Characterization of human thyrotropin receptor-related peptide-like immunoreactivity in peripheral blood of Graves' disease

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Withdrawn: TSH Receptor Cleavage Into Subunits and Shedding of the A-Subunit; A Molecular and Clinical Perspective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

Other miscellaneous hormone binding proteins: attempt at an epilogue

An overview of the detection, mechanism of formation and clinical function of hormone binding proteins shedded from the membrane receptor and detected in the last twenty years is presented. The representatives of such binding proteins are restricted only to human soluble receptors that have been already detected in blood or other intravasal fluids such as soluble receptors for LH/hCG, prolactin, TSH, erythropoietin, insulin and IGF-I. The clinical or diagnostic significance of these putative-detectable or indeed circulating proteins often remains largely unclear.

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.

Identification of unique thyrotropin receptor (TSHR) splice variants in the chicken: the chicken TSHR gene revisited

We previously described the cloning of the full-length chicken thyrotropin receptor (TSHRa) and two splice variants, lacking exon 3 (TSHRb) or both exons 2 and 3 (TSHRc). Here we report the identification of three novel splice variants of the chicken TSHR, named TSHRd, -e and -f, differing in their C-terminal region and containing unique exonic sequences that are not present in the other TSHR variants. This finding suggests a TSHR gene structure with 13 rather than the previously assumed 10 exons. The three novel exons appear to be chicken-specific, as no equivalents of these exons were found in other vertebrate genomes. Like the full-length receptor, the five TSHR splice variants are most abundantly expressed in thyroid gland. TSHRb, -d, -e and -f mRNA was also present in virtually all extra-thyroidal tissues expressing TSHRa, whereas TSHRc shows a more restricted tissue distribution. Whether these receptor transcripts are translated to functional proteins needs to be verified, but if so, they could be attributed various physiological roles.

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.

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

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

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.

Structure and function of the TSH receptor: its suitability as a target for radiotherapy

Systemic radiotherapy, such as radioimmunotherapy, is an exciting and rapidly growing field of medical therapeutics for a variety of solid and diffuse human malignancies. This therapy involves the systemic administration of a radionuclide, attached to a carrier ligand (such as hormone analogue or monoclonal antibody), which becomes directed at the tumor through a target receptor or antigen that resides within the malignant tissue. The thyrotropin receptor (TSHr) is a membrane-bound glycoprotein through which the pituitary communicates with thyroid follicular cells. Because it is a thyroid-specific protein and is expressed frequently in differentiated thyroid cancers, it is a potential candidate target for systemic radiotherapy of these malignancies. I will examine the general structure of TSHr and its potential utility such as a target. Several obstacles regarding the concentration and distribution of TSHr as well as the availability of a suitable carrier ligand must be overcome before radioimmunotherapy of thyroid cancers using TSHr as target becomes a reality.

TSH-receptor expression and human thyroid disease: relation to clinical, endocrine, and molecular thyroid parameters

Thyrotropin receptor (TSH-R) gene expression can be positively or negatively regulated by TSH and stimulating TSH-R antibodies (TSAbs) in immortalized thyroid cell lines such as rat FRTL-5 cells. However, regulation is less clear in other mammalian cells including cultures of human thyroid cells. Additionally, it has been suggested, based on FRTL-5 cell data, that TSH-R gene negative regulation by TSH or TSAbs might be lost in Graves' disease. The present study evaluated TSH-R gene transcript levels in thyroids from patients with Graves' disease to correlate in vivo data with in vitro observations or hypotheses. TSH-R mRNA levels were characterized in a total of 66 human thyroid glands with particular concern to levels in Graves' patients. Results were related to clinical parameters, transcript levels of thyroglobulin (TG), and thyroid peroxidase (TPO), as well as transcript levels of thyroid transcription factor 1 (TTF-1) which regulates the expression of all three genes and paired box-gene 8 (Pax-8) which regulates TG and TPO gene expression. Northern blot analyses showed that TSH-R expression was significantly increased, 2.2-fold, in Graves' thyroids (p = 0.0098, n = 35) by comparison to normals (n = 6). TSH-R mRNA levels were decreased to 30% and 7% of normal levels in Hashimoto's thyroids (p = 0.0281, n = 5) and anaplastic carcinomas (p = 0.0033, n = 6), respectively. No significant changes were seen in endemic goiters (n = 8) and in thyroid autonomy (n = 6). TSH-R RNA levels were higher, 3.6-fold, in thyroids of a subgroup of Graves' patients that had not been pretreated with iodide before surgery (n = 10) by comparison to thyroids from those that had been treated before surgery, 1.7-fold (n = 25). TSH-R antibodies exhibited a nonsignificant tendency toward a negative correlation. All other clinical or endocrine parameters showed no clear relation to TSH-R mRNA levels. Pax-8 and TTF-1 transcripts were detectable in normal thyroids; however, Pax-8 expression was increased in Graves' thyroids (3.8-fold), whereas TTF-1 expression was only minimally changed in all thyroids investigated. Changes of the two did not correlate. Pax-8 expression correlated with TG and TPO expression (in all cases, p = 0.0001); TTF-1, despite its minimal change, still correlated with TG (p = 0.0471) but not with TPO expression (p = 0.0984). TTF-1, again despite its minimal changes, correlated positively with TSH-R gene expression (p = 0.0251); however, surprisingly, Pax-8, which does not regulate TSH-R gene expression, correlated even better with TSH-R transcript levels (p = 0.0001). We conclude that augmentation of TSH-R expression levels, and thus potential ligand binding sites, may indicate an important regulatory principle in the pathogenesis of autoimmune hyperthyroidism in vivo: the responsiveness of the TSH-R to TSH and TSAb induced negative regulation is lost. This increase of TSH-R expression levels is not due to an ongoing transcriptional activation of the TTF-1 gene. Pax-8, though positively correlated with TSH-R RNA levels, cannot be the factor either, because Pax-8 does not upregulate TSH-R expression. This predicts that other factors involved in TSH-R induced negative regulation are abnormal and must be searched for and evaluated.

Thymic hyperplasia in patients with Graves' disease. Identification of thyrotropin receptors in human thymus

Thymic size and density were studied in 23 untreated patients with Graves' disease and 38 control subjects using computed tomography. Both thymic size and density were higher in untreated patients with Graves' disease than in control subjects in the age-matched group. After treatment with antithyroid drugs, both thymic size and density were significantly reduced, with a concomitant decrease in thyrotropin receptor antibodies. PCR of human thymic cDNA using primers for human thyrotropin receptor amplified a fragment in a size expected for the receptor, and its nucleotide sequence was identical to human thyrotropin receptor cDNA in the thyroid. Northern blot analysis of human thymic poly(A)+ RNA demonstrated the presence of the full length form of thyrotropin receptor mRNA. Western blot analysis of human thymic membrane using anti-thyrotropin receptor peptide antibodies demonstrated a band of 100 kD that was also observed in the thyroid membrane. Immunohistochemistry of thymic tissue using mouse antihuman thyrotropin receptor monoclonal antibodies demonstrated the immunostaining of epithelial cells. These results indicate that thymic hyperplasia is apparently associated with Graves' disease and suggest that thymic thyrotropin receptor may act as an autoantigen that may be involved in the pathophysiology of development of Graves' disease.

Shedding of human thyrotropin receptor ectodomain. Involvement of a matrix metalloprotease

The thyrotropin (TSH) receptor in human thyroid glands has been shown to be cleaved into an extracellular alpha subunit and a transmembrane beta subunit held together by disulfide bridges. An excess of the latter component relative to the former suggested the shedding of the ectodomain. Indeed we observed such a shedding in cultures of human thyrocytes and permanently transfected L or Chinese hamster ovary cells. The shedding was increased by inhibitors of endocytosis, recycling, and lysosomal degradation, suggesting that it was dependent on receptor residency at the cell surface. It was slightly increased by TSH and phorbol esters, whereas forskolin and 8-bromo-cyclic AMP were without effect. Decreasing the serum concentration in cell culture medium enhanced the shedding by an unknown mechanism. The shedding of the TSH receptor alpha domain is the consequence of two events: cleavage of the receptor into alpha and beta subunits and reduction of the disulfide bridge(s). The complete inhibition of soluble TSH receptor shedding by the specific inhibitor BB-2116 indicated that the cleavage reaction is catalyzed probably at the cell surface by a matrix metalloprotease. This shedding mechanism may be responsible for the presence of soluble TSH receptor alpha subunit in human circulation.

Novel splicing variants of the human thyrotropin receptor encode truncated polypeptides without a membrane-spanning domain

The thyrotropin receptor is of fundamental importance to normal thyroid function and is considered to be the predominant antigen affected by the autoantibodies of Graves' autoimmune hyperthyroidism. The identification of the epitopes on the receptor to which the autoantibodies bind or the mechanism by which the autoantibodies arise remain to be established. In this report we have analysed in detail thein vivo transcription of the human TSH receptor gene (hTSH-R), demonstrating the presence of numerous novel TSH receptor transcripts. Northern blot analysis of mRNA from human thyroid tissue using a radiolabelled cDNA probe specific for the extracellular domain of the hTSH-R revealed the presence of small polyadenylated mRNAs, in addition to the full-length hTSH-R mRNA. A PCR strategy devised to clone transcripts with 3' polyadenylation and 5' hTSH-R specific sequences was used to clone five different hTSH-R transcripts (hTSH-R. ST1 to ST5; 250bp-1.7 kb) from human thyroid tissue. Sequence analysis demonstrated that the small transcripts arose by alternative splicing of the hTSH-R mRNA. The transcripts were associated with polysomes and were demonstrated in human thyroid tissue from patients suffering from Graves' disease, sporadic goiter as well as in healthy lobes of thyroid tissue.In situ hybridization demonstrated that two of the alternative transcripts adopted a tissue distribution pattern identical to that of the full-length hTSH-R transcript. The two major truncated transcripts ST4 and ST5 contained unique sequences at the 3' end of the mRNAs and thus potentially represent the molecular origin of soluble TSH receptor variants which have been postulated on numerous occasions.

Is TPO detectable in the circulation?

Recent reports have suggested that thyroid peroxidase (TPO) can be detected in the circulation of normal subjects and of patients with Graves' disease and we have attempted to confirm and extend these observations. A TPO radioimmunoassay with a sensitivity of 1 ng/mL was used to measure TPO in the sera from 20 normal subjects and 21 patients with Graves' disease. In addition, TPO was measured in serum samples from six normal subjects before and after oral TRH. We were unable to detect TPO in 46 out of the 47 sera studied (normals and autoimmune thyroid disease). In the one remaining serum (from a normal subject), low levels of TPO were apparently detected, but we demonstrated that this result was due to assay interference from TPO autoantibodies. Overall our studies suggest that (1) thyroid peroxidase is not detectable in normal subjects nor in TPO autoantibody negative patients with Graves' disease; (2) endogenous TPO autoantibodies can interfere in the TPO radioimmunoassay leading to false positive results; and (3) an acute increase of TSH in normal subjects does not result in TPO release into the circulation.

Molecular cloning and sequencing of an alternatively spliced form of the human thyrotropin receptor transcript

In the present study, we report the molecular cloning and sequencing of an alternatively spliced form of the human thyrotropin receptor (hTSHR) mRNA transcript, which has previously been detected on Northern blot analysis of human thyroid cells. The smaller hTSHR cDNA, designated hTSHR cDNA-I, is approximately 1 kb in size and encodes a protein of 253 amino acids. Comparison of the nucleotide sequence of hTSHR cDNA-I with available hTSHR genomic sequence data reveals that the cDNA-I contains exons 1-8 and unidentified DNA tract, presumably an intron. Thus, the hTSHR cDNA-I encodes for the N-terminal half of the extracellular domain of the hTSHR (approximately 60%). The truncated TSHR-I may be secreted and function as a TSH binding protein.