Structural requirements for mutational lutropin/choriogonadotropin receptor activation

Transgenic Drosophila Expressing Active Human LH Receptor in the Gonads Exhibit a Decreased Fecundity: Towards a Platform to Identify New Orally Active Modulators of Gonadotropin Receptor Activity

BACKGROUND/OBJECTIVES

The gonadotropins luteinizing hormone (LH) and follicle-stimulating hormone (FSH) and their receptors are major regulators of reproduction in mammals and are absent in insects. We previously established transgenic Drosophila lines expressing a constitutively active human LH receptor variant (LHRD578Y) and the wild-type receptor (LHRwt; inactive in the absence of an agonist). That study showed that ubiquitously expression of LHRD578Y-but not of LHRwt-resulted in pupal lethality, and targeted expression in midline cells resulted in thorax/bristles defects. To further study the Drosophila model for an in vivo drug screening platform, we investigated here whether expressing LHRD578Y in the fly gonads alters reproduction, as shown in a transgenic mice model.

METHODS

The receptor was expressed in somatic cells of the gonads using the tissue-specific traffic jam-Gal4 driver. Western blot analysis confirmed receptor expression in the ovaries.

RESULTS

A fecundity assay indicated that the ectopic expression of LHRD578Y resulted in a decrease in egg laying compared to control flies carrying, but not expressing the transgene (~40% decrease in two independent fly lines, p < 0.001). No significant reduction in the number of laid eggs was seen in flies expressing the LHRWT (<10% decrease compared to non-driven flies, p > 0.05). The decreased egg laying demonstrates a phenotype of the active receptor in the fly gonads, the prime target organs of the gonadotropins in mammals. We suggest that this versatile Drosophila model can be used for the pharmacological search for gonadotropin modulators.

CONCLUSIONS

This is expected to provide: (a) new mimetic drug candidates (receptor-agonists/signaling-activators) for assisted reproduction treatment, (b) blockers for potential fertility regulation, and (c) leads relevant for the purpose of managing extra gonadotropic reported activities.

7TM domain structures of adhesion GPCRs: what's new and what's missing?

Adhesion-type G protein-coupled receptors (aGPCRs) have long resisted approaches to resolve the structural details of their heptahelical transmembrane (7TM) domains. Single-particle cryogenic electron microscopy (cryo-EM) has recently produced aGPCR 7TM domain structures for ADGRD1, ADGRG1, ADGRG2, ADGRG3, ADGRG4, ADGRG5, ADGRF1, and ADGRL3. We review the unique properties, including the position and conformation of their activating tethered agonist (TA) and signaling motifs within the 7TM bundle, that the novel structures have helped to identify. We also discuss questions that the kaleidoscope of novel aGPCR 7TM domain structures have left unanswered. These concern the relative positions, orientations, and interactions of the 7TM and GPCR autoproteolysis-inducing (GAIN) domains with one another. Clarifying their interplay remains an important goal of future structural studies on aGPCRs.

Mapping the binding sites of UDP and prostaglandin E2 glyceryl ester in the nucleotide receptor P2Y6

Cyclooxygenase‐2 catalyzes the biosynthesis of prostaglandins from arachidonic acid and the biosynthesis of prostaglandin glycerol esters (PG‐Gs) from 2‐arachidonoylglycerol. PG‐Gs are mediators of several biological actions such as macrophage activation, hyperalgesia, synaptic plasticity, and intraocular pressure. Recently, the human UDP receptor P2Y₆ was identified as a target for the prostaglandin E2 glycerol ester (PGE₂‐G). Here, we show that UDP and PGE₂‐G are evolutionary conserved endogenous agonists at vertebrate P2Y₆ orthologs. Using sequence comparison of P2Y₆ orthologs, homology modeling, and ligand docking studies, we proposed several receptor positions participating in agonist binding. Site‐directed mutagenesis and functional analysis of these P2Y₆ mutants revealed that both UDP and PGE₂‐G share in parts one ligand‐binding site. Thus, the convergent signaling of these two chemically very different agonists has already been manifested in the evolutionary design of the ligand‐binding pocket.

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.

Prostaglandin E2 glyceryl ester is an endogenous agonist of the nucleotide receptor P2Y6

Cyclooxygenase-2 catalyses the biosynthesis of prostaglandins from arachidonic acid but also the biosynthesis of prostaglandin glycerol esters (PG-Gs) from 2-arachidonoylglycerol. Previous studies identified PG-Gs as signalling molecules involved in inflammation. Thus, the glyceryl ester of prostaglandin E2, PGE2-G, mobilizes Ca2+ and activates protein kinase C and ERK, suggesting the involvement of a G protein-coupled receptor (GPCR). To identify the endogenous receptor for PGE2-G, we performed a subtractive screening approach where mRNA from PGE2-G response-positive and -negative cell lines was subjected to transcriptome-wide RNA sequencing analysis. We found several GPCRs that are only expressed in the PGE2-G responder cell lines. Using a set of functional readouts in heterologous and endogenous expression systems, we identified the UDP receptor P2Y6 as the specific target of PGE2-G. We show that PGE2-G and UDP are both agonists at P2Y6, but they activate the receptor with extremely different EC50 values of ~1 pM and ~50 nM, respectively. The identification of the PGE2-G/P2Y6 pair uncovers the signalling mode of PG-Gs as previously under-appreciated products of cyclooxygenase-2.

Tethered Agonism: A Common Activation Mechanism of Adhesion GPCRs

Adhesion GPCRs harbor a tethered agonist sequence (reproduced from [24]) As the past years have seen a magnificent increase in knowledge on adhesion GPCR (aGPCR) signal transduction, the time had come to fill the gap on how these receptors can be activated. Based on experimental observations that deletion of the ectodomain can induce signaling, the idea arose that aGPCRs, just like other atypical GPCRs, may harbor a tethered agonist sequence. In this chapter, we describe the recent findings and characteristics of this agonist, called the Stachel sequence, and discuss potential mechanisms that cause liberation of this encrypted sequence. Further, we provide perspectives for application of Stachel-derived synthetic peptides in future studies of aGPCR function.

The Activation Mechanism of Glycoprotein Hormone Receptors with Implications in the Cause and Therapy of Endocrine Diseases

Glycoprotein hormones (GPHs) are the main regulators of the pituitary-thyroid and pituitary-gonadal axes. Selective interaction between GPHs and their cognate G protein-coupled receptors ensure specificity in GPH signaling. The mechanisms of how these hormones activate glycoprotein hormone receptors (GPHRs) or how mutations and autoantibodies can alter receptor function were unclear. Based on the hypothesis that GPHRs contain an internal agonist, we systematically screened peptide libraries derived from the ectodomain for agonistic activity on the receptors. We show that a peptide (p10) derived from a conserved sequence in the C-terminal part of the extracellular N terminus can activate all GPHRs in vitro and in GPHR-expressing tissues. Inactivating mutations in this conserved region or in p10 can inhibit activation of the thyroid-stimulating hormone receptor by autoantibodies. Our data suggest an activation mechanism where, upon extracellular ligand binding, this intramolecular agonist isomerizes and induces structural changes in the 7-transmembrane helix domain, triggering G protein activation. This mechanism can explain the pathophysiology of activating autoantibodies and several mutations causing endocrine dysfunctions such as Graves disease and hypo- and hyperthyroidism. Our findings highlight an evolutionarily conserved activation mechanism of GPHRs and will further promote the development of specific ligands useful to treat Graves disease and other dysfunctions of GPHRs.

The constitutive activity of the adhesion GPCR GPR114/ADGRG5 is mediated by its tethered agonist

Adhesion GPCRs (aGPCRs) form the second largest, yet most enigmatic class of the GPCR superfamily. Although the physiologic importance of aGPCRs was demonstrated in several studies, the majority of these receptors is still orphan with respect to their agonists and signal transduction. Recent studies reported that aGPCRs are activated through a tethered peptide agonist, coined the Stachel sequence. The Stachel sequence is the most C-terminal part of the highly conserved GPCR autoproteolysis-inducing domain. Here, we used cell culture-based assays to investigate 2 natural splice variants within the Stachel sequence of the orphan Gs coupling aGPCR GPR114/ADGRG5. There is 1 variant constitutively active in cAMP assays (∼25-fold over empty vector) and sensitive to mechano-activation. The other variant has low basal activity in cAMP assays (6-fold over empty vector) and is insensitive to mechano-activation. In-depth mutagenesis studies of these functional differences revealed that the N-terminal half of the Stachel sequence confers the agonistic activity, whereas the C-terminal part orientates the agonistic core sequence to the transmembrane domain. Sequence comparison and functional testing suggest that the proposed mechanism of Stachel-mediated activation is relevant not only to GPR114 but to aGPCRs in general.

The LH/CG receptor activates canonical signaling pathway when expressed in Drosophila

G-protein coupled receptors (GPCRs) and their ligands provide precise tissue regulation and are therefore often restricted to specific animal phyla. For example, the gonadotropins and their receptors are crucial for vertebrate reproduction but absent from invertebrates. In mammals, LHR mainly couples to the PKA signaling pathway, and CREB is the major transcription factor of this pathway. Here we present the results of expressing elements of the human gonadotropin system in Drosophila. Specifically, we generated transgenic Drosophila expressing the human LH/CG receptor (denoted as LHR), a constitutively active form of LHR, and an hCG analog. We demonstrate activation-dependent signaling by LHR to direct Drosophila phenotypes including lethality and specific midline defects; these phenotypes were due to LHR activation of PKA/CREB pathway activity. That the LHR can act in an invertebrate demonstrates the conservation of factors required for GPCR function among phylogenetically distant organisms. This novel gonadotropin model may assist the identification of new modulators of mammalian fertility by exploiting the powerful genetic and pharmacological tools available in Drosophila.

A tethered agonist within the ectodomain activates the adhesion G protein-coupled receptors GPR126 and GPR133

Adhesion G protein-coupled receptors (aGPCRs) comprise the second largest yet least studied class of the GPCR superfamily. aGPCRs are involved in many developmental processes and immune and synaptic functions, but the mode of their signal transduction is unclear. Here, we show that a short peptide sequence (termed the Stachel sequence) within the ectodomain of two aGPCRs (GPR126 and GPR133) functions as a tethered agonist. Upon structural changes within the receptor ectodomain, this intramolecular agonist is exposed to the seven-transmembrane helix domain, which triggers G protein activation. Our studies show high specificity of a given Stachel sequence for its receptor. Finally, the function of Gpr126 is abrogated in zebrafish with a mutated Stachel sequence, and signaling is restored in hypomorphic gpr126 zebrafish mutants upon exogenous Stachel peptide application. These findings illuminate a mode of aGPCR activation and may prompt the development of specific ligands for this currently untargeted GPCR family.

Influence of the hinge region and its adjacent domains on binding and signaling patterns of the thyrotropin and follitropin receptor

Glycoprotein hormone receptors (GPHR) have a large extracellular domain (ECD) divided into the leucine rich repeat (LRR) domain for binding of the glycoprotein hormones and the hinge region (HinR), which connects the LRR domain with the transmembrane domain (TMD). Understanding of the activation mechanism of GPHRs is hindered by the unknown interaction of the ECD with the TMD and the structural changes upon ligand binding responsible for receptor activation. Recently, our group showed that the HinR of the thyrotropin receptor (TSHR) can be replaced by those of the follitropin (FSHR) and lutropin receptor (LHCGR) without effects on surface expression and hTSH signaling. However, differences in binding characteristics for bovine TSH at the various HinRs were obvious. To gain further insights into the interplay between LRR domain, HinR and TMD we generated chimeras between the TSHR and FSHR. Our results obtained by the determination of cell surface expression, ligand binding and G protein activation confirm the similar characteristics of GPHR HinRs but they also demonstrate an involvement of the HinR in ligand selectivity indicated by the observed promiscuity of some chimeras. While the TSHR HinR contributes to specific binding of TSH and its variants, no such contribution is observed for FSH and its analog TR4401 at the HinR of the FSHR. Furthermore, the charge distribution at the poorly characterized LRR domain/HinR transition affected ligand binding and signaling even though this area is not in direct contact with the ligand. In addition our results also demonstrate the importance of the TMD/HinR interface. Especially the combination of the TSHR HinR with the FSHR-TMD resulted in a loss of cell surface expression of the respective chimeras. In conclusion, the HinRs of GPHRs do not only share similar characteristics but also behave as ligand specific structural and functional entities.

Differences between lutropin-mediated and choriogonadotropin-mediated receptor activation

The human lutropin/choriogonadotropin receptor (hLHR) for the gonadotropic hormones human luteinizing hormone (hLH; lutropin) and human choriogonadotropin (hCG) is crucial for normal sexual development and fertility. We aimed to unravel differences between the two hLHR hormones in molecular activation mechanisms at hLHR. We utilized a specific hLHR variant that lacks exon 10 (hLHR-delExon10), which maintains full cAMP signaling by hCG, but decreases hLH-induced receptor signaling, resulting in a pathogenic phenotype. Exon 10 encodes 27 amino acids within the hinge region, which is an extracellular segment that is important for signaling and hormone interaction. Initially, we assumed that the lack of exon 10 might disturb intermolecular trans-activation of hLH, a mechanism that has been reported for hCG at hLHR. Coexpression of signaling-deficient hLHR and binding-deficient hLHR can be used to examine the mechanisms of receptor signaling, in particular intermolecular cooperation and intramolecular cis-activation. Therefore, hLHR-delExon10 was combined with the hLHR Lys605→Glu mutant, in which signaling is abolished, and the hLHR mutant Cys131→Arg, in which binding is deficient. We found that hCG signaling was partially rescued, indicating trans-activation. However, the hLH signal could not be restored via forced trans-activation with any construct. Fluorescence cross-correlation spectroscopy detected oligomerization in all combinations, indicating that these functional differences cannot be explained by monomerization of hLHR-delExon10. Thus, our data demonstrate not only that the different behavior of hLH at hLHR-delExon10 is unlikely to be related to modified intermolecular receptor activation, but also that hLH may exclusively stimulate the targeted hLHR by cis-activation, whereas hCG is also capable of inducing trans-activation.

Structural biology of glycoprotein hormones and their receptors: insights to signaling

This article reviews the progress made in the field of glycoprotein hormones (GPH) and their receptors (GPHR) by several groups of structural biologists including ourselves aiming to gain insight into GPH signaling mechanisms. The GPH family consists of four members, with follicle-stimulating hormone (FSH) being the prototypic member. GPH members belong to the cystine-knot growth factor superfamily, and their receptors (GPHR), possessing unusually large N-terminal ectodomains, belong to the G-protein coupled receptor Family A. GPHR ectodomains can be divided into two subdomains: a high-affinity hormone binding subdomain primarily centered on the N-terminus, and a second subdomain that is located on the C-terminal region of the ectodomain that is involved in signal specificity. The two subdomains unexpectedly form an integral structure comprised of leucine-rich repeats (LRRs). Following the structure determination of hCG in 1994, the field of FSH structural biology has progressively advanced. Initially, the FSH structure was determined in partially glycosylated free form in 2001, followed by a structure of FSH bound to a truncated FSHR ectodomain in 2005, and the structure of FSH bound to the entire ectodomain in 2012. Comparisons of the structures in three forms led a proposal of a two-step monomeric receptor activation mechanism. First, binding of FSH to the FSHR high-affinity hormone-binding subdomain induces a conformational change in the hormone to form a binding pocket that is specific for a sulfated-tyrosine found as sTyr 335 in FSHR. Subsequently, the sTyr is drawn into the newly formed binding pocket, producing a lever effect on a helical pivot whereby the docking sTyr provides as the 'pull & lift' force. The pivot helix is flanked by rigid LRRs and locked by two disulfide bonds on both sides: the hormone-binding subdomain on one side and the last short loop before the first transmembrane helix on the other side. The lift of the sTyr loop frees the tethered extracellular loops of the 7TM domain, thereby releasing a putative inhibitory influence of the ectodomain, ultimately leading to the activating conformation of the 7TM domain. Moreover, the data lead us to propose that FSHR exists as a trimer and to present an FSHR activation mechanism consistent with the observed trimeric crystal form. A trimeric receptor provides resolution of the enigmatic, but important, biological roles played by GPH residues that are removed from the primary FSH-binding site, as well as several important GPCR phenomena, including negative cooperativity and asymmetric activation. Further reflection pursuant to this review process revealed additional novel structural characteristics such as the identification of a 'seat' sequence in GPH. Together with the 'seatbelt', the 'seat' enables a common heteodimeric mode of association of the common α subunit non-covalently and non-specifically with each of the three different β subunits. Moreover, it was possible to establish a dimensional order that can be used to estimate LRR curvatures. A potential binding pocket for small molecular allosteric modulators in the FSHR 7TM domain has also been identified.

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.

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

The first version of a glycoprotein hormone receptor (GPHR) information resource was designed to link functional with structural GPHR information, in order to support sequence-structure-function analysis of the LH, FSH, and TSH receptors (http://ssfa-gphr.de). However, structural information on a binding- and signaling-sensitive extracellular fragment (∼100 residues), the hinge region, had been lacking. A new FSHR crystal structure of the hormone-bound extracellular domain has recently been solved. The structure comprises the leucine-rich repeat domain and most parts of the hinge region. We have not only integrated the new FSHR/FSH structure and the derived homology models of TSHR/TSH, LHCGR/CG, and LHCGR/LH into our web-based information resource, but have additionally provided novel tools to analyze the advanced structural features, with the common characteristics and distinctions between GPHRs, in a more precise manner. The hinge region with its second hormone-binding site allows us to assign functional data to the new structural features between hormone and receptor, such as binding details of a sulfated tyrosine (conserved throughout the GPHRs) extending into a pocket of the hormone. We have also implemented a protein interface analysis tool that enables the identification and visualization of extracellular contact points between interaction partners. This provides a starting point for comparing the binding patterns of GPHRs. Together with the mutagenesis data stored in the database, this will help to decipher the essential residues for ligand recognition and the molecular mechanisms of signal transduction, extending from the extracellular hormone-binding site toward the intracellular G protein-binding sites.

G protein-coupled receptor transactivation: from molecules to mice

G protein-coupled receptors (GPCRs) mediate a diverse range of physiological functions via activation of complex signaling systems. Organization of GPCRs in to dimers and oligomers provides a mechanism for both signal diversity and specificity in cellular responses, yet our understanding of the physiological significance of dimerization, particularly homodimerization, has not been forthcoming. This chapter will describe how we have investigated the physiological importance of GPCR homodimerization, using the luteinizing hormone/chorionic gonadotropin receptor as a model GPCR. Using transactivation as a mode of assessing receptor dimerization, we describe our cellular system and functional assays for assessment of transactivation in vitro and detail our strategy for generating a mouse model to assess GPCR transactivation in vivo.

Extended and structurally supported insights into extracellular hormone binding, signal transduction and organization of the thyrotropin receptor

The hormone thyrotropin (TSH) and its receptor (TSHR) are crucial for the growth and function of the thyroid gland. The TSHR is evolutionary linked with the receptors of follitropin (FSHR) and lutropin/choriogonadotropin (LHR) and their sequences and structures are similar. The extracellular region of TSHR contains more than 350 amino acids and binds hormone and antibodies. Several important questions related to functions and mechanisms of TSHR are still not comprehensively understood. One major reason for these open questions is the lack of any structural information about the extracellular segment of TSHR that connects the N-terminal leucine-rich repeat domain (LRRD) with the transmembrane helix (TMH) 1, the hinge region. It has been shown experimentally that this segment is important for fine tuning of signaling and ligand interactions. A new crystal structure containing most of the extracellular hFSHR region in complex with hFSH has recently been published. Now, we have applied these new structural insights to the homologous TSHR and have generated a structural model of the TSHR LRRD/hinge-region/TSH complex. This structural model is combined and evaluated with experimental data including hormone binding (bTSH, hTSH, thyrostimulin), super-agonistic effects, antibody interactions and signaling regulation. These studies and consideration of significant and non-significant amino acids have led to a new description of mechanisms at the TSHR, including ligand-induced displacements of specific hinge region fragments. This event triggers conformational changes at a convergent center of the LRRD and the hinge region, activating an “intramolecular agonistic unit” close to the transmembrane domain.

The hinge region of human thyroid-stimulating hormone (TSH) receptor operates as a tunable switch between hormone binding and receptor activation

The mechanism by which the hinge regions of glycoprotein hormone receptors couple hormone binding to activation of downstream effecters is not clearly understood. In the present study, agonistic (311.62) and antagonistic (311.87) monoclonal antibodies (MAbs) directed against the TSH receptor extracellular domain were used to elucidate role of the hinge region in receptor activation. MAb 311.62 which identifies the LRR/Cb-2 junction (aa 265–275), increased the affinity of TSHR for the hormone while concomitantly decreasing its efficacy, whereas MAb 311.87 recognizing LRR 7–9 (aa 201–259) acted as a non-competitive inhibitor of Thyroid stimulating hormone (TSH) binding. Binding of MAbs was sensitive to the conformational changes caused by the activating and inactivating mutations and exhibited differential effects on hormone binding and response of these mutants. By studying the effects of these MAbs on truncation and chimeric mutants of thyroid stimulating hormone receptor (TSHR), this study confirms the tethered inverse agonistic role played by the hinge region and maps the interactions between TSHR hinge region and exoloops responsible for maintenance of the receptor in its basal state. Mechanistic studies on the antibody-receptor interactions suggest that MAb 311.87 is an allosteric insurmountable antagonist and inhibits initiation of the hormone induced conformational changes in the hinge region, whereas MAb 311.62 acts as a partial agonist that recognizes a conformational epitope critical for coupling of hormone binding to receptor activation. The hinge region, probably in close proximity with the α-subunit in the hormone-receptor complex, acts as a tunable switch between hormone binding and receptor activation.

Cell adhesion receptor GPR133 couples to Gs protein

Adhesion G protein-coupled receptors (GPCR), with their very large and complex N termini, are thought to participate in cell-cell and cell-matrix interactions and appear to be highly relevant in several developmental processes. Their intracellular signaling is still poorly understood. Here we demonstrate that GPR133, a member of the adhesion GPCR subfamily, activates the G(s) protein/adenylyl cyclase pathway. The presence of the N terminus and the cleavage at the GPCR proteolysis site are not required for G protein signaling. G(s) protein coupling was verified by Gα(s) knockdown with siRNA, overexpression of Gα(s), co-expression of the chimeric Gq(s4) protein that routes GPR133 activity to the phospholipase C/inositol phosphate pathway, and missense mutation within the transmembrane domain that abolished receptor activity without changing cell surface expression. It is likely that not only GPR133 but also other adhesion GPCR signal via classical receptor/G protein-interaction.

Mutation analysis of the LH receptor gene in Leydig cell adenoma and hyperplasia and functional and biochemical studies of activating mutations of the LH receptor gene

CONTEXT

Germline and somatic activating mutations in the LH receptor (LHR) gene have been reported.

OBJECTIVE

Our objective was to perform mutation analysis of the LHR gene of patients with Leydig cell adenoma or hyperplasia. Functional studies were conducted to compare the D578H-LHR mutant with the wild-type (WT)-LHR and the D578G-LHR mutant, a classic cause of testotoxicosis. The three main signal transduction pathways in which LHR is involved were studied.

PATIENTS

We describe eight male patients with gonadotropin-independent precocious puberty due to Leydig cell adenoma or hyperplasia.

RESULTS

The D578H-LHR mutation was found in the adenoma or nodule with hyperplasia in all but two patients. D578H-LHR displayed a constitutively increased but noninducible production of cAMP, led to a very high production of inositol phosphates, and induced a slight phosphorylation of p44/42 MAPK in the absence of human chorionic gonadotropin. The D578G-LHR showed a response intermediate between WT-LHR and the D578H-LHR. Subcellular localization studies showed that the WT-LHR was almost exclusively located at the cell membrane, whereas the D578H-LHR showed signs of internalization. D578H-LHR was the only receptor to colocalize with early endosomes in the absence of human chorionic gonadotropin.

CONCLUSIONS

Although several LHR mutations have been reported in testotoxicosis, the D578H-LHR mutation, which has been found only as a somatic mutation, appears up until now to be specifically responsible for Leydig cell adenomas. This is reflected by the different activation of the signal transduction pathways, when compared with the WT-LHR or D578G-LHR, which may explain the tumorigenesis in the D578H mutant.

GPCR oligomers in pharmacology and signaling

G protein-coupled receptors (GPCRs) represent one of the largest families of cell surface receptors, and are the target of more than half of the current therapeutic drugs on the market. When activated by an agonist, the GPCR undergoes conformational changes that facilitate its interaction with heterotrimeric G proteins, which then relay signals to downstream intracellular effectors. Although GPCRs were thought to function as monomers, many studies support the hypothesis that G protein coupling involves the formation of GPCR homo- and/or hetero-complexes. These complex systems have been suggested to exhibit specific signaling cascades, pharmacological, internalization, and recycling properties. In this review, we summarize recent advances in our understanding of the structure, function and dynamics of GPCR complexes, as well as the findings obtained in animal models.

Defining structural and functional dimensions of the extracellular thyrotropin receptor region

The extracellular region of the thyrotropin receptor (TSHR) can be subdivided into the leucine-rich repeat domain (LRRD) and the hinge region. Both the LRRD and the hinge region interact with thyrotropin (TSH) or autoantibodies. Structural data for the TSHR LRRD were previously determined by crystallization (amino acids Glu(30)-Thr(257), 10 repeats), but the structure of the hinge region is still undefined. Of note, the amino acid sequence (Trp(258)-Tyr(279)) following the crystallized LRRD comprises a pattern typical for leucine-rich repeats with conserved hydrophobic side chains stabilizing the repeat fold. Moreover, functional data for amino acids between the LRRD and the transmembrane domain were fragmentary. We therefore investigated systematically these TSHR regions by mutagenesis to reveal insights into their functional contribution and potential structural features. We found that mutations of conserved hydrophobic residues between Thr(257) and Tyr(279) cause TSHR misfold, which supports a structural fold of this peptide, probably as an additional leucine-rich repeat. Furthermore, we identified several new mutations of hydrophilic amino acids in the entire hinge region leading to partial TSHR inactivation, indicating that these positions are important for intramolecular signal transduction. In summary, we provide new information regarding the structural features and functionalities of extracellular TSHR regions. Based on these insights and in context with previous results, we suggest an extracellular activation mechanism that supports an intramolecular agonistic unit as a central switch for activating effects at the extracellular region toward the serpentine domain.

The hinge region: an important receptor component for GPHR function

Glycoprotein hormone receptors (GPHRs) are members of the seven-transmembrane-spanning receptor family characterized by a large ectodomain. The hinge region belongs to a part of the GPHR ectodomain for which the three-dimensional structure has not yet been deciphered, leaving important questions unanswered concerning ligand binding and GPHR activation. Recent publications indicate that specific residues of the hinge region mediate hormone binding, receptor activation and/or intramolecular signaling for the three GPHRs, emphasizing the importance of this region. Based on these findings, the hinge region is involved at least in part in hormone binding and receptor activation. This review summarizes functional data regarding the hinge region, demonstrating that this receptor portion represents a link between ligand binding and subsequent GPHR activation.

Rescue of defective G protein-coupled receptor function in vivo by intermolecular cooperation

G protein-coupled receptors (GPCRs) are ubiquitous mediators of signaling of hormones, neurotransmitters, and sensing. The old dogma is that a one ligand/one receptor complex constitutes the functional unit of GPCR signaling. However, there is mounting evidence that some GPCRs form dimers or oligomers during their biosynthesis, activation, inactivation, and/or internalization. This evidence has been obtained exclusively from cell culture experiments, and proof for the physiological significance of GPCR di/oligomerization in vivo is still missing. Using the mouse luteinizing hormone receptor (LHR) as a model GPCR, we demonstrate that transgenic mice coexpressing binding-deficient and signaling-deficient forms of LHR can reestablish normal LH actions through intermolecular functional complementation of the mutant receptors in the absence of functional wild-type receptors. These results provide compelling in vivo evidence for the physiological relevance of intermolecular cooperation in GPCR signaling.

Thyrotropin and homologous glycoprotein hormone receptors: structural and functional aspects of extracellular signaling mechanisms

The TSH receptor (TSHR) together with the homologous lutropin/choriogonadotropin receptor and the follitropin receptor are glycoprotein hormone receptors (GPHRs). They constitute a subfamily of the rhodopsin-like G protein-coupled receptors with seven transmembrane helices. GPHRs and their corresponding hormones are pivotal proteins with respect to a variety of physiological functions. The identification and characterization of intra- and intermolecular signaling determinants as well as signaling mechanisms are prerequisites to gaining molecular insights into functions and (pathogenic) dysfunctions of GPHRs. Knowledge about activation mechanisms is fragmentary, and the specific aspects have still not been understood in their entirety. Therefore, here we critically review the data available for these receptors and bring together structural and functional findings with a focus on the important large extracellular portion of the TSHR. One main focus is the particular function of structural determinants in the initial steps of the activation such as: 1) hormone binding at the extracellular site; 2) hormone interaction at a second binding site in the hinge region; 3) signal regulation via sequence motifs in the hinge region; and 4) synergistic signal amplification by cooperative effects of the extracellular loops toward the transmembrane region. Comparison and consolidation of data from the homologous glycoprotein hormone receptors TSHR, follitropin receptor, and lutropin/choriogonadotropin receptor provide an overview of extracellular mechanisms of signal initiation, conduction, and regulation at the TSHR and homologous receptors. Finally, we address the issue of structural implications and suggest a refined scenario for the initial signaling process on GPHRs.

Constitutive activation of G protein-coupled receptors and diseases: insights into mechanisms of activation and therapeutics

The existence of constitutive activity for G protein-coupled receptors (GPCRs) was first described in 1980s. In 1991, the first naturally occurring constitutively active mutations in GPCRs that cause diseases were reported in rhodopsin. Since then, numerous constitutively active mutations that cause human diseases were reported in several additional receptors. More recently, loss of constitutive activity was postulated to also cause diseases. Animal models expressing some of these mutants confirmed the roles of these mutations in the pathogenesis of the diseases. Detailed functional studies of these naturally occurring mutations, combined with homology modeling using rhodopsin crystal structure as the template, lead to important insights into the mechanism of activation in the absence of crystal structure of GPCRs in active state. Search for inverse agonists on these receptors will be critical for correcting the diseases cause by activating mutations in GPCRs. Theoretically, these inverse agonists are better therapeutics than neutral antagonists in treating genetic diseases caused by constitutively activating mutations in GPCRs.

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.

Interactions between the extracellular domain and the extracellular loops as well as the 6th transmembrane domain are necessary for TSH receptor activation

OBJECTIVE

The molecular mechanisms of TSH receptor (TSHR) activation and intramolecular signal transduction are largely unknown. Deletion of the extracellular domain (ECD) of the TSHR results in increased constitutive activity, which suggests a self-inhibitory interaction between the ECD and the extracellular loops (ECLs) or the transmembrane domains (TMDs). To investigate these potential interactions and to pursue the idea that mutations in the ECD affect the constitutive activity of mutants in the ECLs or TMDs we generated double mutants between position 281 in the ECD and mutants in all three ECLs as well as the 6th TMD.

DESIGN

We combined mutation S281D, characterized by an impaired TSH-stimulated cAMP response, with the constitutively activating in vivo mutations I486F (1st ECL), I568T (2nd ECL), V656F (3rd ECL) and D633F (6th TMD). Further, we constructed double mutants containing the constitutively activating mutation S281N and one of the inactivating mutations D474E, T477I (1st ECL) and D633K (6th TMD).

RESULTS

The cAMP level of the double mutants with S281N and the inactive mutants in the 1st ECL was decreased below the level of the inactive single mutants, demonstrating that a constitutively activating mutation in the ECD cannot bypass disruption of signal transduction in the serpentine domain. In double mutants with S281D, basal and TSH-induced cAMP and inositol phosphate production of constitutively active mutants was reduced to the level of S281D.

CONCLUSION

The dominance of S281D and the dependence of constitutively activating mutations in the ECLs on the functionally intact ECD strongly suggest that interactions between these receptor domains are required for TSHR activation and intramolecular signal transduction.

Model of Glycoprotein Hormone Receptor Ligand Binding and Signaling

Studies described here were initiated to develop a model of glycoprotein hormone receptor structure and function. We found that the region that links the lutropin receptor leucine-rich repeat domain (LRD) to its transmembrane domain (TMD) has substantial roles in ligand binding and signaling, hence we term it the signaling specificity domain (SSD). Theoretical considerations indicated the short SSDs in marmoset lutropin and salmon follitropin receptors have KH domain folds. We assembled models of lutropin, follitropin, and thyrotropin receptors by aligning models of their LRD, TMD, and shortened SSD in a manner that explains how substitutions in follitropin and thyrotropin receptors distant from their apparent ligand binding sites enable them to recognize lutropins. In these models, the SSD is parallel to the concave surface of the LRD and makes extensive contacts with TMD outer loops 1 and 2. The LRD appears to contact TMD outer loop 3 and a few residues in helices 1, 5, 6, and 7. We propose that signaling results from contacts of the ligands with the SSD and LRD that alter the LRD, which then moves TMD helices 6 and 7. The positions of the LRD and SSD support the notion that the receptor can be activated by hormones that dock with these domains in either of two different orientations. This would account for the abilities of some ligands and ligand chimeras to bind multiple receptors and for some receptors to bind multiple ligands. This property of the receptor may have contributed significantly to ligand-receptor co-evolution.

Identification of a novel epitope in the thyroid-stimulating hormone receptor ectodomain acting as intramolecular signaling interface

Glycoprotein hormone receptors (GPHRs) differ from the other seven transmembrane receptors mainly through a complex activation mechanism that requires the binding of a large hormone toward a large N-terminal ectodomain. The intramolecular mechanism of the signal transduction to the serpentine domain upon hormone binding at the ectodomain is not understood. To identify determinants at the GPHR ectodomain that may be involved in signal transduction, we first searched for homologous structural features. Based on high sequence similarity to the determined structures of the Nogo-receptor ectodomain and the intermolecular complex of the Interleukin-8 ligand (IL8) and the N-terminal peptide of the IL8 receptor (IL8RA), the hypothesis was developed that portions of the intramolecular components, Cysteine-box-2 and Cysteine-box-3, of the GPHR ectodomain interact and localize at the interface between ectodomain and serpentine domain. Indeed, point mutations within the D403EFN406 motif at Cysteine-box-3 of the thyrotropin receptor resulted in increased basal cAMP levels, suggesting that this motif may be important for transduction of the signal from the ectodomain to the transmembrane domain. New indications are provided about the tight spatial cooperation and relative location of the new epitope and other determinants at the thyrotropin receptor ectodomain, such as the leucine-rich repeat motif Ser281 and the cysteine boxes. According to the high sequence conservation, the results are of general relevance for the signal transduction mechanism of other glycoprotein hormone receptors such as choriogonadotrophic/luteinizing hormone receptor and follicle-stimulating hormone receptor.

Thyroid-stimulating hormone receptor and its role in Graves' disease

The thyroid-stimulating hormone (TSH, or thyrotropin) receptor (TSHR) mediates the activating action of TSH to the thyroid gland, resulting in the growth and proliferation of thyrocytes and thyroid hormone production. In Graves' disease, thyroid-stimulating autoantibodies can mimic TSH action and stimulate thyroid cells. This leads to hyperthyroidism and abnormal overproduction of thyroid hormone. TSHR-antibodies-binding epitopes on the receptor molecule are well studied. Mechanism of TSHR-autoantibodies production is more or less clear but a susceptibility gene, which is linked to their production, is still unknown. Genetic studies show no linkage between the TSHR gene and Graves' disease. Among three common polymorphisms in the TSHR gene, only the D727E germline polymorphism in the cytoplasmic tail of the receptor showed an association with the disease, and this association is weak. The absence of a strong genetic effect of the TSHR polymorphisms in such a common and complex disorder as Graves' disease may be explained by a high degree of evolutionary conservation in TSHR. This can be shown by naturally existing germline and somatic mutations in the TSHR gene that cause various types of nonautoimmune and hereditary thyroid disease.

Proper targeting and activity of a nonfunctioning thyroid-stimulating hormone receptor (TSHr) combining an inactivating and activating TSHr mutation in one receptor

Activating mutations of the thyroid-stimulating hormone receptor (TSHr) have been identified as a cause of toxic adenomas. Germline-inactivating TSHr mutations have been described as a cause of congenital hypothyroidism. The effects of combining activating and inactivating mutations within a single receptor was studied. The double mutant T477I/P639S contained an activating TSHr mutation (P639S) together with an inactivating one (T477I). The other one (I486M/P639S) contained two activating mutations. Constructs were expressed in COS-7 cells and basal and TSH-stimulated cyclic AMP (cAMP) accumulation and inositol phosphate (IP) production were determined. The expression at the cell surface was studied both with binding and fluorescence-activated cell scanning analysis. Our results show that the effect of combining the two activating mutations is an increase in the constitutive activity only for the cAMP pathway and not for the IP pathway suggesting that different mutations result in receptor conformations with different relative abilities to couple to Gs-alpha or Gq-alpha. Surprisingly the double mutant containing the T477I behaves as an activating receptor with constitutive activity both for the cAMP and IP pathways. These data show that an inactive form of the TSHr which is trapped inside a cell after transfection is able to gain the membrane surface when combined with an activated form of the receptor.

Role of cleavage and shedding in human thyrotropin receptor function and trafficking

The thyrotropin receptor (TSHR) undergoes a cleavage at the cell membrane, leading to a heterodimer, comprising an alpha extracellular and a beta-transmembrane and intracellular subunits, held together by disulfide bonds. Moreover, part of the alpha-subunit of the receptor is shed from thyroid and transfected L cells. To understand the role of cleavage and shedding, we constructed deletion mutants starting, respectively, at the most N-terminal (S314), and C-terminal (L378) cleavage sites previously mapped, corresponding to free beta1 or beta2-subunits without further modification of receptor structure. Functional studies performed in COS-7 cells showed that both mutants display an increased basal activation of the cAMP pathway when compared with the wild-type receptor. By contrast, deletion of almost the entire extracellular domain of the receptor (TM409 mutant) totally impairs receptor function, thus confirming a role of the juxtamembrane extracellular region in receptor function. The beta1 mutant receptor exhibited an increased internalization when compared with the hormone-activated holoreceptor. Furthermore, no recycling was observed in the case of the beta1 mutant receptor. These observations strongly argue for a different conformation between the receptor activated by cleavage and shedding on the one hand, and the receptor activated by the ligand on the other hand. Cleavage and shedding of a receptor already activated by a transmembrane activating mutation M453T further increase its activity, showing that the extracellular domain still exerts a negative effect in the M453T holoreceptor. An increased internalization of the M453T receptor was observed when compared with the wild-type receptor, which was increased further in the corresponding truncated beta1-M453T receptor. Thus cleavage and shedding yield TSHR activation but also increase internalization of the free beta-subunits of the receptor, the latter mechanism limiting simultaneously excessive receptor signaling. The combined effects may be responsible for the limited basal constitutive activation of the cAMP pathway that is detected for the TSHR.

Glycoprotein hormone receptors: determinants in leucine-rich repeats responsible for ligand specificity

Glycoprotein hormone receptors [thyrotropin (TSHr), luteinizing hormone/chorionic gonadotropin (LH/CGr), follicle stimulating hormone (FSHr)] are rhodopsin-like G protein-coupled receptors with a large extracellular N-terminal portion responsible for hormone recognition and binding. In structural models, this ectodomain is composed of two cysteine clusters flanking nine leucine-rich repeats (LRRs). The LRRs form a succession of beta-strands and alpha-helices organized into a horseshoe-shaped structure. It has been proposed that glycoprotein hormones interact with residues of the beta-strands making the concave surface of the horseshoe. Gain-of-function homology scanning of the beta-strands of glycoprotein hormone receptors allowed identification of the critical residues responsible for the specificity towards human chorionic gonadotropin (hCG). Substitution of eight or two residues of the LH/CGr into the TSHr or FSHr, respectively, resulted in constructs displaying almost the same affinity and sensitivity for hCG as wild-type LH/CGr. Molecular dynamics simulations and additional site-directed mutagenesis provided a structural rationale for the evolution of binding specificity in this duplicated gene family.