disomy, pseudogenes, gene fusion, and gene dosage, need to be elaborated in light of GPCR dysfunctions and possible therapeutic strategies.

The intramolecular agonist is obligate for activation of glycoprotein hormone receptors

In contrast to most rhodopsin-like G protein-coupled receptors, the glycoprotein hormone receptors (GPHR) have a large extracellular N-terminus for hormone binding. The hormones do not directly activate the transmembrane domain but mediate their action via a, thus, far only partially known Tethered Agonistic LIgand (TALI). The existence of such an intramolecular agonist was initially indicated by site-directed mutation studies and activating peptides derived from the extracellular hinge region. It is still unknown precisely how TALI is involved in intramolecular signal transmission. We combined systematic mutagenesis studies at the luteinizing hormone receptor and the thyroid-stimulating hormone receptor (TSHR), stimulation with a drug-like agonist (E2) of the TSHR, and structural homology modeling to unravel the functional and structural properties defining the TALI region. Here, we report that TALI (a) is predisposed to constitutively activate GPHR, (b) can by itself rearrange GPHR into a fully active conformation, (c) stabilizes active GPHR conformation, and (d) is not involved in activation of the TSHR by E2. In the active state conformation, TALI forms specific interactions between the N-terminus and the transmembrane domain. We show that stabilization of an active state is dependent on TALI, including activation by hormones and constitutively activating mutations.

A New Highly Thyrotropin Receptor-Selective Small-Molecule Antagonist with Potential for the Treatment of Graves' Orbitopathy

BACKGROUND

The thyrotropin receptor (TSHR) is the target for autoimmune thyroid stimulating antibodies (TSAb) triggering hyperthyroidism. Whereas elevated thyroid hormone synthesis by the thyroid in Graves' disease can be treated by antithyroid agents, for the pathogenic activation of TSHR in retro-orbital fibroblasts of the eye, leading to Graves' orbitopathy (GO), no causal TSHR directed therapy is available.

METHODS

Due to the therapeutic gap for severe GO, TSHR inhibitors were identified by high-throughput screening in Chinese hamster ovary cells expressing the TSHR. Stereo-selective synthesis of the screening hits led to the molecule S37, which contains seven chiral centers. Enantiomeric separation of the molecule S37 resulted in the enantiopure molecule S37a-a micro-molar antagonist of thyrotropin-induced cyclic adenosine monophosphate accumulation in HEK 293 cells expressing the TSHR.

RESULTS

The unique rigid bent shape of molecule S37a may mediate the observed high TSHR selectivity. Most importantly, the closely related follitropin and lutropin receptors were not affected by this compound. S37a not only inhibits the TSHR activation by thyrotropin itself but also activation by monoclonal TSAb M22 (human), KSAb1 (murine), and the allosteric small-molecule agonist C2. Disease-related ex vivo studies in HEK 293 cells expressing the TSHR showed that S37a also inhibits cyclic adenosine monophosphate formation by oligoclonal TSAb, which are highly enriched in GO patients' sera. Initial in vivo pharmacokinetic studies revealed no toxicity of S37a and a remarkable 53% oral bioavailability in mice.

CONCLUSION

In summary, a novel highly selective inhibitor for the TSHR is presented, which has promising potential for further development for the treatment of GO.

Gonadotropins and Their Analogs: Current and Potential Clinical Applications

The gonadotropin receptors LH receptor and FSH receptor play a central role in governing reproductive competency/fertility. Gonadotropin hormone analogs have been used clinically for decades in assisted reproductive therapies and in the treatment of various infertility disorders. Though these treatments are effective, the clinical protocols demand multiple injections, and the hormone preparations can lack uniformity and stability. The past two decades have seen a drive to develop chimeric and modified peptide analogs with more desirable pharmacokinetic profiles, with some displaying clinical efficacy, such as corifollitropin alfa, which is now in clinical use. More recently, low-molecular-weight, orally active molecules with activity at gonadotropin receptors have been developed. Some have excellent characteristics in animals and in human studies but have not reached the market-largely as a result of acquisitions by large pharma. Nonetheless, such molecules have the potential to mitigate risks currently associated with gonadotropin-based fertility treatments, such as ovarian hyperstimulation syndrome and the demands of injection-based therapies. There is also scope for novel use beyond the current remit of gonadotropin analogs in fertility treatments, including application as novel contraceptives; in the treatment of polycystic ovary syndrome; in the restoration of function to inactivating mutations of gonadotropin receptors; in the treatment of ovarian and prostate cancers; and in the prevention of bone loss and weight gain in postmenopausal women. Here we review the properties and clinical application of current gonadotropin preparations and their analogs, as well as the development of novel orally active, small-molecule nonpeptide analogs.

Structure of a Thyrotropin Receptor Monoclonal Antibody Variable Region Provides Insight into Potential Mechanisms for its Inverse Agonist Activity

BACKGROUND

The high constitutive, or ligand-independent, activity of the thyrotropin receptor (TSHR) is of clinical importance in some thyroid conditions, particularly well-differentiated thyroid carcinoma remnants following incomplete ablative therapy (surgery and radioiodine). Under these conditions, even total suppression of TSH by thyroid hormone administration does not fully reduce TSHR activity, a driver of thyrocyte growth.

METHODS

CS-17 is a murine monoclonal antibody that has inverse agonist activity in that it suppresses TSHR constitutive activity. This study crystallized the CS-17 Fab and determined its atomic structure at a resolution of 3.4 Å.

RESULTS

In silico docking of this structure to that of the TSHR extracellular domain was accomplished by targeting to TSHR residue tyrosine 195 (Y195) known to contribute to the CS-17 epitope. High affinity interaction between these two molecules, primarily by the CS-17 immunoglobulin heavy chain, was validated by energetic analysis (K_D of 8.7 × 10^-11 M), as well as by previously obtained data on a number of individual TSHR amino acids in three regions whose mutagenesis reduced CS-17 binding as detected by flow cytometry.

CONCLUSIONS

Structural insight at atomic resolution of a TSHR antibody with inverse agonist activity opens the way for the development of a molecule with therapeutic potential, particularly in thyroid carcinoma. For this purpose, CS-17 will require "humanization" by substitution of its constant region (Fc component). In addition, with its epitope defined, the CS-17 affinity can be increased further by mutagenesis of selected amino acids in its heavy- and light-chain complementarity determining regions.

Structure-Function Relationships of the Follicle-Stimulating Hormone Receptor

The follicle-stimulating hormone receptor (FSHR) plays a crucial role in reproduction. This structurally complex receptor is a member of the G-protein coupled receptor (GPCR) superfamily of membrane receptors. As with the other structurally similar glycoprotein hormone receptors (the thyroid-stimulating hormone and luteinizing hormone-chorionic gonadotropin hormone receptors), the FSHR is characterized by an extensive extracellular domain, where binding to FSH occurs, linked to the signal specificity subdomain or hinge region. This region is involved in ligand-stimulated receptor activation whereas the seven transmembrane domain is associated with receptor activation and transmission of the activation process to the intracellular loops comprised of amino acid sequences, which predicate coupling to effectors, interaction with adapter proteins, and triggering of downstream intracellular signaling. In this review, we describe the most important structural features of the FSHR intimately involved in regulation of FSHR function, including trafficking, dimerization, and oligomerization, ligand binding, agonist-stimulated activation, and signal transduction.

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.

Minireview: Insights Into the Structural and Molecular Consequences of the TSH-β Mutation C105Vfs114X

Naturally occurring thyrotropin (TSH) mutations are rare, which is also the case for the homologous heterodimeric glycoprotein hormones (GPHs) follitropin (FSH), lutropin (LH), and choriogonadotropin (CG). Patients with TSH-inactivating mutations present with central congenital hypothyroidism. Here, we summarize insights into the most frequent loss-of-function β-subunit of TSH mutation C105Vfs114X, which is associated with isolated TSH deficiency. This review will address the following question. What is currently known on the molecular background of this TSH variant on a protein level? It has not yet been clarified how C105Vfs114X causes early symptoms in affected patients, which are comparably severe to those observed in newborns lacking any functional thyroid tissue (athyreosis). To better understand the mechanisms of this mutant, we have summarized published reports and complemented this information with a structural perspective on GPHs. By including the ancestral TSH receptor agonist thyrostimulin and pathogenic mutations reported for FSH, LH, and choriogonadotropin in the analysis, insightful structure function and evolutionary restrictions become apparent. However, comparisons of immunogenicity and bioactivity of different GPH variants is hindered by a lack of consensus for functional analysis and the diversity of used GPH assays. Accordingly, relevant gaps of knowledge concerning details of GPH mutation-related effects are identified and highlighted in this review. These issues are of general importance as several previous and recent studies point towards the high impact of GPH variants in differential signaling regulation at GPH receptors (GPHRs), both endogenously and under diseased conditions. Further improvement in this area is of decisive importance for the development of novel targeted therapies.

Structural modeling of G-protein coupled receptors: An overview on automatic web-servers

Despite the significant efforts and discoveries during the last few years in G protein-coupled receptor (GPCR) expression and crystallization, the receptors with known structures to date are limited only to a small fraction of human GPCRs. The lack of experimental three-dimensional structures of the receptors represents a strong limitation that hampers a deep understanding of their function. Computational techniques are thus a valid alternative strategy to model three-dimensional structures. Indeed, recent advances in the field, together with extraordinary developments in crystallography, in particular due to its ability to capture GPCRs in different activation states, have led to encouraging results in the generation of accurate models. This, prompted the community of modelers to render their methods publicly available through dedicated databases and web-servers. Here, we present an extensive overview on these services, focusing on their advantages, drawbacks and their role in successful applications. Future challenges in the field of GPCR modeling, such as the predictions of long loop regions and the modeling of receptor activation states are presented as well.

TSH Receptor Signaling Abrogation by a Novel Small Molecule

Pathological activation of the thyroid-stimulating hormone receptor (TSHR) is caused by thyroid-stimulating antibodies in patients with Graves' disease (GD) or by somatic and rare genomic mutations that enhance constitutive activation of the receptor influencing both G protein and non-G protein signaling. Potential selective small molecule antagonists represent novel therapeutic compounds for abrogation of such abnormal TSHR signaling. In this study, we describe the identification and in vitro characterization of a novel small molecule antagonist by high-throughput screening (HTS). The identification of the TSHR antagonist was performed using a transcription-based TSH-inhibition bioassay. TSHR-expressing CHO cells, which also expressed a luciferase-tagged CRE response element, were optimized using bovine TSH as the activator, in a 384 well plate format, which had a Z score of 0.3-0.6. Using this HTS assay, we screened a diverse library of ~80,000 compounds at a final concentration of 16.7 μM. The selection criteria for a positive hit were based on a mean signal threshold of ≥50% inhibition of control TSH stimulation. The screening resulted in 450 positive hits giving a hit ratio of 0.56%. A secondary confirmation screen against TSH and forskolin - a post receptor activator of adenylyl cyclase - confirmed one TSHR-specific candidate antagonist molecule (named VA-K-14). This lead molecule had an IC₅₀ of 12.3 μM and a unique chemical structure. A parallel analysis for cell viability indicated that the lead inhibitor was non-cytotoxic at its effective concentrations. In silico docking studies performed using a TSHR transmembrane model showed the hydrophobic contact locations and the possible mode of inhibition of TSHR signaling. Furthermore, this molecule was capable of inhibiting TSHR stimulation by GD patient sera and monoclonal-stimulating TSHR antibodies. In conclusion, we report the identification of a novel small molecule TSHR inhibitor, which has the potential to be developed as a therapeutic antagonist for abrogation of TSHR signaling by TSHR autoantibodies in GD.

Structure-function relationships of glycoprotein hormones and their subunits' ancestors

Glycoprotein hormones (GPHs) are the most complex molecules with hormonal activity. They exist only in vertebrates but the genes encoding their subunits' ancestors are found in most vertebrate and invertebrate species although their roles are still unknown. In the present report, we review the available structural and functional data concerning GPHs and their subunits' ancestors.

Characterization of tilapia (Oreochromis niloticus) gonadotropins by modeling and immunoneutralization

In fish, both follicle-stimulating hormone (FSH) and luteinizing hormone (LH) play important roles in reproduction. Here we explored the structure and differential specificity of tilapia (t) gonadotropins (GTHs) to delineate their physiological relevance and the nature of their regulation. We generated structural models of tGTHs and GTH receptors (R) that enabled us to better understand the hormone-receptor interacting region. In tilapia, FSH release is under the control of the hypothalamic decapeptide GnRH, an effect that was abolished by specific bioneutralizing antisera [anti-recombinant (r) tFSHβ]. These antisera also reduced the basal secretion and delayed GnRH-stimulated production of 11-ketotestosterone (11KT), and dramatically reduced LH levels. Immunoneutralization of tLH using anti-rtLHβ significantly reduced its GnRH-stimulated levels. Basal 11KT and FSH levels were also reduced. Taken together, these results suggest a feedback mechanism between FSH and LH release in tilapia.

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.