Functional assessment of the thyrotropin receptor-beta subunit

Simultaneous induction of Graves' hyperthyroidism and Graves' ophthalmopathy by TSHR genetic immunization in BALB/c mice

BACKGROUND

Graves' disease is the most common form of autoimmune thyroid disorder, characterized by hyperthyroidism due to circulating autoantibodies. To address the pathological features and establish a therapeutic approach of this disease, an animal model carrying the phenotype of Graves' disease (GD) in concert with Graves' Ophthalmopathy (GO) will be very important. However, there are no ideal animal models that are currently available. The aim of the present study is to establish an animal model of GD and GO disease, and its pathological features were further characterized.

METHODS

A recombinant plasmid pcDNA3.1- T289 was constructed by inserting the TSHR A-subunit gene into the expression vector pcDNA3.1, and genetic immunization was successfully performed by intramuscular injection of the plasmid pcDNA3.1-T289 on female 8-week-old BALB/c mice. Each injection was immediately followed by in vivo electroporation using ECM830 square wave electroporator. Morphological changes of the eyes were examined using 7.0T MRI scanner. Levels of serum T4 and TSHR antibodies (TRAb) were assessed by ELISA. The pathological changes of the thyroid and orbital tissues were examined by histological staining such as H&E staining and Alcian blue staining.

RESULTS

More than 90% of the immunized mice spontaneously developed goiter, and about 80% of the immunized mice manifested increased serum T4 and TRAb levels, combined with hypertrophy and hyperplasia of thyroid follicles. A significantly increased synthesis of hyaluronic acid was detected in in the immunized mice compared with the control groups.

CONCLUSION

We have successfully established an animal model manifesting Graves' hyperthyroidism and ophthalmopathy, which provides a useful tool for future study of the pathological features and the development of novel therapies of the diseases.

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

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

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

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

Transmembrane domains of attraction on the TSH receptor

The TSH receptor (TSHR) has the propensity to form dimers and oligomers. Our data using ectodomain-truncated TSHRs indicated that the predominant interfaces for oligomerization reside in the transmembrane (TM) domain. To map the potentially interacting residues, we first performed in silico studies of the TSHR transmembrane domain using a homology model and using Brownian dynamics (BD). The cluster of dimer conformations obtained from BD analysis indicated that TM1 made contact with TM4 and two residues in TM2 made contact with TM5. To confirm the proximity of these contact residues, we then generated cysteine mutants at all six contact residues predicted by the BD analysis and performed cysteine cross-linking studies. These results showed that the predicted helices in the protomer were indeed involved in proximity interactions. Furthermore, an alternative experimental approach, receptor truncation experiments and LH receptor sequence substitution experiments, identified TM1 harboring a major region involved in TSHR oligomerization, in agreement with the conclusion from the cross-linking studies. Point mutations of the predicted interacting residues did not yield a substantial decrease in oligomerization, unlike the truncation of the TM1, so we concluded that constitutive oligomerization must involve interfaces forming domains of attraction in a cooperative manner that is not dominated by interactions between specific residues.

Targeting the thyrotropin receptor in thyroid disease

INTRODUCTION

The thyrotropin receptor (TSHR) is essential for thyroid growth and for the production of thyroid hormones. It is unique among the glycoprotein hormone receptors, in that some of the TSHRs undergo cleavage and shedding of the alpha subunit.

AREAS COVERED

This review discusses the structure and function of the TSHR, followed by an evaluation of its role in thyroid disease. Possible limitations of the TSHR as a therapeutic target are also discussed.

EXPERT OPINION

The TSHR is involved in a number of hereditary and acquired disorders of the thyroid making it of potential importance as a therapeutic target in thyroid disease. Expression of the TSHR in several non-thyroidal tissues and the development of systemic manifestations of thyroid disease suggest that the TSHR is also of interest as a therapeutic target outside the thyroid.

Subunit interactions influence TSHR multimerization

The TSH receptor (TSHR) is the key molecule influencing thyroid growth and development and is an antigenic target in autoimmune thyroid disease. The TSHR exists in monomeric and multimeric forms, and it has been shown previously that multimeric complexes of the TSHR preferentially localize in lipid rafts. However, unlike other glycoprotein hormone receptors, the TSHR exists in several forms on the cell membrane due to intramolecular cleavage of its ectodomain, which causes the production of α- and β-subunits of various lengths. After cleavage and reduction of disulfide bonds, α-subunits consisting of the receptor ectodomain may be lost from the cell surface by receptor shedding, leading to accumulation of excess β-subunits within the membrane. Because cell surface expression of these various forms of the TSHR is critical to receptor signaling and autoimmune responses, we set out to model the influence of β-subunits on full-length TSHRs. To study this interaction, we generated three truncated ectodomain β-subunits linked to green fluorescent protein (named β-316, -366, and -409) as examples of native cleaved forms of the TSHR. These constructs were transfected into human embryonic kidney 293 cells in the presence and absence of the full-length receptor. Whereas the β-316 and β-366 forms showed cell surface expression, the expression of β-409 was primarily intracellular. Cotransfection of the β-subunits with a full-length hemagglutinin-tagged wild-type (WT) receptor (HT-WT-TSHR) in both transient and stable systems caused a significant decrease in surface expression of the full-length WT receptors. This decrease was not seen with control plasmid consisting of a plasma membrane-targeted protein tagged to red fluorescent protein. To ascertain if this response was due to homointeraction of the truncated β-constructs with the WT-TSHRs, we immunoprecipitated membranes prepared from the cotransfected cells using antihemagglutinin and then probed with anti-green fluorescent protein. These studies confirmed dimerization of the β-subunits with the WT full-length receptor, and this interaction was further observed in vivo by fluorescence resonance energy transfer. We then studied the functional consequences of this interaction on TSHR signaling by examining Gαs-mediated signals. The well-expressed truncated constructs, when coexpressed with full-length TSHR, did not alter constitutive cAMP levels, but there was a significant decrease in TSH-induced cAMP generation. Furthermore, we observed that truncated β-316 and β-366 had faster internalization rate, which may lead to a significant decrease in the expression of the full-length receptor on the cell surface, thus contributing to the decreased signaling response. However, the decrease in surface receptors may also be due to inhibition of newly formed receptors reaching the surface as result of receptor-receptor interaction. It is well known that under normal physiological conditions both cleaved and uncleaved TSHR forms coexist on the cell surface of normal thyrocytes. Our studies allow us to conclude, therefore, that multimerization of cleaved/ truncated forms of the β-subunits with the full-length TSHR has a profound influence on TSHR internalization and signaling. Hence, the degree of intramolecular cleavage must also modulate TSHR signaling.

The cleavage of thyroid-stimulating hormone receptor is dependent on cell-cell contacts and regulates the hormonal stimulation of phospholipase c

Thyroid-stimulating hormone receptor (TSHR) consists of a hormone-binding extracellular subunit and a seven-transmembrane spanning subunit that interacts with the G proteins G(alphas) and G(alphaq). The two subunits, generated by proteolytic cleavage of a single polypeptide chain, are held together by disulphide bridges. The receptor is completely cleaved in thyroid tissue, while in cultured cells (thyrocytes and non-thyroid cells) the cleaved and uncleaved forms coexist. The reasons for these divergent data are not understood. Here we provide an explanation by showing that cleavage depends on cell-cell contacts. An almost complete cleavage was observed in confluent cells, while in sparse cells most of the receptor was in the uncleaved form. We also show that coupling of TSHR to G(alphaq) (as measured by inositolphosphate generation) is markedly reduced when the receptor is not cleaved. In contrast, coupling to G(alphas) [as measured by cyclic adenosine 3',5'-monophosphate (cAMP) synthesis] is unaffected by cleavage of the receptor. These results suggest that the cell-cell contacts are necessary for cleavage of the receptor, which acts as a regulatory step in inositolphosphate production via phospholipase C activation. The latter observation was confirmed using cells that express the uncleavable mutant TSHR-delta50-NET, for which the TSH-stimulated inositolphosphate production was completely abolished.

Clinical review about TRAb assay's history

Commercial assays to measure thyroid stimulating hormone (TSH) receptor (TSHR) autoantibodies (TRAb) have been available for the serological diagnosis of autoimmune thyroid diseases (AITD) for several years. The widespread assessment of this parameter has identified Graves' disease (GD) as a common organ-specific autoimmune disease. Within the present article we aim to review immunobiological and epidemiological aspects as well as diagnostic methods available for the detection of TRAb. Over the last decade, TRAb detection in GD became more sensitive since TRAb assays were being largely improved by named research groups. Therefore, functional assay (fas) and diagnostic sensitivity of current TRAb assays will be discussed. Within the second part of this review we will focus on clinical applications of TRAb measurement for outcome prediction of GD as well as the importance of this method to distinguish GD from other AITD.

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.

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

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

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

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