Significance of ectodomain cysteine boxes 2 and 3 for the activation mechanism of the thyroid-stimulating hormone receptor

Structure of an LGR dimer - an evolutionary predecessor of glycoprotein hormone receptors

The glycoprotein hormones of humans, produced in the pituitary and acting through receptors in the gonads to support reproduction and in the thyroid gland for metabolism, have co-evolved from invertebrate counterparts1,2. These hormones are heterodimeric cystine-knot proteins; and their receptors bind the cognate hormone at an extracellular domain and transmit the signal of this binding through a transmembrane domain that interacts with a heterotrimeric G protein. Structures determined for the human receptors as isolated for cryogenic electron microscopy (cryo-EM) are all monomeric3-6 despite compelling evidence for their functioning as dimers7-10. Here we describe the cryo-EM structure of the homologous receptor from a neuroendocrine pathway that promotes growth in a nematode11. This structure is an asymmetric dimer that can be activated by the hormone from that worm12, and it shares features especially like those of the thyroid stimulating hormone receptor (TSHR). When studied in the context of the human homologs, this dimer provides a structural explanation for the transactivation evident from functional complementation of binding-deficient and signaling-deficient receptors7, for the negative cooperativity in hormone action that is manifest in the 1:2 asymmetry of primary TSH:TSHR complexes8,9, and for switches in G-protein usage that occur as 2:2 complexes form9,10.

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.

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.

Glycosylation pattern analysis of glycoprotein hormones and their receptors

We have studied glycosylation patterns in glycoprotein hormones (GPHs) and glycoprotein hormone receptor (GPHR) extracellular domains (ECD) from different species to identify areas not glycosylated that could be involved in intermolecular or intramolecular interactions. Comparative models of the structure of the TSHR ECD in complex with TSH and in complex with TSHR autoantibodies (M22, stimulating and K1-70, blocking) were obtained based on the crystal structures of the FSH-FSHR ECD, M22-TSHR leucine-rich repeat domain (LRD) and K1-70-TSHR LRD complexes. The glycosylation sites of the GPHRs and GPHs from all species studied were mapped on the model of the human TSH TSHR ECD complex. The areas on the surfaces of GPHs that are known to interact with their receptors are not glycosylated and two areas free from glycosylation, not involved in currently known interactions, have been identified. The concave faces of GPHRs leucine-rich repeats 3-7 are free from glycosylation, consistent with known interactions with the hormones. In addition, four other non-glycosylated areas have been identified, two located on the receptors' convex surfaces, one in the long loop of the hinge regions and one at the C-terminus of the extracellular domains. Experimental evidence suggests that the non-glycosylated areas identified on the hormones and receptors are likely to be involved in forming intramolecular or intermolecular interactions.

Loss-of-Function Mutations in the Human Luteinizing Hormone Receptor Predominantly Cause Intracellular Retention

Mutations in G protein-coupled receptors (GPCRs) have been identified for many endocrine hormone signaling deficiencies. Inactivating mutations can impair ligand binding, receptor activation/coupling to signaling pathways, or can cause receptor misfolding and consequent impaired expression at the cell membrane. Here we examine the cell surface expression, ligand binding, and signaling of a range of mutant human luteinizing hormone receptors (LHRs) identified as causing reproductive dysfunction in human patients. The data obtained reveal how mutations in GPCRs can have diverse and severely deleterious effects on receptor function. Furthermore, it was found that impaired functionality of the majority of the mutant LHRs was due to reduced expression at the cell surface (14/20) while only two mutations caused impaired binding affinity and two impaired in signaling. An additional two mutations were found to cause no impairment of receptor function. These data demonstrate that the majority of LHR mutations lead to intracellular retention and highlight the potential for novel pharmacological chaperone therapeutics that can "rescue" expression/function of retained mutant GPCRs.

TSH Receptor Cleavage Into Subunits and Shedding of the A-Subunit; A Molecular and Clinical Perspective

The TSH receptor (TSHR) on the surface of thyrocytes is unique among the glycoprotein hormone receptors in comprising two subunits: an extracellular A-subunit, and a largely transmembrane and cytosolic B-subunit. Unlike its ligand TSH, whose subunits are encoded by two genes, the TSHR is expressed as a single polypeptide that subsequently undergoes intramolecular cleavage into disulfide-linked subunits. Cleavage is associated with removal of a C-peptide region, a mechanism similar in some respects to insulin cleavage into disulfide linked A- and B-subunits with loss of a C-peptide region. The potential pathophysiological importance of TSHR cleavage into A- and B-subunits is that some A-subunits are shed from the cell surface. Considerable experimental evidence supports the concept that A-subunit shedding in genetically susceptible individuals is a factor contributing to the induction and/or affinity maturation of pathogenic thyroid-stimulating autoantibodies, the direct cause of Graves' disease. The noncleaving gonadotropin receptors are not associated with autoantibodies that induce a "Graves' disease of the gonads." We also review herein current information on the location of the cleavage sites, the enzyme(s) responsible for cleavage, the mechanism by which A-subunits are shed, and the effects of cleavage on receptor signaling.

Withdrawn: TSH Receptor Cleavage Into Subunits and Shedding of the A-Subunit; A Molecular and Clinical Perspective

The TSH receptor (TSHR) on the surface of thyrocytes is unique among the glycoprotein hormone receptors in comprising two subunits: an extracellular A-subunit, and a largely transmembrane and cytosolic B-subunit. Unlike its ligand TSH, whose subunits are encoded by two genes, the TSHR is expressed as a single polypeptide that subsequently undergoes intramolecular cleavage into disulfide-linked subunits. Cleavage is associated with removal of a C-peptide region, a mechanism similar in some respects to insulin cleavage into disulfide linked A- and B-subunits with lossofaC-peptideregion. The potential pathophysiological importance of TSHR cleavage into A-and B-subunits is that some A-subunits are shed from the cell surface. Considerable experimental evidence supports the concept that A-subunit shedding in genetically susceptible individuals is a factor contributing to the induction and/or affinity maturation of pathogenic thyroid-stimulating autoantibodies, the direct cause of Graves' disease. The noncleaving gonadotropin receptors are not associated with autoantibodies that induce a "Graves' disease of the gonads." We also review herein current information on the location of the cleavage sites, the enzyme(s) responsible for cleavage, the mechanism by which A-subunits are shed, and the effects of cleavage on receptor signaling. (Endocrine Reviews 37: 114-134, 2016).

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.

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 thyrotropin receptor hinge region as a surrogate ligand: identification of loci contributing to the coupling of thyrotropin binding and receptor activation

The TSH receptor (TSHR) hinge region, the least well understood component, bridges the leucine-rich repeat and transmembrane domains. We report data on clusters of hinge charged residues the mutation of which to Ala is compatible with cell surface expression and normal, or near normal, TSH binding affinity yet with a relative reduction in receptor activation. Mutation to Ala of E409 at the junction with the transmembrane domain was the most potent in uncoupling TSH binding and signal transduction (~22-fold less sensitive than the wild-type TSHR) and was unique among the residues studied in reducing both the amplitude and the sensitivity of the ligand-induced signal. Unexpectedly, a dual E409A/D410A mutation partially corrected the major suppressive effect of TSHR-E409A. The combined Ala substitution of a cluster of positively charged hinge residues (K287, K290, K291, R293; termed "K3R1") synergistically reduced sensitivity to TSH stimulation approximately 21-fold without altering the TSH binding affinity. Simultaneous Ala substitutions of a cluster of acidic hinge residues D392, E394, and D395 (termed "DE392-5A") partially uncoupled TSH binding from signal transduction (4.4-fold reduction in sensitivity), less than for E409A and K3R1A. Remarkably, the combination of the K3R1A and DE392-5A mutations was not additive but ameliorated the major uncoupling effect of K3R1A. This lack of additivity suggests that these two clusters contribute to a common signaling pathway. In summary, we identify several TSHR hinge residues involved in signal transmission. Our data support the concept that the hinge regions of the TSHR (and other glycoprotein hormone receptors) act as surrogate ligands for receptor activation.

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.

Deletion of thyrotropin receptor residue Asp403 in a hyperfunctioning thyroid nodule provides insight into the role of the ectodomain in ligand-induced receptor activation

Somatic mutations of the TSH receptor (TSHR) gene are the main cause of autonomously functioning thyroid nodules. Except for mutations in ectodomain residue S281, all of the numerous reported activating mutations are in the TSHR membrane-spanning region. Here, we describe a patient with a toxic adenoma with a novel heterozygous somatic mutation caused by deletion of ectodomain residue Asp403 (Del-D403). Subsequent in vitro functional studies of the Del-D403 TSHR mutation demonstrated greatly increased ligand-independent constitutive activity, 8-fold above that of the wild-type TSHR. TSH stimulation had little further effect, indicating that the mutation produced near maximal activation of the receptor. In summary, we report only the second TSHR ectodomain activating mutation (and the first ectodomain deletion mutation) responsible for development of a thyroid toxic adenoma. Because Del-D403 causes near maximal activation, our finding provides novel insight into TSHR structure and function; residue D403 is more likely to be involved in the ligand-mediated activating pathway than in the ectodomain inverse agonist property.

Identification of novel TSH interaction sites by systematic binding analysis of the TSHR hinge region

In which ways the binding of the thyroid stimulating hormone to the extracellular domain of its receptor leads to activation of the thyroid-stimulating hormone receptor (TSHR) is currently only incompletely understood. It is known that TSH binding to the TSHR depends on the interaction with the leucine-rich repeat and sulfation at Y385 of the hinge region. Recently it was also shown that electrostatic interactions between positive charges of bovine (b) TSH and the residues E297, E303, and D382 of the hinge region contribute to hormone-TSHR binding. After the identification of these first TSH binding sites in the hinge region, it was apparent that multiple positions in this region remained to be characterized for their roles in hormone binding. The goal of this study was therefore to clarify whether further contact points of TSH exist in the structurally undefined hinge region. Therefore, we systematically analyzed 41 uncharacterized residues of the TSHR hinge region as single mutants regarding differences between cell surface expression and bTSH binding. Indeed, we identified further amino acids of the hinge region with influence on bTSH binding. Some of these contribute to a new binding domain from human TSHR position F381 to D386. These hinge mutants with influence on bTSH binding were also analyzed for binding of the superagonistic human TSH analog TR1401 demonstrating that these positions also have an impact on TR1401 binding. Moreover, side chain variations revealed that different amino acid properties like the negative charge, aromatic as well as hydrophilic characteristics, contribute to maintain the hormone-TSHR hinge interaction.

Relationship between thyrotropin receptor hinge region proteolytic posttranslational modification and receptor physiological function

The glycoprotein hormone receptor hinge region is the least conserved component and the most variable in size; the TSH receptor (TSHR) being the longest (152 amino acids; residues 261-412). The TSHR is also unique among the glycoprotein hormone receptor in undergoing in vivo intramolecular cleavage into disulfide-linked A- and B-subunits with removal of an intervening 'C-peptide' region. Experimentally, hinge region amino acids 317-366 (50 residues) can be deleted without alteration in receptor function. However, in vivo, more than 50 amino acids are deleted during TSHR intramolecular cleavage; furthermore, the boundaries of this deleted region are ragged and poorly defined. Studies to determine the extent to which hinge region deletions can be tolerated without affecting receptor function ('minimal hinge') are lacking. Using as a template the functionally normal TSHR with residues 317-366 deleted, progressive downstream extension of deletions revealed residue 371 to be the limit compatible with normal TSH binding and coupling with cAMP signal transduction. Based on the foregoing downstream limit, upstream deletion from residue 307 (307-371 deletion) was also tolerated without functional alteration, as was deletion of residues 303-366. Addressing a related issue regarding the functional role of the TSHR hinge region, we observed that downstream hinge residues 377-384 contribute to coupling ligand binding with cAMP signal transduction. In summary, we report the first evaluation of TSHR function in relation to proteolytic posttranslational hinge region modifications. Deletion of TSHR hinge amino acids 303-366 (64 residues) or 307-371 (65 residues) are the maximum hinge region deletions compatible with normal TSHR function.

Thyrotropin (TSH) receptor residue E251 in the extracellular leucine-rich repeat domain is critical for linking TSH binding to receptor activation

The TSH receptor (TSHR) ectodomain comprises a tubular leucine-rich repeat domain (LRD) and a hinge [or signaling specificity domain (SSD)]. TSH binds to both the LRD and SSD, leading to signal transduction by the transmembrane domain. The SSD structure and spatial orientation to the other components are unknown. We exploited a fortuitous observation to obtain mechanistic insight into the relationship between TSH binding and signal transduction. A mouse TSHR cDNA generated by PCR was found to express a receptor with poor TSH-induced cAMP generation despite normal TSH binding. Progressive reversion to wild-type of six mutations revealed E251K in the LRD to be critical for reduced signal transduction in both mouse and human TSHR. An I286F substitution in the SSD had a much weaker effect and was additive with E251K. To our knowledge, there are no previous examples of specific amino acid mutations in the TSHR LRD that dissociate TSH binding from TSHR signal transduction. To prevent flailing of the TSHR LRD, its position vis-à-vis the SSD must be stabilized by multiple amino acid interactions. The present data suggest that TSHR residue E251 is one of these residues involved in stabilizing the LRD relative to the SSD, thereby enabling ligand binding to transduce a signal by the latter. That the E251K mutation can reduce signal transduction despite high-affinity TSH binding comparable with the wild-type TSHR provides mechanistic insight into the coupling between ligand binding and receptor activation.

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.

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.

The superagonistic activity of bovine thyroid-stimulating hormone (TSH) and the human TR1401 TSH analog is determined by specific amino acids in the hinge region of the human TSH receptor

Bovine TSH (bTSH) has a higher affinity to the human TSHR (hTSHR) and a higher signaling activity than human TSH (hTSH). The molecular reasons for these phenomena are unknown. Distinct negatively charged residues (Glu297, Glu303, and Asp382) in the hinge region of the hTSHR are known to be important for bTSH binding and signaling. To investigate the potential relevance of these positions for differences between bTSH and hTSH in the interaction to the hTSHR, we determined bTSH- and hTSH-mediated cAMP production of several substitutions at these three hinge residues. To examine specific variations of hTSH, we also investigated the superagonistic hTSH analog TR1401 (TR1401), whose sequence differs from hTSH by four additional positively charged amino acids that are also present in bTSH. To characterize possible interactions between the acidic hTSHR positions Glu297, Glu303, or Asp382 and the additional basic residues of TR1401, we investigated TR1401 binding and signaling properties. Our data reveal increased cAMP signaling of the hTSHR using TR1401 and bTSH compared with hTSH. Whereas Asp382 seems to be important for bTSH- and TR1401-mediated but not for hTSH-mediated signaling, the substitution E297K exhibits a decreased signaling for all three TSH variants. Interestingly, bTSH and TR1401 showed only a slightly different binding pattern. These observations imply that specific residues of the hinge region are mediators of the superagonistic activity of bTSH and TR1401 in contrast to hTSH. Moreover, the simultaneous localization of binding components in the glycoprotein hormone molecule and the receptor hinge region permits important reevaluation of interacting hormone receptor domains.

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.

Prevalence of TSH receptor and Gsalpha mutations in 45 autonomously functioning thyroid nodules in Japan

Somatic mutations of the thyrotropin receptor (TSHR) gene and the gene encoding the alpha subunit of the stimulatory GTP-binding protein (Gsalpha) are the main cause for autonomously functioning thyroid nodules (AFTN) in iodine-deficient regions of the world. In iodine-sufficient regions, including Japan, the genetic relevance of AFTN is unclear. In a series of 45 Japanese subjects with AFTN, exons 9 and 10 of the TSHR and exons 7-10 of Gsalpha , where the activating mutations have been found, were analyzed using direct sequencing. We found 29 somatic mutations: 22 in the TSHR gene and 7 in the Gsalpha gene. The most frequent mutation in TSHR was Met453Thr (10 cases), followed by clustered residues from codons 630 through 633 on TSHR (7 cases). Mutations of Gsalpha were detected at codon 201 in 5 cases and at codon 227 in 2 cases. No patients had coexistent TSHR and Gsalpha mutations in the same nodule. All mutated residues but one, which was deleted at codon 403 on the TSHR gene, are constitutively active. The prevalences of a germline polymorphism of Asp727Glu on the TSHR gene and incidental papillary thyroid carcinoma in thyroid surgical specimens were similar to those reported in other studies. In the present study, more than half of the cases with AFTN had a somatic activating mutation either of the TSHR or Gsalpha gene, despite their high iodine intake.

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.

Asp330 and Tyr331 in the C-terminal cysteine-rich region of the luteinizing hormone receptor are key residues in hormone-induced receptor activation

The luteinizing hormone (LH) receptor plays an essential role in male and female gonadal function. Together with the follicle-stimulating hormone (FSH) and thyroid stimulating hormone (TSH) receptors, the LH receptor forms the family of glycoprotein hormone receptors. All glycoprotein hormone receptors share a common modular topography, with an N-terminal extracellular ligand binding domain and a C-terminal seven-transmembrane transduction domain. The ligand binding domain consists of 9 leucine-rich repeats, flanked by N- and C-terminal cysteine-rich regions. Recently, crystal structures have been published of the extracellular domains of the FSH and TSH receptors. However, the C-terminal cysteine-rich region (CCR), also referred to as the "hinge region," was not included in these structures. Both structure and function of the CCR therefore remain unknown. In this study we set out to characterize important domains within the CCR of the LH receptor. First, we mutated all cysteines and combinations of cysteines in the CCR to identify the most probable disulfide bridges. Second, we exchanged large parts of the LH receptor CCR by its FSH receptor counterparts, and characterized the mutant receptors in transiently transfected HEK 293 cells. We zoomed in on important regions by focused exchange and deletion mutagenesis followed by alanine scanning. Mutations in the CCR specifically decreased the potencies of LH and hCG, because the potency of the low molecular weight agonist Org 41841 was unaffected. Using this unbiased approach, we identified Asp(330) and Tyr(331) as key amino acids in LH/hCG mediated signaling.

Identification of key amino acid residues in a thyrotropin receptor monoclonal antibody epitope provides insight into its inverse agonist and antagonist properties

CS-17 is a murine monoclonal antibody to the human TSH receptor (TSHR) with both inverse agonist and antagonist properties. Thus, in the absence of ligand, CS-17 reduces constitutive TSHR cAMP generation and also competes for TSH binding to the receptor. The present data indicate that for both of these functions, the monovalent CS-17 Fab (50 kDa) behaves identically to the intact, divalent IgG molecule (150 kDa). The surprising observation that CS-17 competes for TSH binding to the human but not porcine TSHR enabled identification of a number of amino acids in its epitope. Replacement of only three human TSHR residues (Y195, Q235, and S243) with the homologous porcine TSHR residues totally abolishes CS-17 binding as detected by flow cytometry. TSH binding is unaffected. Of these residues, Y195 is most important, with Q235 and S243 contributing to CS-17 binding to a much lesser degree. The functional effects of CS-17 IgG and Fab on constitutive cAMP generation by porcinized human TSHR confirm the CS-17 binding data. The location of TSHR amino acid residues Y195, Q235, and S243 deduced from the crystal structure of the FSH receptor leucine-rich domain provides valuable insight into the CS-17 and TSH binding sites. Whereas hormone ligands bind primarily to the concave surface of the leucine-rich domains, a major portion of the CS-17 epitope lies on the opposite convex surface with a minor component in close proximity to known TSH binding residues.

A new silent germline mutation of the TSH receptor: coexpression in a hyperthyroid family member with a second activating somatic mutation

BACKGROUND

Up to date, three thyroid-stimulating hormone receptor (TSHR) germline variants have been reported for which no functional consequences have been detected by in vitro characterizations. However, familial nonautoimmune hyperthyroidism and hot nodules are clearly associated with constitutively activating TSHR germline mutations. We describe a family with a new TSHR germline mutation that is associated with euthyroidism in 13 family members and hyperthyroidism in 1 family member.

METHODS

Mutation analysis of the TSHR gene was performed by denaturing gradient gel electrophoresis. TSHR constructs were characterized by determination of cell surface expression, 3'-5'-cyclic adenosine monophosphate (cAMP) accumulation, and constitutive cAMP activity.

RESULTS

A novel TSHR germline mutation (N372T) was found in a man who presented with thyrotoxicosis. The mutation was also detected in 13 family members, all of whom were euthyroid. Interestingly, an additional constitutively active somatic mutation (S281N) was identified on the second parental TSHR allele of the hyperthyroid index patient. Linear regression analysis showed a lack of constitutive activity for N372T. Moreover, coexpression studies of N372T with S281N did not reveal any evidence for a functional influence of N372T on the constitutively active mutation (CAM).

CONCLUSIONS

N372T is unlikely to cause altered thyroid function. This is consistent with the finding that only the index patient with the additional somatic mutation S281N was hyperthyroid.

The thyrotropin receptor hinge region is not simply a scaffold for the leucine-rich domain but contributes to ligand binding and signal transduction

The glycoprotein hormone receptor hinge region connects the leucine-rich and transmembrane domains. The prevalent concept is that the hinge does not play a significant role in ligand binding and signal transduction. Portions of the hinge are redundant and can be deleted by mutagenesis or are absent in certain species. A minimal hinge will be more amenable to future investigation of its structure and function. We, therefore, combined and progressively extended previous deletions (Delta) in the TSH receptor (TSHR) hinge region (residues 277-418). TSHRDelta287-366, Delta287-371, Delta287-376, and Delta287-384 progressively lost their response to TSH stimulation of cAMP generation in intact cells, consistent with a progressive loss of TSH binding. The longest deletion (TSHRDelta287-384), reducing the hinge region from 141 to 43 amino acids, totally lost both functions. Surprisingly, however, with deletions extending from residues 371-384, constitutive (ligand-independent) activity increased severalfold, reversing the suppressive (inverse agonist) effect of the TSHR extracellular domain. TSHR-activating point mutations I486F and I568T in the first and second extracellular loops (especially the former) had reduced activity on a background of TSHRDelta287-371. In summary, our data support the concept that the TSHR hinge contributes significantly to ligand binding affinity and signal transduction. Residues within the hinge, particularly between positions 371-384, appear involved in ectodomain inverse agonist activity. In addition, the hinge is necessary for functionality of activating mutations in the first and second extracellular loops. Rather than being an inert linker between the leucine-rich and transmembrane domains, the TSHR hinge is a signaling-specificity domain.

Molecular characterization and seasonal changes in gonadal expression of a thyrotropin receptor in the European sea bass

The thyroid stimulating hormone (TSH) is a glycoprotein synthesized and secreted from thyrotrophs of the anterior pituitary gland. It acts by binding to and activating its specific receptor, the TSHR, to induce the synthesis and secretion of thyroid hormones. Recent studies conducted in diverse fish species suggest a direct role of TSH on gonadal physiology. In this work, we describe the cloning of a cDNA encoding a TSHR which was isolated from the gonads of the European sea bass (Dicentrarchus labrax). The mature protein displays typical features of the members of the glycoprotein hormone receptor family and shows the highest amino acid sequence identity with the TSHRs of other fish species. An insertion of approximately 50 amino acids, specific for the TSHR subfamily is also present in the carboxyl end of the extracellular domain of the sbsTSHR. By RT-PCR analysis, we demonstrate the extrathyroidal expression of sbsTSHR in numerous tissues of the sea bass. Also, two transcripts that differ in the length of their 3' untranslated regions were found. They reflect the use of alternative polyadenylation cleavage sites. Seasonal changes in sbsTSHR mRNA levels in female and male sea bass during the first ovarian and testicular recrudescence suggest that in females the TSHR could participate in active vitellogenesis and in the regulation of gamete maturation and ovulation, whereas in males, the TSHR would be involved in the regulation of processes that occur during the early stages of the gonadal development and also of gamete maturation and spermiation. The results of this work indicate that a sbsTSHR has been cloned from the testis of the European sea bass and they provide the basis for future studies concerning the function of TSHR in this species.