Analysis of the thyrotropin receptor-thyrotropin interaction by comparative modeling

The full-length TSH receptor is stabilized by TSH ligand

The receptor for thyroid stimulating hormone (TSHR), a GPCR, is the primary antigen in autoimmune hyperthyroidism (Graves' disease) caused by stimulating TSHR antibodies. While we have previously published a full length model of the TSHR, including its leucine rich domain (LRD), linker region (LR) and transmembrane domain (TMD), to date, only a partial LRD (aa 21-261) stabilized with TSHR autoantibodies has been crystallized. Recently, however, cryo-EM structures of the full-length TSHR have been published but they include only an incomplete LR. We have now utilized the cryo-EM models, added disulfide bonds to the LR and performed longer (3000 ns) molecular dynamic (MD) simulations to update our previous model of the entire full-length TSHR, with and without the presence of TSH ligand. As in our earlier work, the new model was embedded in a lipid membrane and was solvated with water and counterions. We found that the 3000 ns Molecular Dynamic simulations showed that the structure of the LRD and TMD were remarkably constant while the LR, known more commonly as the "hinge region", again showed significant flexibility, forming several transient secondary structural elements. Analysis of the new simulations permitted a detailed examination of the effect of TSH binding on the structure of the TSHR. We found a structure-stabilizing effect of TSH, including increased stability of the LR, which was clearly demonstrated by analyzing several intrinsic receptor properties including hydrogen bonding, fluctuation of the LRD orientation, and radius of gyration. In conclusion, we were able to quantify the flexibility of the TSHR and show its increased stability after TSH binding. These data indicated the important role of ligands in directing the signaling structure of a receptor.

A Novel TSH Receptor Gene Variant Associated with Non-Autoimmune Hyperthyrotropinemia: A Case Report

BACKGROUND

Resistance to TSH is defined as reduced sensitivity to normal, biologicallyactive TSH, and abnormally high levels of TSH are needed to achieve normal levels of thyroid hormones.

CASE PRESENTATION

A 15-year-old female patient, having been treated since childhood with levothyroxine for hyperthyrotropinemia was referred to our institution complaining of tachycardia after the levothyroxine therapy had been increased. Thyroid ultrasound features were normal, and thyroid antibodies were negative. The therapy was gradually tapered in light of the symptoms, although subclinical hypothyroidism was evident at thyroid function tests. First-degree relatives were tested for thyroid function, and the father was also found to have a previously-unknown subclinical hypothyroidism. The patient underwent genetic testing for TSH receptor (TSHR) gene mutations, which revealed a gene variant hitherto not described: p.C598R (c.1792T>C). The father was also tested and was found to carry the same mutation, while other first-degree relatives were wild-type for the TSHR gene. An in-silico analysis was performed, which revealed a loss-of-function phenotype corresponding to the described variant, suggesting a novel loss-of-function TSH receptor gene mutation.

CONCLUSION

In this case report, we present a novel loss-of-function gene mutation in the TSH receptor gene associated with a TSH resistance phenotype.

Application of machine learning to predict the inhibitory activity of organic chemicals on thyroid stimulating hormone receptor

With the promotion of carbon neutrality, it is also important to synchronously promote the assessment and sustainable management of chemicals so as to protect public health. Humans and animals are possibly exposed to endocrine disruptors that have inhibitory effects on thyroid stimulating hormone receptor (TSHR). As such, it is important to identify chemicals that inhibit TSHR and to develop models to predict their inhibitory activity. In this study, 5952 compounds derived from a cyclic adenosine monophosphate (cAMP) analysis, a key signaling pathway in thyrocytes, were used to establish a binary classification model comparing methods that included random forest (RF), extreme gradient boosting (XGB), and logistic regression (LR). The prediction model based on RF showed the highest identification accuracy for revealing chemicals that may inhibit TSHR. For the RF model, recall was calculated at 0.89, balance accuracy was 0.85, and its receiver operating characteristic (ROC) curve-area under (AUC) was 0.92, indicating that the model had very high predictive capacity. The lowest CDocker energy (CE) and CDocker interaction energy (CIE) for chemicals and TSHR were determined and were subsequently introduced into the predictive model as descriptors. A regression model, extreme gradient boosting-Regression (XGBR), was successfully established yielding an R² = 0.65 to predict inhibitory activity for active compounds. Parameters that included dissociation characteristics, molecular structure, and binding energy were all key factors in the predictive model. We demonstrate that QSAR models are useful approaches, not only for identifying chemicals that inhibit TSHR, but for predicting inhibitory activity of active compounds.

Crystal structure of a ligand-free stable TSH receptor leucine-rich repeat domain

The crystal structures of the thyroid-stimulating hormone receptor (TSHR) leucine-rich repeat domain (amino acids 22-260; TSHR260) in complex with a stimulating human monoclonal autoantibody (M22TM) and in complex with a blocking human autoantibody (K1-70™) have been solved. However, attempts to purify and crystallise free TSHR260, that is not bound to an autoantibody, have been unsuccessful due to the poor stability of free TSHR260. We now describe a TSHR260 mutant that has been stabilised by the introduction of six mutations (H63C, R112P, D143P, D151E, V169R and I253R) to form TSHR260-JMG55TM, which is approximately 900 times more thermostable than wild-type TSHR260. These six mutations did not affect the binding of human TSHR monoclonal autoantibodies or patient serum TSHR autoantibodies to the TSHR260. Furthermore, the response of full-length TSHR to stimulation by TSH or human TSHR monoclonal autoantibodies was not affected by the six mutations. Thermostable TSHR260-JMG55TM has been purified and crystallised without ligand and the structure solved at 2.83 Å resolution. This is the first reported structure of a glycoprotein hormone receptor crystallised without ligand. The unbound TSHR260-JMG55TM structure and the M22 and K1-70 bound TSHR260 structures are remarkably similar except for small changes in side chain conformations. This suggests that neither the mutations nor the binding of M22TM or K1-70TM change the rigid leucine-rich repeat domain structure of TSHR260. The solved TSHR260-JMG55TM structure provides a rationale as to why the six mutations have a thermostabilising effect and provides helpful guidelines for thermostabilisation strategies of other soluble protein domains.

Born This Way: Using Intrinsic Disorder to Map the Connections between SLITRKs, TSHR, and Male Sexual Orientation

Recently, genome-wide association study reveals a significant association between specific single nucleotide polymorphisms (SNPs) in men and their sexual orientation. These SNPs (rs9547443 and rs1035144) reside in the intergenic region between the SLITRK5 and SLITRK6 genes and in the intronic region of the TSHR gene and might affect functionality of SLITRK5, SLITRK6, and TSHR proteins that are engaged in tight control of key developmental processes, such as neurite outgrowth and modulation, cellular differentiation, and hormonal regulation. SLITRK5 and SLITRK6 are single-pass transmembrane proteins, whereas TSHR is a heptahelical G protein-coupled receptor (GPCR). Mutations in these proteins are associated with various diseases and are linked to phenotypes found at a higher rate in homosexual men. A bioinformatics analysis of SLITRK5, SLITRK6, and TSHR proteins is conducted to look at their structure, protein interaction networks, and propensity for intrinsic disorder. It is assumed that this information might improve understanding of the roles that SLITRK5, SLITRK6, and TSHR play within neuronal and thyroidal tissues and give insight into the phenotypes associated with male homosexuality.

Thyrotropin receptor, still much to be learned from the patients

In the absence of crystal available for the full-length thyrotropin receptor, knowledge of its structure and functioning has benefitted from the identification and characterization of mutations in patients with various thyroid dysfunctions. The characterization of activating mutations has contributed to the elaboration of a model involving the extracellular domain of the receptor as an inverse tethered agonist which, upon binding of the ligand, relieves the transmembrane domain from an inhibiting interaction and activates it. The models derived from comparisons with other receptors, enriched with the information provided by the study of mutations, have proven useful for the design of small-molecule agonists and antagonists that may be used in the future to treat thyroid dysfunctions. In this review, extrathyroidal expression of the thyrotropin receptor is described, the role of which is still poorly defined.

Structure and activation of the TSH receptor transmembrane domain

PURPOSE

The thyroid-stimulating hormone receptor (TSHR) is the target autoantigen for TSHR-stimulating autoantibodies in Graves' disease. The TSHR is composed of: a leucine-rich repeat domain (LRD), a hinge region or cleavage domain (CD) and a transmembrane domain (TMD). The binding arrangements between the TSHR LRD and the thyroid-stimulating autoantibody M22 or TSH have become available from the crystal structure of the TSHR LRD-M22 complex and a comparative model of the TSHR LRD in complex with TSH, respectively. However, the mechanism by which the TMD of the TSHR and the other glycoprotein hormone receptors (GPHRs) becomes activated is unknown.

METHODS

We have generated comparative models of the structures of the inactive (TMD_In) and active (TMD_Ac) conformations of the TSHR, follicle-stimulating hormone receptor (FSHR) and luteinizing hormone receptor (LHR) TMDs. The structures of TMD_Ac and TMD_In were obtained using class A GPCR crystal structures for which fully active and inactive conformations were available.

RESULTS

Most conserved motifs observed in GPCR TMDs are also observed in the amino acid sequences of GPHR TMDs. Furthermore, most GPCR TMD conserved helix distortions are observed in our models of the structures of GPHR TMDs. Analysis of these structures has allowed us to propose a mechanism for activation of GPHR TMDs.

CONCLUSIONS

Insight into the mechanism of activation of the TSHR by both TSH and TSHR autoantibodies is likely to be useful in the development of new treatments for Graves' disease.

Dissecting the structural and functional features of the Luteinizing hormone receptor using receptor specific single chain fragment variables

The Luteinizing hormone receptor (LHR) has a large extracellular domain (amino acid residues, a.a.1-355) and a transmembrane domain (TMD; a.a. 356-699), essential for hormone binding and signaling, respectively. The LHR hinge region (a.a. 256-355) connects the two domains and acts as an activating switch for the receptor by an unknown mechanism. LHR hinge-specific Single chain fragment variables (ScFv) stimulated cAMP production by the stable and transiently transfected cell lines expressing LHR in a hormone-independent manner and the C-terminal region of LHR hinge (a.a. 313-349) was identified as the probable epitope for one agonistic ScFv. This epitope attained a helical conformation upon agonistic ScFv binding and the activity of the ScFv was dependent on Y331 sulfation. ScFv was also able to activate TMD mutants, D578Y and A593P, reemphasizing the role of TM helix VI in LHR 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.

Thyroid and bone: macrophage-derived TSH-β splice variant increases murine osteoblastogenesis

It is now firmly established that TSH may influence the physiology and patho-physiology of bone by activating osteoblasts and inhibiting osteoclast activity resulting in relative osteoprotection. Whether this influence is directly exerted by pituitary-derived TSH in vivo is less certain, because we have previously reported that the suppression of pituitary TSH does not remove such protection. Here, we have characterized the functional relevance of a novel form of the TSH-β subunit, designated TSH-βv, known to be produced by murine bone marrow cells. We found that fresh bone marrow-derived macrophages (MØs) preferentially produced TSH-βv and, when cocultured with CHO cells engineered to overexpress the full-length TSH receptor, were able to generate the production of intracellular cAMP; a phenomenon not seen in control CHO cells, such results confirmed the bioactivity of the TSH variant. Furthermore, cocultures of MØs and osteoblasts were shown to enhance osteoblastogenesis, and this phenomenon was markedly reduced by antibody to TSH-β, suggesting direct interaction between MØs and osteoblasts as observed under the electron microscope. These data suggest a new paradigm of local modulation of bone biology by a MØ-derived TSH-like molecule and raise the question of the relative contribution of local vs pituitary-derived TSH in osteoprotection.

Thyroid autoantibodies in pregnancy: their role, regulation and clinical relevance

Autoantibodies to thyroglobulin and thyroid peroxidase are common in the euthyroid population and are considered secondary responses and indicative of thyroid inflammation. By contrast, autoantibodies to the TSH receptor are unique to patients with Graves' disease and to some patients with Hashimoto's thyroiditis. Both types of thyroid antibodies are useful clinical markers of autoimmune thyroid disease and are profoundly influenced by the immune suppression of pregnancy and the resulting loss of such suppression in the postpartum period. Here, we review these three types of thyroid antibodies and their antigens and how they relate to pregnancy itself, obstetric and neonatal outcomes, and the postpartum.

Blocking type TSH receptor antibodies

TSH receptor (TSHR) autoantibodies (TRAbs) play a key role in the pathogenesis of Graves' disease. In the majority of patients, TRAbs stimulate thyroid hormone synthesis via activation of the TSHR (stimulating TRAbs, TSHR agonists). In some patients, TRAbs bind to the receptor but do not cause activation (blocking TRAbs, TSHR antagonists). Isolation of human TSHR monoclonal antibodies (MAbs) with either stimulating (M22 and K1-18) or blocking activities (5C9 and K1-70) has been a major advance in studies on the TSHR. The binding characteristics of the blocking MAbs, their interaction with the TSHR and their effect on TSHR constitutive activity are summarised in this review. In addition, the binding arrangement in the crystal structures of the TSHR in complex with the blocking MAb K1-70 and with the stimulating MAb M22 (2.55 Å and 1.9 Å resolution, respectively) are compared. The stimulating effect of M22 and the inhibiting effect of K1-70 on thyroid hormone secretion in vivo is discussed. Furthermore the ability of K1-70 to inhibit the thyroid stimulating activity of M22 in vivo is shown. Human MAbs which act as TSHR antagonists are potentially important new therapeutics. For example, in Graves' disease, K1-70 may well be effective in controlling hyperthyroidism and the eye signs caused by stimulating TRAb. In addition, hyperthyroidism caused by autonomous TSH secretion should be treatable by K1-70, and 5C9 has the potential to control hyperthyroidism associated with TSHR activating mutations. Furthermore, K1-70 has potential applications in thyroid imaging as well as targeted drug delivery to TSHR expressing tissues.

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.

Similarities and differences in interactions of thyroid stimulating and blocking autoantibodies with the TSH receptor

Binding of a new thyroid-stimulating human monoclonal autoantibody (MAb) K1-18 to the TSH receptor (TSHR) leucine-rich domain (LRD) was predicted using charge-charge interaction mapping based on unique complementarities between the TSHR in interactions with the thyroid-stimulating human MAb M22 or the thyroid-blocking human MAb K1-70. The interactions of K1-18 with the TSHR LRD were compared with the interactions in the crystal structures of the M22-TSHR LRD and K1-70-TSHR LRD complexes. Furthermore, the predicted position of K1-18 on the TSHR was validated by the effects of TSHR mutations on the stimulating activity of K1-18. A similar approach was adopted for predicting binding of a mouse thyroid-blocking MAb RSR-B2 to the TSHR. K1-18 is predicted to bind to the TSHR LRD in a similar way as TSH and M22. The binding analysis suggests that K1-18 light chain (LC) mimics binding of the TSH-α chain and the heavy chain (HC) mimics binding of the TSH-β chain. By contrast, M22 HC mimics the interactions of TSH-α while M22 LC mimics TSH-β in interactions with the TSHR. The observed interactions in the M22-TSHR LRD and K1-70-TSHR LRD complexes (crystal structures) with TSH-TSHR LRD (comparative model) and K1-18-TSHR LRD (predictive binding) suggest that K1-18 and M22 interactions with the receptor may reflect interaction of thyroid-stimulating autoantibodies in general. Furthermore, K1-70 and RSR-B2 interactions with the TSHR LRD may reflect binding of TSHR-blocking autoantibodies in general. Interactions involving the C-terminal part of the TSHR LRD may be important for receptor activation by autoantibodies.

Novel information on the epitope of an inverse agonist monoclonal antibody provides insight into the structure of the TSH receptor

The TSH receptor (TSHR) comprises an extracellular leucine-rich domain (LRD) linked by a hinge region to the transmembrane domain (TMD). Insight into the orientation of these components to each other is required for understanding how ligands activate the receptor. We previously identified residue E251 at the LRD-hinge junction as contributing to coupling TSH binding with receptor activation. However, a single residue cannot stabilize the LRD-hinge unit. Therefore, based on the LRD crystal structure we selected for study four other potential LRD-hinge interface charged residues. Alanine substitutions of individual residues K244, E247, K250 and R255 (as well as previously known E251A) did not affect TSH binding or function. However, the cumulative mutation of these residues in varying permutations, primarily K250A and R255A when associated with E251A, partially uncoupled TSH binding and function. These data suggest that these three residues, spatially very close to each other at the LRD base, interact with the hinge region. Unexpectedly and most important, monoclonal antibody CS-17, a TSHR inverse agonist whose epitope straddles the LRD-hinge, was found to interact with residues K244 and E247 at the base of the convex LRD surface. These observations, together with the functional data, exclude residues K244 and E247 from the TSHR LRD-hinge interface. Further, for CS-17 accessibility to K244 and E247, the concave surface of the TSHR LRD must be tilted forwards towards the hinge region and plasma membrane. Overall, these data provide insight into the mechanism by which ligands either activate the TSHR or suppress its constitutive activity.

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.

Evidence that the thyroid-stimulating hormone (TSH) receptor transmembrane domain influences kinetics of TSH binding to the receptor ectodomain

Thyroid-stimulating hormone (TSH)-induced reduction in ligand binding affinity (negative cooperativity) requires TSH receptor (TSHR) homodimerization, the latter involving primarily the transmembrane domain (TMD) but with the extracellular domain (ECD) also contributing to this association. To test the role of the TMD in negative cooperativity, we studied the TSHR ECD tethered to the cell surface by a glycosylphosphatidylinositol (GPI) anchor that multimerizes despite the absence of the TMD. Using the infinite ligand dilution approach, we confirmed that TSH increased the rate of dissociation (k(off)) of prebound (125)I-TSH from CHO cells expressing the TSH holoreceptor. Such negative cooperativity did not occur with TSHR ECD-GPI-expressing cells. However, even in the absence of added TSH, (125)I-TSH dissociated much more rapidly from the TSHR ECD-GPI than from the TSH holoreceptor. This phenomenon, suggesting a lower TSH affinity for the former, was surprising because both the TSHR ECD and TSH holoreceptor contain the entire TSH-binding site, and the TSH binding affinities for both receptor forms should, theoretically, be identical. In ligand competition studies, we observed that the TSH binding affinity for the TSHR ECD-GPI was significantly lower than that for the TSH holoreceptor. Further evidence for a difference in ligand binding kinetics for the TSH holoreceptor and TSHR ECD-GPI was obtained upon comparison of the TSH K(d) values for these two receptor forms at 4 °C versus room temperature. Our data provide the first evidence that the wild-type TSHR TMD influences ligand binding affinity for the ECD, possibly by altering the conformation of the closely associated hinge region that contributes to the TSH-binding site.

Identification of outer membrane porin f protein of Yersinia enterocolitica recognized by antithyrotopin receptor antibodies in Graves' disease and determination of its epitope using mass spectrometry and bioinformatics tools

CONTEXT

In addition to genetic susceptibility, Yersinia enterocolitica (YE) infection played an important causative role in the pathogenesis of Graves' disease (GD) through molecular mimicry. However, the specific YE proteins and epitopes recognized by anti-TSH receptor (TSHR) autoantibodies (TRAb) have not been fully clarified, resulting in conflicting results from clinical research.

OBJECTIVE

Our aim was to explore the roles of YE in the pathogenesis of GD and identify the YE proteins and epitopes that are similar to the TSHR and are recognized by TRAb.

DESIGN

Assays of YE antibodies, TRAb, thyroglobulin antibodies, and thyroid microsomal antibodies as well as cross-absorption and two-way immunodiffusion were performed in patients with GD. Using mass spectrometry and the bioinformatics tools of protein structure modeling and epitope prediction, we identified the YE protein and its epitope, which was recognized by TRAb and was similar to TSHR.

RESULTS

Our study demonstrated for the first time that the YE protein outer membrane porin F protein (ompF) shared cross-immunogenicity with a leucine-rich domain of TSHR. The epitope recognized by antihuman TSHR antibody is located within the ompF region of amino acids 190-197, and the polyantibody against ompF protein showed TSAb activity.

CONCLUSIONS

Our results suggest that YE ompF is involved in the production of TRAb and the pathogenesis of GD through molecular mimicry. These findings are potentially important for understanding the role molecular mimicry plays in the disturbance of immune tolerance and the induction of autoimmunity to the TSHR.

The genetics of the thyroid stimulating hormone receptor: history and relevance

BACKGROUND

The thyroid stimulating hormone receptor (TSHR) is the key regulator of thyrocyte function. The gene for the TSHR on chromosome 14q31 has been implicated as coding for the major autoantigen in the autoimmune hyperthyroidism of Graves' disease (GD) to which T cells and autoantibodies are directed.

SUMMARY

The TSHR is a seven-transmembrane domain receptor that undergoes complex posttranslational processing. In this brief review, we look at the genetics of this important autoantigen and its influence on a variety of tissue functions in addition to its role in the induction of GD.

CONCLUSIONS

There is convincing evidence that the TSH receptor gene confers increased susceptibility for GD, but not Hashimoto's thyroiditis. GD is associated with polymorphisms in the intron 1 gene region. How such noncoding nucleotide changes influence disease susceptibility remains uncertain, but is likely to involve TSHR splicing variants and/or microRNAs arising from this gene region. Whether such influences are confined to the thyroid gland or whether they influence cell function in the many extrathyroidal sites of TSHR expression remains unknown.

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.

TSH receptor monoclonal antibodies with agonist, antagonist, and inverse agonist activities

Autoantibodies in autoimmune thyroid disease (AITD) bind to the TSH receptor (TSHR) and can act as either agonists, mimicking the biological activity of TSH, or as antagonists inhibiting the action of TSH. Furthermore, some antibodies with antagonist activity can also inhibit the constitutive activity of the TSHR, that is, act as inverse agonists. The production of animal TSHR monoclonal antibodies (MAbs) with the characteristics of patient autoantibodies and the isolation of human autoantibodies from patients with AITD has allowed us to analyze the interactions of these antibodies with the TSHR at the molecular level. In the case of animal MAbs, advances such as DNA immunization allowed the production of the first MAbs which showed the characteristics of human TSHR autoantibodies (TRAbs). Mouse MAbs (TSMAbs 1-3) and a hamster MAb (MS-1) were obtained that acted as TSHR agonists with the ability to stimulate cyclic AMP production in CHO cells expressing the TSHR. In addition, a mouse TSHR MAb (MAb-B2) that had the ability to act as an antagonist of TRAbs and TSH was isolated and characterized. Also, a mouse TSHR MAb that showed TSH antagonist and TSHR inverse agonist activity (CS-17) was described. Furthermore, a panel of human TRAbs has been obtained from the peripheral blood lymphocytes of patients with AITD and extensively characterized. These MAbs have all the characteristics of TRAbs and are active at ng/mL levels. To date, two human MAbs with TSHR agonist activity (M22 and K1-18), one human MAb with TSHR antagonist activity (K1-70) and one human MAb (5C9) with both TSHR antagonist and TSHR inverse agonist activity have been isolated. Early experiments showed that the binding sites for TSH and for TRAbs with thyroid stimulating or blocking activities were located on the extracellular domain of the TSHR. Extensive studies using TSHRs with single amino acid mutations identified TSHR residues that were important for binding and biological activity of TSHR MAbs (human and animal) and TSH. The structures of several TSHR MAb Fab fragments were solved by X-ray crystallography and provided details of the topography of the antigen binding sites of antibodies with either agonist or antagonist activity. Furthermore stable complexes of the leucine-rich repeat domain (LRD) of the TSHR with a human MAb (M22) with agonist activity and with a human MAb (K1-70) with antagonist activity have been produced and their structures solved by X-ray crystallography at 2.55 and 1.9Å resolution, respectively. Together these experiments have given detailed insights into the interactions of antibodies with different biological activities (agonist, antagonist, and inverse agonist) with the TSHR. Although the nature of ligand binding to the TSHR is now understood in some detail, it is far from clear how these initial interactions lead to functional effects on activation or inactivation of the receptor.

A monoclonal antibody with thyrotropin (TSH) receptor inverse agonist and TSH antagonist activities binds to the receptor hinge region as well as to the leucine-rich domain

Monoclonal antibody CS-17 is a TSH receptor (TSHR) inverse agonist (suppresses constitutive activity) and a TSH antagonist. Elucidation of the CS-17 epitope will provide insight into TSHR structure and function. Present information on its epitope conflicts with recent data regarding another TSHR inverse agonist antibody. To characterize further the CS-17 epitope, we exploited the observation that CS-17 does not recognize a chimeric receptor with TSHR hinge region residues 261-289 replaced with homologous rat LH receptor residues (13 mismatches). We generated individual and double TSHR mutations corresponding to these mismatches. On flow cytometry, only T273L/R274V reduced CS-17 recognition. No mutation affected TSH-stimulated cAMP generation. Because the immunogen for CS-17 generation was highly glycosylated, we also investigated whether the glycan moiety at N198, topologically adjacent to Y195 (a previously identified epitopic component), could contribute to the CS-17 epitope. Elimination of this N-linked glycan (mutations of N198 and T200) abrogated CS-17 binding without altering TSH responsiveness. However, studies with tunicamycin suggested that these mutations affected CS-17 binding by altering the polypeptide backbone rather than eliminating the glycan moiety. TSHR residues N198 and T200, like Y195, are on the convex facet of the leucine-rich domain. In summary, the present data indicate that the discontinuous epitope of CS-17, a TSHR inverse agonist and TSH antagonist, includes a component in the hinge region as well as the convex surface of the TSHR leucine-rich domain. These findings expand our present concept of glycoprotein hormone binding and function.

The thyroid-stimulating hormone receptor: impact of thyroid-stimulating hormone and thyroid-stimulating hormone receptor antibodies on multimerization, cleavage, and signaling

The thyroid-stimulating hormone receptor (TSHR) has a central role in thyrocyte function and is also one of the major autoantigens for the autoimmune thyroid diseases. We review the post-translational processing, multimerization, and intramolecular cleavage of TSHR, all of which may modulate its signal transduction. The recent characterization of monoclonal antibodies to the TSHR, including stimulating, blocking, and neutral antibodies, have also revealed unique biologic insights into receptor activation and the variety of these TSHR antibodies may help explain the multiple clinical phenotypes seen in autoimmune thyroid diseases. Knowledge of the structure/function relationship of the TSHR is beginning to provide a greater understanding of thyroid physiology and thyroid autoimmunity.

Loss-of-function mutations in the thyrotropin receptor gene as a major determinant of hyperthyrotropinemia in a consanguineous community

CONTEXT

Resistance to TSH (RTSH) is a condition of impaired responsiveness of the thyroid gland to TSH, characterized by elevated serum TSH, low or normal thyroid hormone levels, and hypoplastic or normal-sized thyroid gland.

OBJECTIVES

The aim of the study was to evaluate the clinical course and the genotype-phenotype relationship of RTSH caused by two different TSH receptor (TSHR) gene mutations in a consanguineous population.

PATIENTS AND METHODS

We conducted a clinical and genetic investigation of 46 members of an extended family and 163 individuals living in the same town. In vitro functional studies of the mutant TSHRs were also performed.

RESULTS

Two TSHR gene mutations (P68S and L653V) were identified in 33 subjects occurring as homozygous L653V (five subjects), heterozygous L653V (20 subjects), heterozygous P68S (four subjects), and compound heterozygous L653V/P68S (four subjects). With the exception of one individual with concomitant autoimmune thyroid disease, all homozygotes and compound heterozygotes presented with compensated RTSH (high TSH with free T(4) and T(3) in the normal range). Only nine of 24 heterozygotes had mild hyperthyrotropinemia. The L653V mutation resulted in a higher serum TSH concentration and showed a more severe in vitro abnormality than P68S. Haplotype analysis predicted a founder of the L653V six to seven generations earlier, whereas the P68S is older. Cross-sectional and prospective longitudinal studies indicate that TSH and T(4) concentrations remain stable over time.

CONCLUSIONS

High frequency hyperthyrotropinemia in an Israeli Arab-Muslim consanguineous community is attributed to two inactivating TSHR gene mutations. Concordant genotype-phenotype was demonstrated clinically and by in vitro functional analysis. Retrospective and prospective studies indicate that in the absence of concomitant autoimmune thyroid disease, elevated TSH levels reflect stable compensated RTSH.

Effect of recombinant human thyroid stimulating hormone on serum thyroxin and thyroid scintigraphy in euthyroid cats

This study investigated the thyroidal response to administration of recombinant human thyroid stimulating hormone (rhTSH) by means of serum total thyroxine (TT(4)) concentration and pertechnetate uptake by the thyroid gland in six healthy euthyroid spayed female cats. A pertechnetate scan was performed on day 1 to calculate thyroid/salivary gland (T/S) uptake ratio. On day 3, 25 microg rhTSH was injected intravenously. Six hours later the thyroid scan was repeated as on day 1. Blood was drawn for serum TT(4) measurement prior to injection of rhTSH and performance of the pertechnetate scan. Statistically significant differences in mean serum TT(4) concentration, T/S uptake ratio before and 6h after rhTSH administration and T/S uptake ratio between left and right lobes were noted. We can conclude that 25 microg rhTSH increases pertechnetate uptake in the thyroid glands of cats, this should be taken into account when thyroid scintigraphy after rhTSH administration is interpreted.

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.

TSH signalling and cancer

Thyroid cancers are the most frequent endocrine neoplasms and mutations in the thyrotropin receptor (TSHR) are unusually frequent. Here we present the state-of-the-art concerning the role of TSHR in thyroid cancer and discuss it in light of the cancer stem cell theory or the classical view. We briefly review the gene and protein structure updating the cancer related TSHR mutations database. Intriguingly, hyperfunctioning TSHR mutants characterise differentiated cancers in contrast to undifferentiated thyroid cancers which very often bear silenced TSHR. It remains unclear whether TSHR alterations in thyroid cancers play a role in the onset or they appear as a consequence of genetic instability during evolution, but the presence of functional TSHR is exploited in therapy. We outline the signalling network build up in the thyrocyte between TSHR/PKA and other proliferative pathways such as Wnt, PI3K and MAPK. This networks integrity surely plays a role in the onset/evolution of thyroid cancer and needs further research. Lastly, future investigation of epigenetic events occurring at the TSHR and other loci may give better clues for molecular based therapy of undifferentiated thyroid carcinomas. Targeted demethylating agents, histone deacetylase inhibitors combined with retinoids and specific RNAis may help treatment in the future.

Cleavage of the human thyrotropin receptor by ADAM10 is regulated by thyrotropin

The thyrotropin receptor (TSHR) has a unique 50 residue (317-366) ectodomain insertion that sets it apart from other glycoprotein hormone receptors (GPHRs). Other ancient members of the leucine-rich repeat G protein-coupled receptor (GPCR) (LGR) family do exhibit ectodomain insertions of variable lengths and sequences. The TSHR-specific insert is digested, apparently spontaneously, to release the ectodomain (A-subunit) leaving the balance of the ectodomain attached to the serpentine (B-subunit). Despite concerted efforts for the last 12 years by many laboratories, the enzyme involved in TSHR cleavage has not been identified and a physiologic role for this process remains unclear. Several lines of evidence had suggested that the TSHR protease is likely a member of the a disintegrin and metalloprotease (ADAM) family of metalloproteases. We show here that the expression of ADAM10 was specific to the thyroid by specially designed DNA microarrays. We also show that TSH increases TSHR cleavage in a dose-dependent manner. To prove that ADAM10 is indeed the TSHR cleavage enzyme, we investigated the effect of TSH-induced cleavage by a peptide based on a motif (TSHR residues 334-349), shared with known ADAM10 substrates. TSH increased dose dependently TSHR ectodomain cleavage in the presence of wild-type peptide but not a scrambled control peptide. Interestingly, TSH increased the abundance of non-cleaved single chain receptor, as well higher molecular forms of the A-subunit, despite their enhancement of the appearance of the fully digested A-subunit. This TSH-related increase in TSHR digested forms was further increased by wild-type peptide. We have identified for the first time ADAM10 as the TSHR cleavage enzyme and shown that TSH regulates its activation.

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.

The thyrotropin receptor in Graves' disease

The application of molecular biology to the study of the thyrotropin receptor (TSHR) has led to major advances in our understanding of its structure, function, and relationship to the pathogenesis of Graves' disease. This review summarizes many of these features and also provides a personal perspective, questioning some assumptions and general concepts, as well as describing remaining challenges. Among the issues raised are the limits in our understanding of the spatial orientation of the structural domains of the TSHR, including the enigmatic hinge region. We review the phenomenon of TSHR intramolecular cleavage, the shedding of the A-subunit component of the ectodomain, and the importance of the latter in generating thyroid-stimulating antibodies. The epitopes of thyroid-stimulating and -blocking autoantibodies have been a confusing and controversial subject that requires review and evaluation of available data. Finally, we address the potential physiological or pathophysiological significance of TSHR multimerization in TSHR. Taken together, this review will, hopefully, convey the fascination and excitement that molecular biology has contributed to the study of the TSHR, especially as it relates to the pathogenesis of Graves' disease.

TSH receptor antibodies

The discovery of thyroid-stimulating autoantibodies by Adams and Purves 50 years ago was one of the most important observations in the history of thyroidology. Since that time, the thyroid-stimulating hormone receptor (TSHR) has been shown to be the antigen recognized by these autoantibodies (1974) and the receptor cloned (1989). More recently, different mouse monoclonal antibodies (MAbs) to the TSHR have been produced, culminating in 2002 in the preparation of mouse and hamster MAbs with strong thyroid-stimulating activity. Further, in 2003 a human MAb to the TSHR (M22) with the characteristics of patient thyroid-stimulating autoantibodies was described. M22 has been particularly useful in advancing our knowledge of the TSHR and TSHR autoimmunity, including the development of new assays for TSHR autoantibodies (2004) and determination of a high-resolution (2.55 A) crystal structure of the TSHR leucine-rich domain in combination with M22 (2007). The structure shows that M22 positions itself on the TSHR in an almost identical way to the native hormone TSH but the evolutionary forces that have resulted in production of a common autoantibody that mimics the actions of TSH so well are far from clear at this time. Very recently, a human MAb (5C9) with the characteristics of blocking-type patient serum TSHR autoantibodies has been isolated (2007). Studies on how 5C9 interacts with the TSHR at the molecular level are planned and should provide key insights as to the differences between TSHR autoantibodies with blocking and with stimulating activities. Also, 5C9 and similar MAbs have considerable potential as drugs to inhibit TSHR stimulation by autoantibodies. Further, now the M22-TSHR structure is known at the atomic level, rational design of specific low-molecular-weight inhibitors of the TSHR-TSHR autoantibody interaction is feasible.

Molecular interactions between the TSH receptor and a Thyroid-stimulating monoclonal autoantibody

OBJECTIVE

To study the molecular interactions between the thyroid-stimulating hormone (TSH) receptor (TSHR) and a human thyroid-stimulating monoclonal autoantibody (M22).

DESIGN

Amino acid mutations were introduced in the variable region gene sequences of M22 and the wild-type (WT) or mutated M22 Fab expressed in Escherichia coli. The ability of WT or mutated M22 Fab to inhibit binding of (125)I-TSH or (125)I-M22 to the TSHR and to stimulate cyclic adenosine monophosphate (AMP) production in Chinese hamster ovary cells expressing WT TSHRs was studied. Mutated TSHRs were also used in these studies in combination with WT or mutated M22 Fab to further identify interacting residues in the TSHR-M22 complex.

MAIN OUTCOME

Out of 11 amino acid changes in the heavy chain (HC) of M22, 7 had an effect on M22 Fab biological activity, while in the case of 1 mutation the Fab was not expressed. In particular, stimulating activity of M22 Fab mutated at HC residues, D52, D54, and Y56 was markedly reduced. Mutation of M22 light chain (LC) D52 also reduced M22 Fab stimulating activity, while mutations at two further residues (LC D51 and LC D93) showed no effect. Reverse charge mutations at M22 HC D52 and TSHR R80 provided experimental evidence that these two residues interacted strongly with each other.

CONCLUSION

Mutation of both the TSHR and M22 Fab has allowed identification of some residues critical for the receptor-autoantibody interaction. This approach should lead to detailed mapping of the amino acids important for M22 biological activity.

Crystal structure of the TSH receptor in complex with a thyroid-stimulating autoantibody

OBJECTIVE

To analyze interactions between the thyroid-stimulating hormone receptor (TSHR) and a thyroid-stimulating human monoclonal autoantibody (M22) at the molecular level.

DESIGN

A complex of part of the TSHR extracellular domain (amino acids 1-260; TSHR260) bound to M22 Fab was prepared and purified. Crystals suitable for X-ray diffraction analysis were obtained and the structure solved at 2.55 A resolution.

MAIN OUTCOME

TSHR260 comprises of a curved helical tube and M22 Fab clasps its concave surface at 90 degrees to the tube length axis. The interface buried in the complex is large (2,500 A(2)) and an extensive network of ionic, polar, and hydrophobic bonding is involved in the interaction. There is virtually no movement in the atoms of M22 residues on the binding interface compared to unbound M22 consistent with "lock and key" binding. Mutation of residues showing strong interactions in the structure influenced M22 activity, indicating that the binding detail observed in the complex reflects interactions of M22 with intact, functionally active TSHR. The receptor-binding arrangements of the autoantibody are very similar to those reported for follicle-stimulating hormone (FSH) binding to the FSH receptor (amino acids 1-268) and consequently to those of TSH itself.

CONCLUSIONS

It is remarkable that the thyroid-stimulating autoantibody shows almost identical receptor-binding features to TSH although the structures and origins of these two ligands are very different. Furthermore, our structure of the TSHR and its complex with M22 provide foundations for developing new strategies to understand and control both glycoprotein hormone receptor activation and the autoimmune response to the TSHR.

Molecular characterization of a disease associated conformational epitope on GAD65 recognised by a human monoclonal antibody b96.11

Autoantibodies to the 65kDa isoform of glutamate decarboxylase (GAD65) are associated with type I diabetes and recognise highly conformational epitope(s) that remain to be defined. The human recombinant Fab from mAb b96.11 inhibits binding of most GAD65 antibody positive sera from patients and its epitope has previously been localized to the middle region of GAD65. Recent studies indicate that b96.11 antibody specificity predicts the risk of developing type 1 diabetes in prediabetic individuals. We describe the use homology modelling, protein-protein docking simulations and biopanning of random peptide phage displayed libraries with b96.11 to predict contact amino acids on the interface of GAD65/Fab b96.11 complex. Further analysis by in vitro mutagenesis of GAD65 followed by radioimmunoprecipitation refined the amino acids contributing to the b96.11 epitope. Our studies show an interface characterized by a protruding antibody-combining site centered on the long heavy chain CDR3 loop of Fab b96.11 establishing interactions with the critical residue Phe(344) in the core of the epitope on GAD65, surrounded by charged sites within (375)RK(376) and (305)DER(307). The epitope requires residues from both middle and the C-terminal domains, and is the first precise definition of an epitope on GAD65. The nature of the b96.11 epitope leads to considerations of potential structural variations for differences in antigenicity between the isoforms GAD65 and GAD67. The study shows the utility of using a combination of in silico techniques and experimental data for molecular characterization and localization of conformational epitopes for which crystal structures are lacking.

Effects of TSH receptor mutations on binding and biological activity of monoclonal antibodies and TSH

The effects of an extensive series of mutations in the TSH receptor (TSHR) leucine-rich domain (LRD) on the ability of thyroid-stimulating monoclonal antibodies (TSMAbs) and TSH to bind to the receptor and stimulate cyclic AMP production in TSHR-transfected CHO cells has been investigated. In addition, the ability of a mouse monoclonal antibody with blocking (i.e., antagonist) activity (RSR-B2) to interact with mutated receptors has been studied. Several amino acids distributed along an extensive part of the concave surface of the LRD were found to be important for binding and stimulation by the thyroid-stimulating human MAb M22 but did not appear to be important for TSH binding and stimulation. Most of these amino acids important for M22 interactions were also found to be important for the stimulating activity of six different mouse TSMAbs and a hamster TSMAb. Furthermore, most of these same amino acids were important for stimulation by TSHR autoantibodies in a panel of sera from patients with Graves' disease. Amino acid R255 was the only residue found to be unimportant for TSH stimulation but critical for stimulation by all thyroid-stimulating antibodies tested (23 patient serum TSHR autoantibodies, M22, and all seven animal TSMAbs). About half the amino acids (all located in the N-terminal part of the LRD) found to be important for M22 activity were also important for the blocking activity of RSR-B2 and although the epitopes for the two MAbs overlap they are different. As the two MAbs have similar affinities, their epitope differences are probably responsible for their different activities. Overall our results indicate that different TSMAbs and different patient sera thyroid-stimulating autoantibodies interact with the same region of the TSHR, but there are subtle differences in the actual amino acids that make contact with the different stimulators.

Structural Differences in the Hinge Region of the Glycoprotein Hormone Receptors: Evidence from the Sulfated Tyrosine Residues

Tyrosine sulfation is a late posttranslational modification of proteins that takes place in the Golgi network. In the past few years, this process has been identified as an important modulator of protein-protein interactions. Sulfated tyrosine residues have recently been identified in the C-terminal, so-called hinge region of the ectodomain of glycoprotein hormone receptors [TSH, LH/chorionic gonadotropin (CG), and FSH receptors] and were shown to play an important role in the interaction with their natural ligands. The position of two sulfated tyrosine residues in a Y-D/E-Y motif appears perfectly conserved in the alignment of TSH and LH receptors from different species, and site-directed mutagenesis experiments demonstrated that sulfation of the first residue of this motif was responsible for the functional effect on hormone binding. In contrast, the corresponding motif is not conserved in the FSH receptor, in which the first tyrosine residue is missing: the Y-D/E-Y motif is replaced by F(333)DY(335). We extend here our previous observation that, in this case, it is sulfation of the second sole tyrosine residue in the motif that is functionally important. An LH/CG receptor harboring an F(331)DY(333) motif (i.e. displaying decreased sensitivity to human CG) was used as a backbone in which short portions of the FSH receptor were substituted. Segments from the FSH receptor capable of restoring sensitivity to human CG were identified by transfection of the chimeras in COS-7 cells. These experiments identified key amino acid residues in the hinge region of the FSH receptor associated with the functional role of the second sulfated tyrosine residue in a Y-D/E-Y motif, allowing for efficient hormone binding. The experiments represent strong evidence that structural differences in the hinge regions of FSH and LH/CG receptors play a significant role in hormone-receptor-specific recognition.

Effects of a thyroid-stimulating human monoclonal autoantibody (M22) on functional activity of LH and FSH receptors

OBJECTIVE

The glycoprotein hormones luteinizing hormone (LH), follicle-stimulating hormone (FSH), and thyrotropin (TSH) show low-level cross-reactivity between their respective receptors (R). Patient serum autoantibodies to the thyrotropin receptor (TSHR) do not appear to cross-react with the luteinizing hormone receptor (LHR) or follicle-stimulating hormone receptor (FSHR), although the concentrations of autoantibody with which it is feasible to carry out experiments of this type are limited. Consequently, we have studied the effects of high doses of the thyroid-stimulating human monoclonal autoantibody (M22) on the LHR and FSHR.

DESIGN

Chinese Hamster ovary (CHO) cells stably expressing the TSHR, LHR, and FSHR and purified M22 IgG preparations were used in the study.

METHODS

CHO-TSHR, CHO-LHR, and CHO-FSHR cells were incubated with bovine TSH (0.1-25mU/mL), human recombinant chorionic gonadotropin (hCG; 0.5-10mU/mL) or human recombinant FSH (100-5000mU/mL) or with M22 IgG (0.001-5.0 microg/mL), and the extracellular cyclic AMP was measured by radioimmunoassay.

RESULTS

Cyclic AMP levels increased in a dose-dependent manner after incubation of CHO-TSHR cells with TSH or M22 IgG, and on a molar basis the effects of TSH and M22 were similar. Cyclic AMP stimulation was not detectable in CHO-LHR and CHO-FSHR cells after incubation with M22 IgG, whereas incubation with hCG or FSH, respectively, caused dose-dependent cyclic AMP stimulation. On a molar basis, concentrations of M22 IgG approximately 100x those of FSH causing clear stimulation were ineffective with CHO-FSHR cells. Similarly, molar concentration of M22 IgG 20,000x those of hCG causing clear stimulation had no effect on CHO-LHR cells.

CONCLUSIONS

This study shows that at relatively high concentrations, M22 IgG is unable to stimulate cyclic AMP levels in CHO-LHR or CHO-FSHR cells, suggesting that TSHR autoantibodies have greater specificity for the TSHR than TSH itself.

Expression and purification of feline thyrotropin (fTSH): immunological detection and bioactivity of heterodimeric and yoked glycoproteins

The goal of this study was to express and purify recombinant feline TSH as a possible immunoassay standard or pharmaceutical agent. Previously cloned feline common glycoprotein alpha (CGA) and beta subunits were ligated into the mammalian expression vector pEAK10. The feline CGA-FLAG and beta subunits were cloned separately into the pEAK10 expression vector, and transiently co-transfected into PEAK cells. Similarly, previously cloned and sequenced yoked (single chain) fTSH (yfTSH) and the CGA-FLAG sequences were ligated into the same vector, and stable cell lines selected by puromycin resistance. Expression levels of at least 1 microg/ml were achieved for both heterodimeric and yoked fTSH forms. The glycoproteins were purified in one step using anti-FLAG immunoaffinity column chromatography to high purity. The molecular weights of feline CGA-FLAG subunit, beta subunit and yfTSH were 20.4, 17, and 45 kDa, respectively. Both heterodimeric and yoked glycoproteins were recognized with approximately 40% detection by both a commercial canine TSH immunoassay and an in-house canine TSH ELISA. The yoked glycoprotein exhibited parallelism with the heterodimeric form in the in-house ELISA, supporting their possible use as immunoassay standards. In bioactivity assays, the heterodimeric and yoked forms of fTSH were 12.5 and 3.4% as potent as pituitary source bovine TSH at displacing (125)I-bTSH and 45 and 24% as potent in stimulating adenylate cyclase activity in human TSH receptor-expressing JP09 cells. However, in addition to reduced receptor binding affinity, the recombinant glycohormones produced a reduced maximal effect at maximal concentration (E(max)) suggesting the possibility of the recombinant glycohormone constructs acting as partial agonists at the human TSH receptor.

Analysis of Epitopes on the Unrelated Proteins Thyrotropin Receptor and alpha1-Antitrypsin which are Recognized by A10 Monoclonal Antibody

In a previous study, we noticed an unexpected reaction of an antithyrotropin receptor ectodomain (ETSHR)-reactive monoclonal antibody, A10, with alpha1-antitrypsin (antitrypsin). Presently, we decided to probe the structural basis of this cross-reactivity. Recombinant ETSHR, antitrypsin, synthetic peptides corresponding to the region of similarity in these proteins (EEDFRV and EEDFHV, respectively) and a set of peptides related to this region, N- and C-terminally elongated, were used in the study. Comparing the values of the dissociation constants, we found that the affinity of peptides corresponding to the region of similarity to monoclonal antibody A10 was the same in spite of a difference in one residue (R 38 in ETSHR and H 209 in antitrypsin), whereas a change of E 206 to R in antitrypsin-related peptide dramatically decreased the affinity. The whole binding site of A10 in ETSHR as well as in antitrypsin was larger than the region of similarity. We propose that residues ECHQEEDFV represent the monoclonal antibody A10 epitope. They form an almost continuous sequence of residues 30-37 and 39 in ETSHR. The monoclonal antibody A10 binding site on antitrypsin is shorter. It comprises amino acids 205-208 and 210, from the region of similarity with, probably, additional two residues, H-287 and E 363.

Designing scaffolds of peptides for phage display libraries

Phage display is a powerful method for the discovery of peptide ligands that are used for analytical tools, drug discovery, and target validations. Phage display technology can produce a huge number of peptides and generate novel peptide ligands. Recently, phage display technology has successfully managed to create peptide ligands that bind to pharmaceutically difficult targets such as the erythropoietin receptor. As a result of the structural analysis of their ligands, we found that the conformational design of peptides in library is important for selecting high-affinity ligands that bind to every target from a phage peptide library. Key issues concern constraints on the conformation of peptides on the phage and the development of chemically synthesized peptides derived from peptides on phage. This review discusses studies related to the conformation of peptides selected from phage display peptide libraries in addition to the conversion from peptides to non-peptides.

Thyrotropin receptor-associated diseases: from adenomata to Graves disease

The thyroid-stimulating hormone receptor (TSHR) is a G protein-linked, 7-transmembrane domain (7-TMD) receptor that undergoes complex posttranslational processing unique to this glycoprotein receptor family. Due to its complex structure, TSHR appears to have unstable molecular integrity and a propensity toward over- or underactivity on the basis of point genetic mutations or antibody-induced structural changes. Hence, both germline and somatic mutations, commonly located in the transmembrane regions, may induce constitutive activation of the receptor, resulting in congenital hyperthyroidism or the development of actively secreting thyroid nodules. Similarly, mutations leading to structural alterations may induce constitutive inactivation and congenital hypothyroidism. The TSHR is also a primary antigen in autoimmune thyroid disease, and some TSHR antibodies may activate the receptor, while others inhibit its activation or have no influence on signal transduction at all, depending on how they influence the integrity of the structure. Clinical assays for such antibodies have improved significantly and are a useful addition to the investigative armamentarium. Furthermore, the relative instability of the receptor can result in shedding of the TSHR ectodomain, providing a source of antigen and activating the autoimmune response. However, it may also provide decoys for TSHR antibodies, thus influencing their biological action and clinical effects. This review discusses the role of the TSHR in the physiological and pathological stimulation of the thyroid.

Insight into Graves' hyperthyroidism from animal models

Graves' hyperthyroidism can be induced in mice or hamsters by novel approaches, namely injecting cells expressing the TSH receptor (TSHR) or vaccination with TSHR-DNA in plasmid or adenoviral vectors. These models provide unique insight into several aspects of Graves' disease: 1) manipulating immunity toward Th1 or Th2 cytokines enhances or suppresses hyperthyroidism in different models, perhaps reflecting human disease heterogeneity; 2) the role of TSHR cleavage and A subunit shedding in immunity leading to thyroid-stimulating antibodies (TSAbs); and 3) epitope spreading away from TSAbs and toward TSH-blocking antibodies in association with increased TSHR antibody titers (as in rare hypothyroid patients). Major developments from the models include the isolation of high-affinity monoclonal TSAbs and analysis of antigen presentation, T cells, and immune tolerance to the TSHR. Studies of inbred mouse strains emphasize the contribution of non-MHC vs. MHC genes, as in humans, supporting the relevance of the models to human disease. Moreover, other findings suggest that the development of Graves' disease is affected by environmental factors, including infectious pathogens, regardless of modifications in the Th1/Th2 balance. Finally, developing immunospecific forms of therapy for Graves' disease will require painstaking dissection of immune recognition and responses to the TSHR.