Epitope analysis of the human thyrotropin (TSH) receptor using monoclonal antibodies

Structure of full-length TSH receptor in complex with antibody K1-70™

Determination of the full-length thyroid-stimulating hormone receptor (TSHR) structure by cryo-electron microscopy (cryo-EM) is described. The TSHR complexed with human monoclonal TSHR autoantibody K1-70™ (a powerful inhibitor of TSH action) was detergent solubilised, purified to homogeneity and analysed by cryo-EM. The structure (global resolution 3.3 Å) is a monomer with all three domains visible: leucine-rich domain (LRD), hinge region (HR) and transmembrane domain (TMD). The TSHR extracellular domain (ECD, composed of the LRD and HR) is positioned on top of the TMD extracellular surface. Extensive interactions between the TMD and ECD are observed in the structure, and their analysis provides an explanation of the effects of various TSHR mutations on TSHR constitutive activity and on ligand-induced activation. K1-70™ is seen to be well clear of the lipid bilayer. However, superimposition of M22™ (a human monoclonal TSHR autoantibody which is a powerful stimulator of the TSHR) on the cryo-EM structure shows that it would clash with the bilayer unless the TSHR HR rotates upwards as part of the M22™ binding process. This rotation could have an important role in TSHR stimulation by M22™ and as such provides an explanation as to why K1-70™ blocks the binding of TSH and M22™ without activating the receptor itself.

Presence of TSH receptors in discrete areas of the hypothalamus and caudal brainstem with relevance for feeding controls-Support for functional significance

AIMS

Previous studies have shown that brain-derived thyroid-stimulating hormone (TSH) and its receptor (TSHr) are present in hypothalamic extracts. No studies investigating both the anatomical location and functional significance of putative TSHr proteins in specific central nervous system (CNS) nuclei involved in feeding controls have yet been conducted. The aim was thus to determine whether TSHr are present in nuclei associated with feeding behavior, and if such receptors may be functional.

METHODS

Brain tissue from adult rats was analyzed for gene expression and receptor protein expression was investigated with immunohistochemistry and western blotting. To investigate whether putative TSHr may be functional, we evaluated food intake of rats given intraparenchymal nanoinjections of TSH into the nucleus of the solitary tract (NTS).

RESULTS

RT-qPCR confirmed previous reports that TSHr mRNA is expressed in CNS tissues of the adult rat. Immunohistochemistry showed TSHr-immunoreactivity in the arcuate, the ventromedial, the dorsomedial, and the paraventricular hypothalamic nuclei. We also found TSHr-ir in the dorsal hindbrain to be localized to the area postrema, NTS, dorsal motor nucleus of the vagus, and the hypoglossal motor nucleus. Further protein analysis with western blotting showed 120kDa TSHr-ir proteins present in the hypothalamus and brainstem. Injections of TSH into the NTS reduced food intake similar to the positive control, urocortin.

CONCLUSIONS

These data suggest that functional TSHr are present in the caudal brainstem and hypothalamic nuclei of relevance for feeding control as a possibly uncleaved holoreceptor, and highlights a hindbrain component to central TSH inhibition of food intake.

Engineering Multi-Walled Carbon Nanotube Therapeutic Bionanofluids to Selectively Target Papillary Thyroid Cancer Cells

BACKGROUND

The incidence of papillary thyroid carcinoma (PTC) has risen steadily over the past few decades as well as the recurrence rates. It has been proposed that targeted ablative physical therapy could be a therapeutic modality in thyroid cancer. Targeted bio-affinity functionalized multi-walled carbon nanotubes (BioNanofluid) act locally, to efficiently convert external light energy to heat thereby specifically killing cancer cells. This may represent a promising new cancer therapeutic modality, advancing beyond conventional laser ablation and other nanoparticle approaches.

METHODS

Thyroid Stimulating Hormone Receptor (TSHR) was selected as a target for PTC cells, due to its wide expression. Either TSHR antibodies or Thyrogen or purified TSH (Thyrotropin) were chemically conjugated to our functionalized Bionanofluid. A diode laser system (532 nm) was used to illuminate a PTC cell line for set exposure times. Cell death was assessed using Trypan Blue staining.

RESULTS

TSHR-targeted BioNanofluids were capable of selectively ablating BCPAP, a TSHR-positive PTC cell line, while not TSHR-null NSC-34 cells. We determined that a 2:1 BCPAP cell:α-TSHR-BioNanofluid conjugate ratio and a 30 second laser exposure killed approximately 60% of the BCPAP cells, while 65% and >70% of cells were ablated using Thyrotropin- and Thyrogen-BioNanofluid conjugates, respectively. Furthermore, minimal non-targeted killing was observed using selective controls.

CONCLUSION

A BioNanofluid platform offering a potential therapeutic path for papillary thyroid cancer has been investigated, with our in vitro results suggesting the development of a potent and rapid method of selective cancer cell killing. Therefore, BioNanofluid treatment emphasizes the need for new technology to treat patients with local recurrence and metastatic disease who are currently undergoing either re-operative neck explorations, repeated administration of radioactive iodine and as a last resort external beam radiation or chemotherapy, with fewer side effects and improved quality of life.

Thyrotropin Receptor Epitope and Human Leukocyte Antigen in Graves' Disease

Graves' disease (GD) is an organ-specific autoimmune disease, and thyrotropin (TSH) receptor (TSHR) is a major autoantigen in this condition. Since the extracellular domain of human TSHR (TSHR-ECD) is shed into the circulation, TSHR-ECD is a preferentially immunogenic portion of TSHR. Both genetic factors and environmental factors contribute to development of GD. Inheritance of human leukocyte antigen (HLA) genes, especially HLA-DR3, is associated with GD. TSHR-ECD protein is endocytosed into antigen-presenting cells (APCs), and processed to TSHR-ECD peptides. These peptide epitopes bind to HLA-class II molecules, and subsequently the complex of HLA-class II and TSHR-ECD epitope is presented to CD4+ T cells. The activated CD4+ T cells secrete cytokines/chemokines that stimulate B-cells to produce TSAb, and in turn hyperthyroidism occurs. Numerous studies have been done to identify T- and B-cell epitopes in TSHR-ECD, including (1) in silico, (2) in vitro, (3) in vivo, and (4) clinical experiments. Murine models of GD and HLA-transgenic mice have played a pivotal role in elucidating the immunological mechanisms. To date, linear or conformational epitopes of TSHR-ECD, as well as the molecular structure of the epitope-binding groove in HLA-DR, were reported to be related to the pathogenesis in GD. Dysfunction of central tolerance in the thymus, or in peripheral tolerance, such as regulatory T cells, could allow development of GD. Novel treatments using TSHR antagonists or mutated TSHR peptides have been reported to be effective. We review and update the role of immunogenic TSHR epitopes and HLA in GD, and offer perspectives on TSHR epitope specific treatments.

Epitope recognition in HLA-DR3 transgenic mice immunized to TSH-R protein or peptides

Development of Graves' disease is related to HLA-DR3. The extracellular domain (ECD) of human TSH receptor (hTSH-R) is a crucial antigen in Graves' disease. hTSH-R peptide 37 (amino acids 78-94) is an important immunogenic peptide in DR3 transgenic mice immunized to hTSH-R. This study examined the epitope recognition in DR3 transgenic mice immunized to hTSH-R protein and evaluated the ability of a mutant hTSH-R peptide to attenuate the immunogenicity of hTSH-R peptide 37. DR3 transgenic mice were immunized to recombinant hTSH-R-ECD protein or peptides. A mutant hTSH-R 37 peptide (ISRIYVSIDATLSQLES: 37 m), in which DR3 binding motif position 5 was mutated V>A, and position 8 Q>S, was synthesized. 37 m should bind to HLA-DR3 but not bind T cell receptors. DR3 transgenic mice were immunized to hTSH-R 37 and 37 m. Mice immunized to hTSH-R-ECD protein developed strong anti-hTSH-R antibody, and antisera reacted strongly with hTSH-R peptides 1-5 (20-94), 21 (258-277), 41 (283-297), 36 (376-389), and 31 (399-418). Strikingly, antisera raised to hTSH-R peptide 37 bound to hTSH-R peptides 1-7 (20-112), 10 (132-50), 33 (137-150), 41, 23 (286-305), 24 (301-320), 36, and 31 as well as to hTSH-R-ECD protein. Both antibody titers to hTSH-R 37 and reaction of splenocytes to hTSH-R 37 were significantly reduced in mice immunized to hTSH-R 37 plus 37 m, compared with mice immunized to hTSH-R 37 alone. The ability of immunization to a single peptide to induce antibodies that bind hTSH-R-ECD protein, and multiple unrelated peptides, is a unique observation. Immunogenic reaction to hTSH-R peptide 37 was partially suppressed by 37 m, and this may contribute to immunotherapy of autoimmune thyroid disease.

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.

Central dogma in thyroid dysfunction: a review on structure modification of TSHR as a cornerstone for thyroid abnormalities

Thyroid stimulating hormone receptor (TSHR) is a vital thyrocyte membrane protein in the thyroid gland. Thyroid Stimulating Hormone (TSH) which is a pituitary hormone is the main stimulator of thyroid gland to produce thyroid hormones, it binds with high affinity to the TSHR through weak bonds including hydrophobic, ionic, hydrogen bonds and trigger the initial steps in thyroid gland stimulation to produce the related hormones. This study was carried out at department of biochemistry of Golestan university of medical sciences. All the related articles related to TSHR modification happened due to mutations and any other alterations which affect the level of TSH-TSHR complex were studied and the main points were extracted out of the pile of information and were organized as present review. TSH-TSHR is the initial and vital step of a long process of thyroid hormone production within the thyroid gland. Any alteration on the TSH-TSHR affinity which may happen due to the direct effect of TSHR modification eventually lead to the serious adverse effects of either hypothyroidism or hyperthyroidism if the TSH-TSHR level are suppressed or elevated, respectively. The prime cause of the thyroid disorders relay on the possible modification on the biochemical structure of TSHR with subsequent alteration on the level of TSH-TSHR complex. TSHR mutation accompanied by biochemical modification, unable it to bind properly to TSH. In some other conditions such mutation leave a TSHR with either of higher affinity towards to TSH or even TSHR which can be activated in the absence of TSH. The structural modification of TSHR and alteration in the level of TSH-TSHR in the thyroid gland eventually lead to thyroid disorders either of hypothyroidism or hyperthyroidism.

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.

Human TSH receptor ligands as pharmacological probes with potential clinical application

The biologic role of thyroid-stimulating hormone (TSH; thyrotropin) as an activator (agonist) of the TSH receptor (TSHR) in the hypothalamic-pituitary-thyroid axis is well known and activation of TSHR by recombinant human TSH is used clinically in patients with thyroid cancer. TSHR ligands other than TSH could be used to probe TSHR biology in thyroidal and extrathyroidal tissues, and potentially be employed in patients. A number of different TSHR ligands have been reported, including TSH analogs, antibodies and small-molecule, drug-like compounds. In this review, we will provide an update on all these classes of TSHR agonists and antagonists but place emphasis on small-molecule ligands.

Autoimmune thyroid diseases: genetic susceptibility of thyroid-specific genes and thyroid autoantigens contributions

Autoimmune thyroid diseases are common polygenic multifactorial disorders with the environment contributing importantly to the emergence of the disease phenotype. Some of the disease manifestations, such as severe thyroid-associated ophthalmopathy, pretibial myxedema and thyroid antigen/antibody immune complex nephritis are unusual to rare. The spectrum of autoimmune thyroid diseases includes: Graves' disease (GD), Hashimoto's thyroiditis (HT), atrophic autoimmune thyroiditis, postpartum thyroiditis, painless thyroiditis unrelated to pregnancy and thyroid-associated ophthalmopathy. This spectrum present contrasts in terms of thyroid function, disease duration and spread to other anatomic location. The genetic basis of autoimmune thyroid disease (AITD) is complex and likely to be due to genes of both large and small effects. In GD the autoimmune process results in the production of thyroid-stimulating antibodies and lead to hyperthyroidism, whereas in HT the end result is destruction of thyroid cells and hypothyroidism. Recent studies in the field of autoimmune thyroid diseases have largely focused on (i) the genes involved in immune response and/or thyroid physiology with could influence susceptibility to disease, (ii) the delineation of B-cell autoepitopes recognized by the main autoantigens, thyroglobulin, thyroperoxidase and TSH receptor, to improve our understanding of how these molecules are seen by the immune system and (iii) the regulatory network controlling the synthesis of thyroid hormones and its dysfunction in AITD. The aim of the present review is to summarize the current knowledge regarding the relation existing between some susceptibility genes, autoantigens and dysfunction of thyroid function during AITD.

TSH receptor autoantibodies

Thyrotropin receptor autoantibodies (TSHR-Abs) of the stimulating variety are the hallmark of Graves' disease. The presence of immune defects leading to synthesis of TSHR-Abs causes hyperthyroidism and is associated with other extrathyroidal manifestations. Further characterization of these antibodies has now been made possible by the generation of monoclonal antibodies with this unique stimulating capacity as well as similar TSHR-Abs not associated with hyperthyroidism. Their present classification divides TSHR-Abs into stimulating, blocking (competing with TSH binding) and neutral (no signaling). Recent studies using monoclonal TSHR-Abs has revealed that stimulating and blocking antibodies bind to the receptor using mostly conformational epitopes, whilst neutral antibodies utilize exclusively linear peptides. Subtle differences in epitopes for stimulating and blocking antibodies account for the diversity of their biological actions. Recently non-classical signaling elicited by neutral antibodies has also been described, raising the need for a new classification of TSHR-Abs.

A human monoclonal autoantibody to the thyrotropin receptor with thyroid-stimulating blocking activity

BACKGROUND

Human monoclonal autoantibodies (MAbs) are valuable tools to study autoimmune responses. To date only one human MAb to the thyrotropin (TSH) receptor (TSHR) with stimulating activity has been available. We now describe the detailed characterization of a blocking type human MAb to the TSHR.

METHODS

A single heterohybridoma cell line was isolated from the peripheral blood lymphocytes of a patient with severe hypothyroidism (TSH 278 mU/L) using standard techniques. The line stably expresses a TSHR autoantibody (5C9; IgG1/kappa). Ability of 5C9 to bind and compete with 125I-TSH or TSHR antibodies binding to the TSHR was tested using tubes coated with solubilized TSHR. Furthermore, the blocking effects of 5C9 on stimulation of cyclic AMP production was assessed using Chinese hamster ovary (CHO) cells expressing the wild-type human TSHR or TSHRs with amino acid mutations.

MAIN OUTCOME

5C9 IgG bound to the TSHR with high affinity (4 x 10(10) L/mol) and inhibited binding of TSH and a thyroid-stimulating human monoclonal autoantibody (M22) to the receptor. 5C9 IgG preparations inhibited the cyclic AMP-stimulating activities of TSH, M22, serum TSHR autoantibodies and thyroid-stimulating mouse monoclonal antibodies. Furthermore 5C9 reduced the constitutive activity of wild-type TSHR and TSHR with some activating mutations. The effect of different amino acid mutations in the TSHR on 5C9 biological activity was studied and TSHR Lys129Ala or Asp203Ala completely abolished the ability of 5C9 to block TSH-mediated stimulation of cyclic AMP production.

CONCLUSIONS

The availability of 5C9 provides new opportunities to investigate the binding and biological activity of TSHR blocking type autoantibodies including studies at the molecular level. Furthermore, monoclonal antibodies such as 5C9 may well provide the basis of new drugs to control TSHR activity including applications in thyroid cancer and Graves' ophthalmopathy.

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 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.

Suppression of thyrotropin receptor constitutive activity by a monoclonal antibody with inverse agonist activity

TSH binding to the TSH receptor (TSHR) induces thyrocyte growth and proliferation primarily by activating the adenylyl cyclase signaling pathway. Relative to the other glycoprotein hormone receptors, the TSHR has considerable ligand-independent (constitutive) activity. We describe a TSHR monoclonal antibody (CS-17) with the previously unrecognized property of being an inverse agonist for TSHR constitutive activity. This property is retained, even when constitutive activity is extremely high consequent to diverse TSHR extracellular region mutations. A similar effect on an activating mutation at the base of the sixth transmembrane helix (not accessible to direct CS-17 contact) indicates that CS-17 is acting allosterically. Administered to mice in vivo, CS-17 reduces serum T(4) levels. The CS-17 epitope is conformational and a significant portion lies in the C-terminal region of the TSHR leucine-rich domain (residues 260-289). By interacting with the large TSHR extracellular domain, CS-17 is, to our knowledge, the first antibody reported to be an inverse agonist for a member of the G protein receptor superfamily. After humanization of its murine constant region, CS-17 has the potential to be an adjunctive therapeutic agent in athyreotic patients with residual well-differentiated thyroid carcinoma as well as pending definitive treatment in some selected hyperthyroidism states.

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.

Evidence that human thyroid cells express uncleaved, single-chain thyrotropin receptors on their surface

The prevailing concept is that, in human thyroid tissue in vivo, all cell-surface TSH receptors (TSHR) cleave into disulfide linked A and B subunits. Because this viewpoint is based on studies using homogenized thyroid tissue and because of TSHR fragility, we studied TSHR subunit structure in intact thyroid cells, primary human thyrocyte cultures, FRTL-5 rat thyroid cells, and WRO (follicular) and NPA (papillary) thyroid cancer cell lines. To overcome the handicap of very low TSHR expression in thyroid cells, we generated a TSHR-expressing adenovirus (TSHR-Ad-RGD) with an integrin-binding RGD motif enabling efficient entry into cells lacking the coxsackie-adenovirus receptor. Two days after TSHR-Ad-RGD infection, [125I]TSH cross-linking to intact cells revealed uncleaved, single-chain TSHR as well as cleaved TSHR A subunits on the surface of all four thyroid cell types. The extent of TSHR cleavage, which is independent of the level of TSHR expression, was consistently lower in the human thyroid cancer cell lines than in the other cell lines. In flow cytometry studies after TSHR-Ad-RGD infection, strong signals were detected in all four thyroid cell types using a monoclonal antibody that primarily recognizes the uncleaved TSHR. Finally, using the same monoclonal antibody, confocal microscopy confirmed the presence of single-chain TSHR on TSHR-Ad-RGD-infected thyroid cells. In summary, TSH covalent cross-linking, flow cytometry, and confocal microscopy demonstrate the presence of uncleaved TSHR on the human thyrocyte surface. These data provide stronger evidence for this alternative than the contrary view based on the finding of only cleaved TSHR in homogenized thyroid cells.

Human thyroid autoantigens and proteins of Yersinia and Borrelia share amino acid sequence homology that includes binding motifs to HLA-DR molecules and T-cell receptor

We previously reported that the spirochete Borrelia burgdorferi could trigger autoimmune thyroid diseases (AITD). Subsequently, we showed local amino acid sequence homology between all human thyroid autoantigens (human thyrotropin receptor [hTSH-R], human thyroglobulin [hTg], human thyroperoxidase [hTPO], human sodium iodide symporter [hNIS]) and Borrelia proteins (n = 6,606), and between hTSH-R and Yersinia enterocolitica (n = 1,153). We have now updated our search of homology with Borrelia (n = 11,198 proteins) and extended our search on Yersinia to the entire species (n = 40,964 proteins). We also searched the homologous human and microbial sequences for peptide-binding motifs of HLA-DR molecules, because a number of these class II major histocompatibility complex (MHC) molecules (DR3, DR4, DR5, DR8, and DR9) are associated with AITD. Significant homologies were found for only 16 Borrelia proteins (5 with hTSH-R, 2 with hTg, 3 with hTPO, and 6 with hNIS) and only 19 Yersinia proteins (4 with hTSH-R, 2 with hTg, 2 with hTPO, and 11 with hNIS). Noteworthy, segments of thyroid autoantigens homologous to these microbial proteins are known to be autoantigenic. Also, the hTSH-R homologous region of one Borrelia protein (OspA) contains an immunodominant epitope that others have found to be homologous to hLFA-1. This is of interest, as the hLFA-1/ICAM-1 ligand/receptor pair is aberrantly expressed in the follicular cells of thyroids affected by Hashimoto's thyroiditis. A computer-assisted search detected antigenic peptide binding motifs to the DR molecules implicated in AITD. In conclusion, our in silico data do not directly demonstrate that Borrelia and Yersinia proteins trigger AITD but suggest that a restricted number of them might have the potential to, at least in persons with certain HLA-DR alleles.

New insights into antibody-mediated hyperthyroidism

The most common cause of hyperthyroidism is Graves' disease, which represents a typical example of an organ-specific autoimmune condition. The exact triggers for the disease remain unknown, but are likely to involve a complex interaction between multiple environmental factors in a genetically predisposed individual. The main feature of the condition is the presence of thyroid-stimulating antibodies, which activate the thyroid- stimulating hormone receptor, resulting in hyperthyroidism. These antibodies may also be involved in the extrathyroidal complications of the disease. The recent generation of thyroid-stimulating antibodies in animal models and the isolation of monoclonal thyroid-stimulating antibodies from a patient with Graves' disease should allow the detailed study of thyroid-stimulating antibodies-thyroid-stimulating hormone receptor interactions. This will help to shed more light on disease pathogenesis and may offer new treatment strategies in difficult cases, particularly in patients with extrathyroidal complications.

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.

Monoclonal antibodies to the thyrotropin receptor

The thyrotropin receptor (TSHR) is a seven transmembrane G-protein linked glycoprotein expressed on the thyroid cell surface and which, under the regulation of TSH, controls the production and secretion of thyroid hormone from the thyroid gland. This membrane protein is also a major target antigen in the autoimmune thyroid diseases. In Graves' disease, autoantibodies to the TSHR (TSHR-Abs) stimulate the TSHR to produce thyroid hormone excessively. In autoimmune thyroid failure, some patients exhibit TSHR-Abs which block TSH action on the receptor. There have been many attempts to generate human stimulating TSHR-mAbs, but to date, only one pathologically relevant human stimulating TSHR-mAb has been isolated. Most mAbs to the TSHR have been derived from rodents immunized with TSHR antigen from bacteria or insect cells. These antigens lacked the native conformation of the TSHR and the resulting mAbs were exclusively blocking or neutral TSHR-mAbs. However, mAbs raised against intact native TSHR antigen have included stimulating mAbs. One such stimulating mAb has demonstrated a number of differences in its regulation of TSHR post-translational processing. These differences are likely to be reflective of TSHR-Abs seen in Graves' disease.

Characteristics of a monoclonal antibody to the thyrotropin receptor that acts as a powerful thyroid-stimulating autoantibody antagonist

Analysis of nine mouse monoclonal antibodies (mAbs) to the thyrotropin receptor (TSHR) with TSH antagonist activity showed that only one of the mAbs (RSR B2) was an effective antagonist of the human thyroid stimulating autoantibody M22. Crystals of B2 Fab were analyzed by x-ray diffraction and a crystal structure at 3.3 A resolution was obtained. The surface charge and topography of the B2 antigen binding site were markedly different from those of the thyroid-stimulating mAb M22 and these differences might contribute to the different properties of the two mAbs. B2 (but not other mouse TSHR-specific mAbs) was also an effective antagonist of thyroid stimulating autoantibody activity in 14 of 14 different sera from patients with Graves' disease. 125I-labeled B2 bound to the TSHR with high affinity (2 x 10(10) L/mol) and patient serum TSHR autoantibodies inhibited labeled B2 binding to the receptor in a similar way to inhibition of labeled TSH binding (r = 0.75; n = 20). Furthermore, labeled B2 binding was inhibited by patient serum TSHR autoantibodies with TSH antagonist activity and also by mouse and human thyroid stimulating mAbs. Overall, mAb B2 is a powerful antagonist of thyroid stimulating autoantibodies (and TSH) thus resembling closely patient serum TSH antagonist TSHR autoantibodies. Furthermore, B2 might have potentially important in vivo applications when tissues containing the TSHR (including those in the orbit) need to be made unresponsive to stimulating autoantibodies.

Thyrotropin receptor antibodies: new insights into their actions and clinical relevance

The thyrotropin receptor (TSHR) is a G-protein-coupled receptor with a large ectodomain. TSH, acting via TSHR, regulates thyroid growth and thyroid hormone production and secretion. The TSHR undergoes complex post-translational processing involving dimerization, intramolecular cleavage, and shedding of its ectodomain, and each of these processes may influence the antigenicity of the TSHR. The TSHR is also the major autoantigen in Graves' disease, as well as a leading candidate autoantigen in both Graves' ophthalmopathy and pretibial myxedema. The naturally conformed TSHR is most effectively presented as an autoantigen to the immune system, causing the production of stimulating TSHR-Abs. There are also autoantibodies which block the TSHR from TSH action, and neutral TSHR-Abs which have no influence on TSH action. TSHR-Abs can be detected by competition assays of TSHR-Abs for labeled TSH, or monoclonal TSHR-Ab binding to solubilized TSHRs, or by bioassays using thyroid cells or mammalian cells expressing recombinant TSHRs.

Analysis of the thyrotropin receptor-thyrotropin interaction by comparative modeling

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

Dissecting linear and conformational epitopes on the native thyrotropin receptor

The TSH receptor (TSHR) is the primary antigen in Graves' disease. In this condition, autoantibodies to the TSHR that have intrinsic thyroid-stimulating activity develop. We studied the epitopes on the native TSHR using polyclonal antisera and monoclonal antibodies (mAbs) derived from an Armenian hamster model of Graves' disease. Of 14 hamster mAbs analyzed, five were shown to bind to conformational epitopes including one mAb with potent thyroid-stimulating activity. Overlapping conformational epitopes were determined by cell-binding competition assays using fluorescently labeled mAbs. We identified two distinct conformational epitopes: epitope A for both stimulating and blocking mAbs and epitope B for only blocking mAbs. Examination of an additional three mouse-derived stimulating TSHR-mAbs also showed exclusive binding to epitope A. The remaining nine hamster-derived mAbs were neutral or low-affinity blocking antibodies that recognized linear epitopes within the TSHR cleaved region (residues 316-366) (epitope C). Serum from the immunized hamsters also recognized conformational epitopes A and B but, in addition, also contained high levels of TSHR-Abs interacting within the linear epitope C region. In summary, these studies indicated that the natively conformed TSHR had a restricted set of epitopes recognized by TSHR-mAbs and that the binding site for stimulating TSHR-Abs was highly conserved. However, high-affinity TSHR-blocking antibodies recognized two conformational epitopes, one of which was indistinguishable from the thyroid-stimulating epitope. Hence, TSHR-stimulating and blocking antibodies cannot be distinguished purely on the basis of their conformational epitope recognition.

Antibodies to TSH-receptor in thyroid autoimmune disease interact with monoclonal antibodies whose epitopes are broadly distributed on the receptor

The hyperthyroidism of Graves' disease (GD) is caused by TSH-receptor (TSH-R) stimulating autoantibodies (TSAb), leading to overproduction of thyroid hormones. We present evidence for TSAb interaction with three distinct regions of the TSH-R, one in immediate vicinity of the carboxy terminal serpentine. Three murine monoclonal antibodies (MoAbs 28.1, A9 and 31.7) directed to amino acids 36-40, 147-228 and 382-415 were labelled and tested for their binding to human recombinant TSH-R on solid phase. All MoAbs bound to TSH-R with a K(d) of 8-12 nm and showed no competition among themselves. We tested 114 sera from euthyroid controls, 118 TBII positive sera from patients with GD (containing TSAb confirmed by bioassays), 16 TBII positive sera from patients with autoimmune thyroid disease (AIT), who were hypothyroid and had TSH blocking antibodies (TBAb), and 20 patients with AIT, who were hypothyroid but negative for all TRAb. Mid-regional MoAb A9 tracer achieved the highest sensitivity in the GD group (72.0%), whereas C-terminal MoAb 31.7 found most sera positive in the AIT group (87.5%). Surprisingly, the N-terminal MoAb 28.1 had the lowest sensitivity in the GD (10.4%) and AIT group (43.8%). Using a mixture of all three tracer MoAbs did not increase the sensitivity in the GD or AIT group, compared to the best single MoAb alone. Median inhibition of MoAb A9 was significantly (P < 0.001) higher than inhibition of MoAbs 28.1 or 31.7 in the group of GD patients but not in other groups. Almost all patient sera with positive reactivity in the MoAb tracer assays had TBII values in the higher range. However, there were many highly TBII positive sera, which did not show a displacement of the MoAb tracers. We conclude that, contrary to some reports, the binding of TSAb and TBAb to the TSH-R is not restricted to distinct and distant epitopes. The middle part of the TSH-R seems to be more relevant for TSAb binding than the N-terminal part, while a proportion of TSAb autoantibodies also binds to a C-terminal epitope of the TSH-R. The method described here is a TSH independent competitive assay for the detection of TSH-R autoantibodies.

The effect of thyrotropin-receptor blocking antibodies on stimulating autoantibodies from patients with Graves' disease

The hyperthyroidism of Graves' disease (GD) is caused by thyrotropin-receptor (TSHR) stimulating autoantibodies (TSAb), which lead to overproduction of thyroid hormones. In this study we tried to block the stimulatory effect of patients' TSAb to the TSHR with monoclonal antibodies (mAbs) and sera from hypothyroid patients. Two groups of blocking mAbs raised by different methods from two independent groups were tested for their ability to inhibit TSH binding to the TSHR, and also the binding of TSAb from the serum of patients with GD. Group 1 mAbs (7E3, 3H10, 4C1, 1B1, 4E9) bind to amino acids 378-387 and group 2 mAbs (23.1 and 31.7) to amino acids 382-415 of the human TSHR. These results were compared to the TSH- and TSAb-inhibiting effect of sera from hypothyroid patients containing bona fide thyroid blocking antibodies (TBAb) without agonistic activity. All studies were done in a conventional cyclic adenosine monophosphate (cAMP) or a modified luciferase reporter gene bioassay. TSH-induced cAMP/luciferase signal was reduced (> 70% inhibition) by all 7 mAbs, verifying the blocking nature. Comparable results (82.2%-96.3% inhibition) were seen when cells were preincubated with 8 TBAb sera. These TBAb sera also inhibited cAMP/luciferase induction of TSAb-positive sera from patients with GD (median of 27 experiments 62.2% inhibition; range, 26.8%-93.9%), and maintained inhibition greater than 20% even when diluted 1:150. However, when mAbs were incubated with these sera, results were heterogeneous: 17 of 30 sera (57%) incubated with mAb 31.7 caused reduced cAMP production compared to incubation with the control antibody, as did 18 of 34 sera (53%) incubated with mAb 7E3, 17 of 33 sera (52%) incubated with mAb 3H10, and 16 of 31 (52%) with mAb 23.1. Mixing all four mAbs did not enhance the cAMP-reductive effect (16/27 sera; 59% inhibited). Inhibition was less pronounced than with TBAb sera (0%-76% of a control antibody) and only present at antibody concentrations greater than 10 microg/mL. We conclude that despite the strong TBAb activity of the mAbs, their effect on TSAb-induced TSHR activation of sera from patients with GD was weaker than that of human TBAb autoantibodies. Thus, the latter are not only strong inhibitors of TSH activity, but also block the stimulatory effect of autoantibodies from patients with GD. However, this effect could not be reproduced by experimental mAbs to the same extent, because it may be the result of a broader spectrum of antibodies present in the TBAb sera, interacting with or in the vicinity of TSAb epitopes. Also of interest, when a TBAb serum was added to a TSAb serum, the TBAb effect was predominant even at high dilutions.

Thyroid-stimulating monoclonal antibodies

Thyrotropin (TSH) receptor monoclonal antibodies (TSHR mAbs) were obtained from cDNA-immunized NMRI mice. Three mAb immunoglobulin Gs (IgGs) (TSmAbs 1-3) that had distinct V(H )and V(L) region sequences stimulated cyclic adenosine monophosphate (cAMP) production in isolated porcine thyroid cells greater than 10x basal and as little as 20 ng/mL (0.13 nmol/L) of TSmAb 1 IgG caused a 2x basal stimulation. TSmAb 1 and 2 Fab fragments were also effective stimulators and thyroid-stimulating activities of the IgGs and Fabs were confirmed using TSHR transfected Chinese hamster ovary (CHO) cells. The TSmAbs also inhibited (125)I-labeled TSH binding to TSHR-coated tubes by 50% or more at concentrations of 1 microg/mL or less and gave 15%-20% inhibition at 20-50 ng/mL. (125)I-labeled TSmAbs bound to TSHR-coated tubes with high affinity (approximately 10(10) L/mol) and this binding was inhibited by TSHR autoantibodies with both TSH agonist and antagonist activities. Inhibition of labeled TSmAb binding by Graves' sera correlated well with inhibition of TSH binding (r = 0.96; n = 18; p < 0.001 for TSmAb 2). The TSmAbs have considerable potential as (1) new probes for TSHR structure-function studies, (2) reagents for new assays for TSHR autoantibodies, and (3) alternatives to recombinant TSH in various in vivo applications.

Characterization of the thyrotropin binding pocket

A panel of monoclonal antibodies (mAbs) to the thyrotropin receptor (TSHR) was prepared using three different immunization strategies. The mAbs obtained (n = 138) reacted with linear epitopes covering most of the TSHR extracellular domain and with conformational epitopes. mAbs that bound to five different regions of the TSHR (amino acids [aa] 32-41, aa 36-42, aa 246-260, aa 277-296, and aa 381-385) were able to inhibit (125)I-labeled thyrotropin (TSH) binding to solubilized TSHR preparations. Fab and immunoglobulin G (IgG) preparations were similarly effective inhibitors for mAbs reactive with aa 246-260, aa 277-291 and aa 381-385 suggesting that these three regions of the TSHR are involved in TSH binding. In contrast mAbs reactive with aa 32-41 and aa 36-42 were not effective at inhibiting TSH binding when Fab preparations were used, suggesting that these N terminal regions of the TSHR were less critical for TSH binding. Our studies suggest that three distinct and discontinuous regions of the TSHR (aa 246-260 and 277-296 on the TSHR A subunit) and aa 381-385 (on the TSHR B subunit) fold together to form a complex TSH binding pocket. Alignment of the aa sequences of these three regions in TSHRs from different species indicates that they are highly conserved.

A monoclonal thyroid-stimulating antibody

The thyrotropin receptor, also known as the thyroid-stimulating hormone receptor (TSHR), is the primary antigen of Graves disease. Stimulating TSHR antibodies are the cause of thyroid overstimulation and were originally called long-acting thyroid stimulators due to their prolonged action. Here we report the successful cloning and characterization of a monoclonal antibody (MS-1) with TSHR-stimulating activity. The thyroid-stimulating activity of MS-1 was evident at IgG concentrations as low as 20 ng/ml. MS-1 also competed for radiolabeled TSH binding to the native TSHR and was able to compete for TSH-induced stimulation. MS-1 recognized a conformational epitope within the TSHR alpha (or A) subunit but excluding the receptor cleavage region. Using an assay measuring loss of antibody recognition after cleavage we demonstrated that MS-1, in contrast to TSH, was unable to enhance TSHR posttranslational cleavage. Since receptor cleavage is followed by alpha subunit shedding and receptor degradation, the functional half-life of the receptor may be extended. The isolation and characterization of MS-1 provides a novel explanation for the prolonged thyroid stimulation in this disease which may be secondary to the lack of receptor cleavage in addition to the prolonged half-life of IgG itself.

Cross-reactivity of a monoclonal antibody to the amino terminal region of thyrotropin receptor with the serum protein alpha(1)-antitrypsin

In a study designed to detect the presence of soluble, secreted A subunit of thyrotropin hormone receptor (TSHR) in serum, using anti-TSHR murine antibodies (mAbs) and peptide specific antiserum for Western blotting of human serum proteins fractionated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) it was consistently observed that only one mAb, termed A10, reacted strongly with the 53 kd serum protein. The reaction was specific with the mAb A10 only, but not with another mAb or polyclonal antiserum. Furthermore, A10 immunoreactivity was documented in a variety of sera from healthy donors and patients, including patients whose thyroid gland was ablated during treatment for thyroid cancer. This suggests that the A10 cross-reactive protein was not derived from thyroid cells. The A10 cross-reactive protein was purified from normal serum and subjected to N-terminal sequence analysis, which identified the protein as alpha(1)-antitrypsin. Further experiments by enzyme-linked immunosorbent assay (ELISA) and the binding of antibody with deglycosylated or elastase-treated purified serum protein confirmed the cross-reactivity of mAb A10 with alpha(1)-antitrypsin. Alignment of the TSHR amino acid sequence with that of alpha(1)-antitrypsin identified five identical amino acids in a short stretch of residues 34-39 (EEDFRV) in TSHR and residues 205-210 (EEDFHV) in alpha(1)-antitrypsin. Analysis of the structural model of alpha(1)-antitrypsin revealed that these residues were exposed on the surface of alpha(1)-antitrypsin and were accessible for antibodies. Autoantibodies in patients with Graves' disease do not appear to recognize this region of the receptor and hence do not react with serum alpha(1)-antitrypsin.

Thyroid-stimulating hormone and thyroid-stimulating hormone receptor structure-function relationships

This review focuses on recent advances in the structure-function relationships of thyroid-stimulating hormone (TSH) and its receptor. TSH is a member of the glycoprotein hormone family constituting a subset of the cystine-knot growth factor superfamily. TSH is produced by the pituitary thyrotrophs and released to the circulation in a pulsatile manner. It stimulates thyroid functions using specific membrane TSH receptor (TSHR) that belongs to the superfamily of G protein-coupled receptors (GPCRs). New insights into the structure-function relationships of TSH permitted better understanding of the role of specific protein and carbohydrate domains in the synthesis, bioactivity, and clearance of this hormone. Recent progress in studies on TSHR as well as studies on the other GPCRs provided new clues regarding the molecular mechanisms of receptor activation. Such advances are a result of extensive site-directed mutagenesis, peptide and antibody approaches, detailed sequence analyses, and molecular modeling as well as studies on naturally occurring gain- and loss-of-function mutations. This review integrates expanding information on TSH and TSHR structure-function relationships and summarizes current concepts on ligand-dependent and -independent TSHR activation. Special emphasis has been placed on TSH domains involved in receptor recognition, constitutive activity of TSHR, new insights into the evolution of TSH bioactivity, and the development of high-affinity TSH analogs. Such structural, physiological, pathophysiological, evolutionary, and therapeutic implications of TSH-TSHR structure-function studies are frequently discussed in relation to concomitant progress made in studies on gonadotropins and their receptors.

Antithyrotropin receptor antibody: an update

Numerous studies have reported the characteristics and significance concerning antithyrotropin receptor antibodies (TSHR-Abs), which cause Graves' disease and in some cases primary hypothyroidism. However, many unsolved questions concerning those antibodies remain. Here, recent developments in the study of TSHR-Abs are reviewed based on three aspects: mechanisms of TSHR-Ab production, antibody binding epitopes, and clinical TSHR-Ab assays. Mechanisms of TSHR-Ab production are discussed from five points of view: aberrant expression of the major histocompatibility complex, dysregulation of T cells, molecular mimicry, bystander effect, and expansion of autoreactive B cells. Regarding epitopes, unique TSHR-Abs have been reported that may explain the complicated pathophysiology of patients with TSHR-Ab diseases. Finally, recent efforts to improve TSHR-Ab measurements are introduced. Such efforts will contribute to clinical examinations and treatments for thyroid diseases as well as experimental methods of thyroidology.

Oligomerization of the human thyrotropin receptor: fluorescent protein-tagged hTSHR reveals post-translational complexes

To examine thyrotropin (TSH) receptor homophilic interactions we fused the human TSH receptor (hTSHR) carboxyl terminus to green fluorescent protein (GFP) and the corresponding chimeric cDNA was expressed in Chinese hamster ovary cells. Fluorescent TSH receptors on the plasma membrane were functional as assessed by TSH-induced cAMP synthesis. The binding of TSH, as well as TSHR autoantibodies, induced time- and dose-dependent receptor capping. Fluorescence resonance energy transfer between receptors differentially tagged with GFP variants (RFP and YFP) provided evidence for the close proximity of individual receptor molecules. This was consistent with previous studies demonstrating the presence of TSHR dimers and oligomers in thyroid tissue. Co-immunoprecipitation of GFP-tagged and Myc-tagged receptor complexes was performed using doubly transfected cells with Myc antibody. Western blotting of the immunoprecipitated complex revealed the absence of noncleaved TSH holoreceptors. This further suggested that cleavage of the holoreceptor into its two-subunit structure, comprising disulfide-linked TSHR-alpha and TSHR-beta subunits, was required for the formation of TSHR dimers and higher order complexes.

Reactivity of thyrotropin receptor autoantibodies with the thyrotropin receptor on western blots

Affinity purified recombinant human thyrotropin receptor (TSHR) was run on sodium dodecyl sulfate (SDS) gels and subjected to a renaturing and blotting procedure. Twenty sera from thyrotropin receptor autoantibodies (TRAb)-positive patients with a history of hyperthyroidism and 20 sera with high levels of TSH blocking activity were analyzed. Four of 20 sera with blocking-type of TRAb (i.e., TSH antagonist activity) were able to recognize the mature, fully glycosylated 120-kd form of the receptor on blots of gels run under reducing conditions. No sera recognized the 100-kd high mannose precursor form of the TSHR. Three of the four recognized a 74-kd band and 2 of the 4 recognized a 50-kd band. These bands are probably proteolytic cleavage fragments of the mature 120-kd TSHR. In the absence of reducing agent the same 4 of 20 sera described above together with a further serum sample (i.e., 5/20 in total) reacted with the 120-kd form of the receptor. No specific reaction with the TSHR was observed on Western blots with the remaining 15 sera with TSH blocking activity, nor with 20 sera from patients with a history of hyperthyroidism, nor with sera from 10 healthy blood donors, 10 Hashimoto sera (negative for TRAb) and 10 systemic lupus erythematosus sera. No clear differences were observed in the TRAb positive sera that were reactive and nonreactive on Western blots in terms of their ability to inhibit TSH binding or to immunoprecipitate 125I-labeled TSHR. Overall, our results indicate that the mature 120-kd form of the TSHR that is principally responsible for binding TSH is also responsible for binding TRAb (when this binding can be detected). These observations together with immunoprecipitation and TSH binding inhibition studies, emphasize the close relationship between the receptor's binding sites for TSH and TRAb.