Production of bioactive amino-terminal domain of the thyrotropin receptor via insertion in the plasma membrane by a glycosylphosphatidylinositol anchor

Thyrotropin Receptor p.N432D Retained Variant Is Degraded Through an Alternative Lysosomal/Autophagosomal Pathway and Can Be Functionally Rescued by Chemical Chaperones

Background: Loss-of-function mutations of thyrotropin receptor (TSHR) are one of the main causes of congenital hypothyroidism. As for many disease-associated G-protein coupled receptors (GPCRs), these mutations often affect the correct trafficking and maturation of the receptor, thus impairing the expression on the cell surface. Several retained GPCR mutants are able to effectively bind their ligands and to transduce signals when they are forced to the cell surface by degradation inhibition or by treatment with chaperones. Despite the large number of well-characterized retained TSHR mutants, no attempts have been made for rescue. Further, little is known about TSHR degradation pathways. We hypothesize that, similar to other GPCRs, TSHR retained mutants may be at least partially functional if their maturation and membrane expression is facilitated by chaperones or degradation inhibitors. Methods: We performed in silico predictions of the functionality of known TSHR variants and compared the results with available in vitro data. Western blot, confocal microscopy, enzyme-linked immunosorbent assays, and dual luciferase assays were used to investigate the effects of degradation pathways inhibition and of chemical chaperone treatments on TSHR variants' maturation and functionality. Results: We found a high discordance rate between in silico predictions and in vitro data for retained TSHR variants, a fact indicative of a conserved potential to initiate signal transduction if these mutants were expressed on the cell surface. We show experimentally that some maturation defective TSHR mutants are able to effectively transduce Gs/cAMP signaling if their maturation and expression are enhanced by using chemical chaperones. Further, through the characterization of the intracellular retained p.N432D variant, we provide new insights on the TSHR degradation mechanism, as our results suggest that aggregation-prone mutant can be directed toward the autophagosomal pathway instead of the canonical proteasome system. Conclusions: Our study reveals alternative pathways for TSHR degradation. Retained TSHR variants can be functional when expressed on the cell surface membrane, thus opening the possibility of further studies on the pharmacological modulation of TSHR expression and functionality in patients in whom TSHR signaling is disrupted.

Deleting the Redundant TSH Receptor C-Peptide Region Permits Generation of the Conformationally Intact Extracellular Domain by Insect Cells

The TSH receptor (TSHR) extracellular domain (ECD) comprises a N-terminal leucine-rich repeat domain and an hinge region (HR), the latter contributing to ligand binding and critical for receptor activation. The crystal structure of the leucine-rich repeat domain component has been solved, but previous attempts to generate conformationally intact complete ECD or the isolated HR component for structural analysis have failed. The TSHR HR contains a C-peptide segment that is removed during spontaneous TSHR intramolecular cleavage into disulfide linked A- and B-subunits. We hypothesized that deletion of the redundant C-peptide would overcome the obstacle to generating conformationally intact TSHR ECD protein. Indeed, lacking the C-peptide region, the TSHR ECD (termed ECD-D1) and the isolated HR (termed HR-D1) were secreted into medium of insect cells infected with baculoviruses coding for these modified proteins. The identities of TSHR ECD-D1 and HR-D1 were confirmed by ELISA and immunoblotting using TSHR-specific monoclonal antibodies. The TSHR-ECD-D1 in conditioned medium was folded correctly, as demonstrated by its ability to inhibit radiolabeled TSH binding to the TSH holoreceptor. The TSHR ECD-D1 purification was accomplished in a single step using a TSHR monoclonal antibody affinity column, whereas the HR-D1 required a multistep protocol with a low yield. In conclusion, we report a novel approach to generate the TSHR ECD, as well as the isolated HR in insect cells, the former in sufficient amounts for structural studies. However, such studies will require previous complexing of the ECD with a ligand such as TSH or a thyroid-stimulating antibody.

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

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

CODA-RET reveals functional selectivity as a result of GPCR heteromerization

Here we present a novel method that combines protein complementation with resonance energy transfer to study conformational changes in response to activation of a defined G protein-coupled receptor heteromer, and we apply the approach to the putative dopamine D1-D2 receptor heteromer. Remarkably, the potency of the D2 receptor (D2R) agonist R(–)-Propylnorapomorphine (NPA) to change the Gαi conformation via the D2R protomer in the D1-D2 heteromer was enhanced 10-fold relative to that observed in the D2R homomer. In contrast, the potencies of the D2R agonists dopamine and quinpirole were the same in the homomer and heteromer. Thus, we have uncovered a molecular mechanism for functional selectivity, in which a drug acts differently at a GPCR protomer depending on the identity of the second protomer that participates in forming the signaling unit, opening the door to enhanced pharmacological specificity through targeting differences between homomeric and heteromeric signaling.

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

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

Endocrine autoimmune disease: genetics become complex

The endocrine system is a frequent target in pathogenic autoimmune responses. Type 1 diabetes and autoimmune thyroid disease are the prevailing examples. When several diseases cluster together in one individual, the phenomenon is called autoimmune polyglandular syndrome. Progress has been made in understanding the genetic factors involved in endocrine autoimmune diseases. Studies on monogenic autoimmune diseases such as autoimmune polyglandular syndrome type 1, immunodysregulation, polyendocrinopathy, enteropathy, X-linked and primary immune deficiencies helped uncover the role of key regulators in the preservation of immune tolerance. Alleles of the major histocompatibility complex have been known to contribute to the susceptibility to most forms of autoimmunity for more than 3 decades. Furthermore, sequencing studies revealed three non-major histocompatibility complex loci and some disease specific loci, which control T lymphocyte activation or signalling. Recent genome-wide association studies (GWAS) have enabled acceleration in the identification of novel (non-HLA) loci and hence other relevant immune response pathways. Interestingly, several loci are shared between autoimmune diseases, and surprisingly some work in opposite direction. This means that the same allele which predisposes to a certain autoimmune disease can be protective in another. Well powered GWAS in type 1 diabetes has led to the uncovering of a significant number of risk variants with modest effect. These studies showed that the innate immune system may also play a role in addition to the adaptive immune system. It is anticipated that next generation sequencing techniques will uncover other (rare) variants. For other autoimmune disease (such as autoimmune thyroid disease) GWAS are clearly needed.

Association of the thyroid stimulating hormone receptor gene (TSHR) with Graves' disease

Graves' disease (GD) is a common autoimmune disease (AID) that shares many of its susceptibility loci with other AIDs. The thyroid stimulating hormone receptor (TSHR) represents the primary autoantigen in GD, in which autoantibodies bind to the receptor and mimic its ligand, thyroid stimulating hormone, causing the characteristic clinical phenotype. Although early studies investigating the TSHR and GD proved inconclusive, more recently we provided convincing evidence for association of the TSHR region with disease. In the current study, we investigated a combined panel of 98 SNPs, including 70 tag SNPs, across an extended 800 kb region of the TSHR to refine association in a cohort of 768 GD subjects and 768 matched controls. In total, 28 SNPs revealed association with GD (P < 0.05), with strongest SNP associations at rs179247 (chi(2) = 32.45, P = 8.90 x 10(-8), OR = 1.53, 95% CI = 1.32-1.78) and rs12101255 (chi(2) = 30.91, P = 1.95 x 10(-7), OR = 1.55, 95% CI = 1.33-1.81), both located in intron 1 of the TSHR. Association of the most associated SNP, rs179247, was replicated in 303 GD families (P = 7.8 x 10(-4)). In addition, we provide preliminary evidence that the disease-associated genotypes of rs179247 (AA) and rs12101255 (TT) show reduced mRNA expression ratios of flTSHR relative to two alternate TSHR mRNA splice variants.

Effects of mutations involving cysteine residues distal to the S281HCC motif at the C-terminus on the functional characteristics of a truncated ectodomain-only thyrotropin receptor anchored on glycosylphosphatidyl-inositol

BACKGROUND

Cysteine (Cys) residues pair to form disulfide bonds that are important in maintaining structure and function of the thyrotropin receptor (TSHR). There are 11 Cys residues in the ectodomain (ECD). Cys 41 at the N-terminus and Cys 283 at the SHCC motif have been identified as important for ligand binding. The present study evaluated the effects of mutating Cys distal to the S281HCC motif at the C-terminus of the ECD on the functional characteristics of TSHR.

METHODS

We introduced (i) individual Cys and (ii) consecutive cumulative Cys mutations into the starting template SHCS-TSHR, a truncated TSHR-ECD moiety previously shown to behave like the wild-type TSHR. Each mutant receptor was evaluated for relative specific binding (RSB), calculated as a measure of TSH-binding ability after normalization with receptor surface expression.

RESULTS

In the first approach, RSB was severely affected when Cys 390 and Cys 398 were individually switched to serine. Failed receptor trafficking occurred with Cys 408 mutation. These findings were likely results of altered receptor conformation due to illegitimate disulfide bridge formation. Only SHCS-301 TSHR bound TSH in a specific manner, and it formed the base for sequential Cys mutations. Through this second approach, both Cys 301 and 390 could be removed simultaneously without hindering TSH binding significantly. Cys 398, however, was shown to be critical. Its absence resulted in huge loss of TSH binding. Leaving Cys 283 and 398 as the only Cys pair in the C-terminus alone could support 40% of the total ligand-binding capacity.

CONCLUSIONS

From these data, we proposed Cys 398 as a stable disulfide bond partner of Cys 283, corroborating with a model based on evolutionary history of TSHR across species. This pairing of Cys 283 and Cys 398 also provides an objective alternative to conventional hypotheses on Cys coupling based on other predictive models.

A family with a novel TSH receptor activating germline mutation (p.Ala485Val)

Autosomal dominant nonautoimmune hyperthyroidism (ADNAH) is caused by gain of function mutations in the TSH receptor (TSHr) gene and characterized by toxic thyroid hyperplasia with a variable age of onset in the absence of thyroid antibodies and clinical symptoms of autoimmune thyroid disease in at least two generations. We report here a Turkish family with a novel TSHr gene mutation with distinct features all consistent with ADNAH. Thyroid function tests of the proband were as follows: free T3: 13.1 pg/ml (N: 1.8-4.6); free T4: 5.1 ng/dl (N: 0.9-1.7); TSH: 0.01 microIU/ml (N: 0.2-4.2); and TSH receptor antibody: 2 IU/ml (N: 0-10). A heterozygous missense mutation in exon 10 of the TSHr gene (c.1454C>T) resulting in the substitution of valine for alanine at codon 485 (p.Ala485Val) was found in the father and his son and daughter. This mutation had arisen de novo in the father. Functional studies of the novel TSHr germline mutation demonstrated a higher constitutive activation of adenyl cyclase than wild type without any effect on phospholipase C activity. In conclusion, our data indicate that gain of function germline mutations in the TSHr gene should be investigated in families with members suffering from thyrotoxicosis and progressive growth of goiter, but without clinical and biochemical evidence of autoimmune thyroid disease. In addition, patients harboring the same mutation of the TSHr gene may show wide phenotypic variability with respect to the age at onset, and severity of hyperthyroidism and thyroid growth.

The cloned equine thyrotropin receptor is hypersensitive to human chorionic gonadotropin; identification of three residues in the extracellular domain involved in ligand specificity

The receptors for TSH, LH/chorionic gonadotropin (CG), and FSH belong to the same subfamily of G protein-coupled receptors. The specificity of recognition of their cognate hormone involves a limited number of residues in the leucine-rich repeats present in the N-terminal ectodomain of the receptor. It is admitted that receptors of this subfamily coevoluted with their respective ligands. The secretion of CG is restricted to gestation of primates and Equidae. We hypothesized that, facing the challenge of a new hormone, the glycoprotein hormone receptors would have evolved differently in Equidae and human so that distinct residues are involved in hormone specificity. In particular, it is known that equine CG has a dual (FSH and LH) activity when administered to other species. In the present work, we cloned and characterized functionally the equine TSH receptor (TSHR), which shares 89% homology with the human TSHR. The equine TSHR is not responsive to equine CG but is more sensitive to human CG than the human TSHR. Three residues, at positions 60, 229, and 235 of the ectodomain, are responsible for this difference in sensitivity as shown by modelization and targeted mutagenesis, followed by in vitro functional characterization. The phylogenetic approach is a suitable approach to identify determinants of specificity of receptors.

Dopamine D2 receptors form higher order oligomers at physiological expression levels

G-protein-coupled receptors are generally thought to be organized as dimers; whether they form higher order oligomers is a topic of much controversy. We combined bioluminescence/fluorescence complementation and energy transfer to demonstrate that at least four dopamine D2 receptors are located in close molecular proximity in living mammalian cells, consistent with their organization as higher order oligomers at the plasma membrane. This implies the existence of multiple receptor interfaces. In addition to the symmetrical interface in the fourth transmembrane segment (TM4) we identified previously by cysteine (Cys) crosslinking, we now show that a patch of residues at the extracellular end of TM1 forms a second symmetrical interface. Crosslinking of D2 receptor with Cys substituted simultaneously into both TM1 and TM4 led to higher order species, consistent with our novel biophysical results. Remarkably, the rate and extent of crosslinking at both interfaces were unaltered over a 100-fold range of receptor expression. Thus, at physiological levels of expression, the receptor is organized in the plasma membrane into a higher order oligomeric structure.

TSH signalling and cancer

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

A lack of thyroid hormones rather than excess thyrotropin causes abnormal skeletal development in hypothyroidism

By proposing TSH as a key negative regulator of bone turnover, recent studies in TSH receptor (TSHR) null mice challenged the established view that skeletal responses to disruption of the hypothalamic-pituitary-thyroid axis result from altered thyroid hormone (T(3)) action in bone. Importantly, this hypothesis does not explain the increased risk of osteoporosis in Graves' disease patients, in which circulating TSHR-stimulating antibodies are pathognomonic. To determine the relative importance of T(3) and TSH in bone, we compared the skeletal phenotypes of two mouse models of congenital hypothyroidism in which the normal reciprocal relationship between thyroid hormones and TSH was intact or disrupted. Pax8 null (Pax8(-/-)) mice have a 1900-fold increase in TSH and a normal TSHR, whereas hyt/hyt mice have a 2300-fold elevation of TSH but a nonfunctional TSHR. We reasoned these mice must display opposing skeletal phenotypes if TSH has a major role in bone, whereas they would be similar if thyroid hormone actions predominate. Pax8(-/-) and hyt/hyt mice both displayed delayed ossification, reduced cortical bone, a trabecular bone remodeling defect, and reduced bone mineralization, thus indicating that the skeletal abnormalities of congenital hypothyroidism are independent of TSH. Treatment of primary osteoblasts and osteoclasts with TSH or a TSHR-stimulating antibody failed to induce a cAMP response. Furthermore, TSH did not affect the differentiation or function of osteoblasts or osteoclasts in vitro. These data indicate the hypothalamic-pituitary-thyroid axis regulates skeletal development via the actions of T(3).

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.

Pituitary-thyroid setpoint and thyrotropin receptor expression in consomic rats

The genetic basis for differences in TSH sensitivity between two rat strains was examined using consomic rats generated from original strains salt-sensitive Dahl (SS) (TSH 1.8 +/- 0.1 ng/ml; free T(4) index 4.9 +/- 0.4) and Brown Norwegian (BN) (TSH 5.5 +/- 0.6 ng/ml, P < 0.05; free T(4) index 4.3 +/- 0.1, P not significant). Consomic rats SSBN6 [BN chromosome (CH) 6 placed in SS rat] and SSBN2 (BN CH 2 placed in SS rat) have TSH concentrations intermediate between pure SS and BN strains (2.9 +/- 0.3 and 3.1 +/- 0.3 ng/ml, respectively; P < 0.05). Candidate genes on rat CH 2 included TSH beta-subunit and on CH 6 the TSH receptor (TSHR). TSH from sera of BN, SS, SSBN6, and SSBN2 strains had similar in vitro bioactivity suggesting that the cause for the variable TSH concentrations was not due to an altered TSH. Physiological response to TSH was measured by changes in serum T(4) concentrations upon administration of bovine TSH (bTSH). Rat strain SS had a greater T(4) response to bTSH than BN (change in T(4), 1.3 +/- 0.1 vs. 0.4 +/- 0.1 microg/dl, P < 0.005), suggesting reduced thyrocyte sensitivity to TSH in BN. Sequencing of the TSHR coding region revealed an amino acid difference in BN (Q46R). This substitution is unlikely to contribute to the strain difference in serum TSH because both TSHR variants were equally expressed at the cell surface of transfected cells and responsive to bTSH. Given similar TSH activity and similar TSHR structure, TSHR mRNA expression in thyroid tissue was quantitated by real-time PCR. BN had 54 +/- 5% the total TSHR expression compared to SS (100 +/- 7%, P < 0.0001), when corrected for GAPDH expression, a difference confirmed at the protein level. Therefore, the higher TSH level in the BN strain appears to reflect an adjustment of the feedback loop to reduced thyrocyte sensitivity to TSH secondary to reduced TSHR expression. These strains of rat provide a model to study the cis- and trans-acting factors underlying the difference in TSHR expression.

A fast method to detect cell surface expression of thyrotropin receptor (TSHr): the microchip flow cytometry analysis

Loss-of-function mutations of the thyrotropin receptor (TSHr) may be responsible for congenital hypothyroidism or isolated hyperthyreotropinemia. To study cell surface expression of inactivating TSHr mutations detected in patients with isolated hyperthyreotropinemia (L252P, Q8fsX62, P27T, E34K, R46P, D403N, W488R, and M527T), we used the Agilent 2100 bioanalyzer to perform microchip flow cytometry analysis. The previously described TSHr inactivating mutation T477I was used as control. The level of receptor expression in COS-7 cells transfected with the T477I measured by binding assay was four times lower with respect to the wild-type TSHr. The very low expression of T477I was confirmed by fluorescence-activated cell sorting (FACS) analysis and by microchip flow cytometry analysis, suggesting that this method can be a reliable system to measure receptor cell surface expression. Other inactivating TSHr mutations were expressed in COS-7 cells for binding studies, FACS analysis, and microchip flow cytometry analysis. Binding studies showed that L252P, Q8fsX62, P27T, E34K, R46P, D403N, W488R, and M527T mutants had a low expression at the cell surface, as demonstrated by Bmax values. Data obtained by binding studies were in good agreement with data obtained by FACS analysis and microchip flow cytometry analysis. In conclusion, the low number of cells required for analysis and the ease of use make the microchip flow cytometry analysis a very reliable and favorable system to study cell surface expression of TSHr mutations.

A familial thyrotropin (TSH) receptor mutation provides in vivo evidence that the inositol phosphates/Ca2+ cascade mediates TSH action on thyroid hormone synthesis

CONTEXT

In the human thyroid gland, TSH activates both the cAMP and inositol phosphates (IP) signaling cascades via binding to the TSH receptor (TSHR). Biallelic TSHR loss-of-function mutations cause resistance to TSH, clinically characterized by hyperthyrotropinemia, and normal or reduced thyroid gland volume, thyroid hormone output, and iodine uptake.

OBJECTIVE

We report and study a novel familial TSHR mutation (L653V).

RESULTS

Homozygous individuals expressing L653V had euthyroid hyperthyrotropinemia. Paradoxically, patients had significantly higher 2-h radioiodide uptake and 2- to 24-h radioiodide uptake ratios compared with heterozygous, unaffected family members, suggesting an imbalance between iodide trapping and organification. In transfected COS-7 cells, the mutant TSHR had normal surface expression, basal activity, and TSH-binding affinity, equally (2.2-fold) increased EC50 values for TSH-induced cAMP and IP accumulation, and normal maximum cAMP generation. In contrast, the efficacy of TSH for generating IP was more than 7-fold lower with the mutant compared with wild-type TSHR.

CONCLUSIONS

We identified and characterized a TSHR defect, preferentially affecting the IP pathway, with a phenotype distinct from previously reported loss-of-function mutations. Results provide the first in vivo evidence for the physiological role of the TSHR/IP/Ca2+ cascade in regulating iodination. According to systematic in vitro mutagenesis studies, other TSHR mutations can result in even complete loss of IP signaling with retained cAMP induction. We hypothesize that such TSHR mutations could be the cause in unexplained partial organification defects.

Ligand-selective determinants in gonadotropin receptors

In mammals, the interactions between gonadotropins and their cognate receptors are highly specific; unintended cross-reactivity under normal physiological conditions has not been observed. This paper summarizes the comparative structure-function studies that aim at elucidating the molecular basis of the ligand selectivity.

Cysteine 390 mutation of the TSH receptor modulates its ectodomain as an inverse agonist on the serpentine domain with decrease in basal constitutive activity

Mutations of individual cysteine residues at codon 301, 390, 398 and 408 of the thyrotropin receptor (TSHr) to serine resulted in cell surface expression of only C301S and C390S mutants. C390S mutation was a silencing mutation with decreased basal constitutive activity. Although the C301S and C390S mutants did not show any significant TSH binding, they generated cyclic AMP upon TSH stimulation. These mutants were also able to interact with stimulating and blocking anti-TSHr antibodies. In fact, C390S receptor is a more sensitive tool for blocking antibody detection than wild type receptor. Introduction of C390S to activating mutations in the ectodomain (S281N), exloop (I486F) and transmembrane (D633H) segments could not mute/nullify receptor activation. These data indicate that the C390S ectodomain behaves as a more effective inverse agonist on the noisy transmembrane segment and suggest that the basal and activated states of the receptor operate through two independent pathways.

Thyrotropin receptor-associated diseases: from adenomata to Graves disease

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

Glycoprotein hormone receptors: link between receptor homodimerization and negative cooperativity

The monomeric model of rhodopsin-like G protein-coupled receptors (GPCRs) has progressively yielded the floor to the concept of GPCRs being oligo(di)mers, but the functional correlates of dimerization remain unclear. In this report, dimers of glycoprotein hormone receptors were demonstrated in living cells, with a combination of biophysical (bioluminescence resonance energy transfer and homogenous time resolved fluorescence/fluorescence resonance energy transfer), functional and biochemical approaches. Thyrotropin (TSHr) and lutropin (LH/CGr) receptors form homo- and heterodimers, via interactions involving primarily their heptahelical domains. The large hormone-binding ectodomains were dispensable for dimerization but modulated protomer interaction. Dimerization was not affected by agonist binding. Observed functional complementation indicates that TSHr dimers may function as a single functional unit. Finally, heterologous binding-competition studies, performed with heterodimers between TSHr and LH/CG-TSHr chimeras, demonstrated the unsuspected existence of strong negative cooperativity of hormone binding. Tracer desorption experiments indicated an allosteric behavior in TSHr and, to a lesser extent, in LH/CGr and FSHr homodimers. This study is the first report of homodimerization associated with negative cooperativity in rhodopsin-like GPCRs. As such, it may warrant revisitation of allosterism in the whole GPCR family.

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.

TSH receptor protein is selectively expressed in normal human extraocular muscle

Thyroid-associated ophthalmopathy is a common manifestation of Graves' disease. Its pathophysiology is not well understood but an antigen shared between the thyroid and orbit is thought to play a role. Using immunohistochemistry, we have demonstrated the presence of the autoantigenic target of Graves' disease, the thyroid-stimulating hormone receptor, in normal human extraocular muscle. These results support previous findings of the full length and splice variant thyroid-stimulating hormone receptor cDNA in extraocular muscle. The observation of the autoimmune target--the thyroid-stimulating hormone receptor protein--being shared between the thyroid and extraocular muscle lends greater support to the notion that an extraocular muscle thyroid-stimulating hormone receptor is also a likely target autoantigen in thyroid-associated ophthalmopathy.

Evidence that the thyrotropin receptor protease is membrane-associated and is not within lipid rafts

The thyrotropin receptor (TSHR) cleaves to a variable extent within the ectodomain into a ligand-binding A subunit linked by disulfide bonds to the largely transmembrane B subunit. To obtain insight into this variability, we examined the extent of cleavage of TSHR ectodomains tethered to the plasma membrane by different means: (1) the wild-type, serpentine region, (2) a glycosylphosphatidylinositol (GPI) anchor, and (3) a single CD8alpha transmembrane region. For this purpose, we covalently cross-linked(125)I-TSH to the TSHR ectodomain expressed on the surface of intact cell monolayers. The extent of cleavage of the CD8alpha-tethered ectodomain was similar to the wild-type TSHR (approximately 50%) whereas the same ectodomain with a GPI anchor remained almost entirely (approximately 90%) uncleaved. These findings have three possible implications. First, differential cleavage of the TSHR ectodomain depending on its attachment to the plasma membrane suggests that the TSHR protease is membrane-associated and is not a soluble (secreted or shed) protease. Second, because GPI-anchored proteins (unlike CD8alpha) segregate in membrane lipid rafts, the TSHR protease appears not to be associated with lipid rafts. Finally, the similar extent of cleavage of the wild-type TSHR and the CD8alpha (not the GPI) tethered ectodomain supports the concept that the wild-type TSHR resides largely outside lipid rafts.

Modulation of ligand selectivity associated with activation of the transmembrane region of the human follitropin receptor

Recently, three naturally occurring mutations in the serpentine region of the FSH receptor (FSHr) (D567N and T449I/A) have been identified in three families with spontaneous ovarian hyperstimulation syndrome (OHSS). All mutant receptors displayed abnormally high sensitivity to human chorionic gonadotropin and, in addition, D567N and T449A displayed concomitant increase in sensitivity to TSH and detectable constitutive activity. In the present study, we have used a combination of site-directed mutagenesis experiments and molecular modeling to explore the mechanisms responsible for the phenotype of the three OHSS FSHr mutants. Our results suggest that all mutations lead to weakening of interhelical locks between transmembrane helix (TM)-VI and TM-III, or TM-VI and TM-VII, which contributes to maintaining the receptor in the inactive state. They also indicate that broadening of the functional specificity of the mutant FSHr constructs is correlated to their increase in constitutive activity. This relation between basal activity and functional specificity is a characteristic of the FSHr, which is not shared by the other glycoprotein hormone receptors. It leads to the interesting suggestion that different pathways have been followed during primate evolution to avoid promiscuous stimulation of the TSHr and FSHr by human chorionic gonadotropin. In the hFSHr, specificity would be exerted both by the ectodomain and the serpentine portion.

Sphingolipid-cholesterol domains (lipid rafts) in normal human and dog thyroid follicular cells are not involved in thyrotropin receptor signaling

Partition of signaling molecules in sphingolipid-cholesterol-enriched membrane domains, among which are the caveolae, may contribute to signal transduction efficiency. In normal thyroid, nothing is known about a putative TSH/cAMP cascade compartmentation in caveolae or other sphingolipid-cholesterol-enriched membrane domains. In this study we show for the first time that caveolae are present in the apical membrane of dog and human thyrocytes: caveolin-1 mRNA presence is demonstrated by Northern blotting in primary cultures and that of the caveolin-1 protein by immunohistochemistry performed on human thyroid tissue. The TSH receptor located in the basal membrane can therefore not be located in caveolae. We demonstrate for the first time by biochemical methods the existence of sphingolipid-cholesterol-enriched domains in human and dog thyroid follicular cells that contain caveolin, flotillin-2, and the insulin receptor. We assessed a possible sphingolipid-cholesterol-enriched domains compartmentation of the TSH receptor and the alpha- subunit of the heterotrimeric G(s) and G(q) proteins using two approaches: Western blotting on detergent-resistant membranes isolated from thyrocytes in primary cultures and the influence of 10 mm methyl-beta-cyclodextrin, a cholesterol chelator, on basal and stimulated cAMP accumulation in intact thyrocytes. The results from both types of experiments strongly suggest that the TSH/cAMP cascade in thyroid cells is not associated with sphingolipid-cholesterol-enriched membrane domains.

A functional site on the human TSH receptor: a potential therapeutic target in Graves' disease

OBJECTIVE

Identifying sites on the TSH-receptor that are involved in the pathological stimulation of the thyroid by autoantibodies in Graves' disease would aid the development of new therapies. We tested a series of monoclonal antibodies that recognize the native receptor for their ability to inhibit stimulation of the receptor in vitro.

PATIENTS AND METHODS

Heterologous cells expressing the recombinant human TSH-receptor were stimulated with TSH or serum samples from 13 Graves' disease patients or the MRC Long-Acting Thyroid Stimulator standard B (LATS-B) and their cAMP responses measured. The effect on this stimulation of various doses of purified monoclonal antibodies with defined epitopes was determined.

RESULTS

Antibodies against one epitope (residues 381-384) inhibited TSH-stimulated cyclic adenosine monophosphate (cAMP) production (1 microg/ml causing 50% inhibition of the response to 100 microU/ml TSH) and also inhibited cAMP production induced by sera from approximately 40% (6/14) of Graves' disease patients, including the MRC LATS-B standard.

CONCLUSIONS

Residues 381-384 of the human TSH-receptor are important in the physiological and pathological stimulation of the thyroid. This opens the possibility of more specific therapy of some Graves' disease patients by agents directed against this epitope.

Glycoprotein hormone receptors: determinants in leucine-rich repeats responsible for ligand specificity

Glycoprotein hormone receptors [thyrotropin (TSHr), luteinizing hormone/chorionic gonadotropin (LH/CGr), follicle stimulating hormone (FSHr)] are rhodopsin-like G protein-coupled receptors with a large extracellular N-terminal portion responsible for hormone recognition and binding. In structural models, this ectodomain is composed of two cysteine clusters flanking nine leucine-rich repeats (LRRs). The LRRs form a succession of beta-strands and alpha-helices organized into a horseshoe-shaped structure. It has been proposed that glycoprotein hormones interact with residues of the beta-strands making the concave surface of the horseshoe. Gain-of-function homology scanning of the beta-strands of glycoprotein hormone receptors allowed identification of the critical residues responsible for the specificity towards human chorionic gonadotropin (hCG). Substitution of eight or two residues of the LH/CGr into the TSHr or FSHr, respectively, resulted in constructs displaying almost the same affinity and sensitivity for hCG as wild-type LH/CGr. Molecular dynamics simulations and additional site-directed mutagenesis provided a structural rationale for the evolution of binding specificity in this duplicated gene family.

Generation of a mouse monoclonal TSH receptor antibody with stimulating activity

A Balb/c mouse was subjected to genetic immunization with a cDNA construct encoding the human thyrotropin receptor (TSHr). The immune response of the mouse resulted in the production of immunoglobulins recognizing the TSHr in three different assays: (1) flow immunocytometry (FACS) with CHO cells expressing the receptor; (2) receptor-dependent stimulation of cAMP production in the same cell line; and (3) competition with labeled TSH for binding to the receptor. One thousand hybridomas were generated from the spleen of the mouse and their supernatants were screened. A single monoclonal, IRI-SAb1, scored positive in all three assays and was studied further. It stimulated 13-fold cAMP production in TSHr-expressing CHO cells, with an EC50 in the low nanomolar range. When compared with bovine TSH, IRI-SAb1 behaved as a partial agonist. Contrary to the expectation from the characteristic of autoantibodies of Graves' patients, IRI-SAb1 recognized a linear epitope, which was localized in a segment encompassing the first 281 residues of the receptor.

Identification of a novel GPI-anchored CRISP glycoprotein, MAK248, located on the posterior head and equatorial segment of cynomolgus macaque sperm

To identify a sperm-surface component that is highly antigenic, we immunized female cynomolgus macaques with glycosylphosphatidylinositol (GPI)-anchored sperm surface proteins that were released following treatment with phosphatidylinositol-specific phospholipase C (PI-PLC). Five different adjuvants were used in combination with the PI-PLC-released proteins, and three of these proteins (24, 48, and 53 kDa) were shown to be potent antigens for immunization of female monkeys. The 53 kDa protein was found to be a surface coating protein and not a GPI-anchored protein. Polyclonal antibodies to the 24 kDa protein and the 48 kDa protein were produced in rabbits. The two antibodies recognized both proteins on Western blots. The same rabbit antibodies recognized 28, 18, and 10 kDa bands on a Western blot of chemically reduced PI-PLC-released proteins, suggesting that the 48 kDa protein is a dimer of the 24 kDa protein, which we refer to as MAK248. Rabbit polyclonal antibodies developed to reduced fragments of the 24 kDa protein showed that the 18 and 10 kDa bands are proteolytic peptide fragments of the 24 kDa protein. Screening of tissues from male macaques showed that MAK248 is expressed only in the epididymis. Microsequencing of two proteolytic fragments of the 18 kDa component showed 100% amino acid homology to a 233 deduced amino acid sequence previously identified in human testes genome. Antibodies to MAK248 recognized a 24 kDa protein released from human sperm exposed to PI-PLC. Antibodies to MAK248 recognized the equatorial segment and posterior head regions of capacitated cynomolgus macaque sperm. Structural analysis suggests that MAK248 is a novel CRISP protein and a member of the CAP (CRISP, Ag 5, PR-1) family of proteins. Based on amino acid sequence homology, it is possible that MAK248 functions as a protease inhibitor.

Association of the thyrotropin receptor with calnexin, calreticulin and BiP. Efects on the maturation of the receptor

The thyrotropin receptor (TSHR) is a member of the G protein-coupled receptor superfamily. It has by now been clearly established that the maturation of the glycoproteins synthesized in the endoplasmic reticulum involves interactions with molecular chaperones, which promote the folding and assembly of the glycoproteins. In this study, we investigated whether calnexin (CNX), calreticulin (CRT) and BiP, three of the main molecular chaperones present in the endoplasmic reticulum, interact with the TSHR and what effects these interactions might have on the folding of the receptor. In the first set of experiments, we observed that in a K562 cell line expressing TSHR, about 50% of the receptor synthesized was degraded by the proteasome after ubiquitination. In order to determine whether TSHR interact with CNX, CRT and BiP, coimmunoprecipitation experiments were performed. TSHR was found to be associated with all three molecular chaperones. To study the role of the interactions between CNX and CRT and the TSHR, we used castanospermine, a glucosidase I and II inhibitor that blocks the interactions between these chaperones and glycoproteins. In K562 cells expressing the TSHR, these drugs led to a faster degradation of the receptor, which indicates that these interactions contribute to stabilizing the receptor after its synthesis. The overexpression of calnexin and calreticulin in these cells stabilizes the receptor during the first hour after its synthesis, whereas the degradation of TSHR increased in a cell line overexpressing BiP and the quantity of TSHR able to acquire complex type oligosaccharides decreased. These results show that calnexin, calreticulin and BiP all interact with TSHR and that the choice made between these two chaperone systems is crucial because each of them has distinct effects on the folding and stability of this receptor at the endoplasmic reticulum level.

Thyroid-stimulating autoantibodies in Graves disease preferentially recognize the free A subunit, not the thyrotropin holoreceptor

Graves disease is directly caused by thyroid-stimulating autoantibodies (TSAb's) that activate the thyrotropin receptor (TSHR). We observed upon flow cytometry using intact cells that a mouse mAb (3BD10) recognized the TSHR ectodomain with a glycosidylphosphatidylinositol (ECD-GPI) anchor approximately tenfold better than the same ectodomain on the wild-type TSHR, despite the far higher level of expression of the latter. The 3BD10 epitope contains the N-terminal cysteine cluster critical for TSAb action. Consequently, we hypothesized and confirmed that TSAb (but not thyrotropin-blocking autoantibodies [TBAb's]) also poorly recognize the wild-type TSHR relative to the ECD-GPI. Despite poor recognition by TSAb of the holoreceptor, soluble TSHR A subunits (known to be shed from surface TSHR) fully neutralized autoantibody-binding activity. These data indicate that the epitope(s) for TSAb's, but not for TBAb's, are partially sterically hindered on the holoreceptor by the plasma membrane, the serpentine region of the TSHR, or by TSHR dimerization. However, the TSAb epitope on the soluble A subunit is freely accessible. This observation, as well as other evidence, supports the concept that A subunit shedding either initiates or amplifies the autoimmune response to the TSHR, thereby causing Graves disease in genetically susceptible individuals.

A conserved Asn in TM7 of the thyrotropin receptor is a common requirement for activation by both mutations and its natural agonist

The wide spectrum of naturally occurring mutations able to activate the thyrotropin (TSH) receptor provides a useful tool to approach the structure of the active state(s) of the glycoprotein hormone receptors. Here we show that the side-chain of the highly conserved N7.49 (Asn 674) in TM7 is mandatory for activation of the TSH receptor, not only by TSH, but also by a panel of eight natural and two artificial activating mutations. Basal activity levels of the mutants were significantly decreased by suppression of the side-chain of N7.49 (N7.49A double mutants). In addition, comparative effects of the N7.49A substitution on the ten mutants demonstrate that basal activity and agonist- or mutation-stimulated activity might involve different structural changes.

Lysine 183 and glutamic acid 157 of the TSH receptor: two interacting residues with a key role in determining specificity toward TSH and human CG

A naturally occurring mutation in the ectodomain of the TSH receptor (TSHr), K183R, has been described recently in a familial case of gestational hyperthyroidism. Hyperthyroidism was explained by the widening of the specificity of the mutant receptor toward human CG (hCG). In the present study, we attempted to understand in molecular terms the structure-phenotype relationships of this mutant in light of the available structural model of TSHr ectodomain established on the template of the atomic structure of the porcine ribonuclease inhibitor. To this aim, we studied by site-directed mutagenesis and functional assays in transfected COS cells the effects of substituting amino acids with different physicochemical properties for lysine 183. Unexpectedly, all TSHr mutants displayed widening of their specificity toward hCG. Molecular dynamics simulations suggested that the gain of function would be secondary to the release of a nearby glutamate residue (E157) from a salt bridge with K183. This hypothesis was supported by further site-directed mutagenesis experiments showing that the presence of an acidic residue in position 157, or in its vicinity, was required to observe the increase in sensitivity to hCG (an acidic residue in position 183 can partially fulfill the role of a free acidic residue in position 157 when tested on the background of a E157A mutant). Our results suggest also that additional natural mutations (especially K183M, N, or Q) in position 183 of TSHr are expected to be found in gestational hyperthyroidism.

Demonstration of immunoglobulin G, A, and E autoantibodies to the human thyrotropin receptor using flow cytometry

Human TSH receptor (TSHR) autoantibodies with biological activity result in thyroid dysfunction, but antibodies that simply bind do not. We have applied flow cytometry to the measurements of IgG, IgA, and IgE immunoreactivity to the TSHR in patients with Graves' disease (GD) and thyroid eye disease (TED) and in normal controls. CHO cells stably expressing the extracellular domain of the TSHR with a glycophosphatidylinositol anchor were produced and found to express approximately 4 times as many receptors, but of similar affinity, as JP09 in TSH binding studies. Substantial increases in median fluorescence and peak channel fluorescence were obtained by flow cytometry using TSHR monoclonal antibodies on the glycophosphatidylinositol cells. IgG autoantibodies were demonstrated in 55 of 65 untreated GD patients, 3 of 25 normal subjects, and 4 of 8 atypical TED sera (negative for TSHR autoantibodies with biological activity) by flow cytometry and correlated poorly with thyroid-stimulating antibodies. IgA antibodies were present in 1 of 12 normal, 1 of 7 treated GD with TED, and 3 of 8 atypical TED sera. IgE binding was observed in 1 of 12 normal, 2 of 8 treated GD without TED, 1 of 6 treated GD with TED, and 0 of 8 atypical TED sera. In conclusion, we have demonstrated autoantibodies that bind directly to the TSHR in the majority of GD patients and in 50% of patients with atypical TED and a small number of normal controls lacking TSHR antibodies that affect function. Although predominantly IgG lambda, TSHR autoantibodies of the IgA and IgE isotypes are also detectable.

Activation of the cAMP pathway by the TSH receptor involves switching of the ectodomain from a tethered inverse agonist to an agonist

Several lines of evidence indicate that constraining intramolecular interactions between transmembrane domains are required to maintain G protein-coupled receptors in an inactive conformation in the absence of agonist. For the glycoprotein hormone receptors, which harbor a long amino-terminal ectodomain responsible for hormone binding, it has been suggested that the ectodomain could contribute to these negative constraints. To test this hypothesis, we expressed at the surface of COS-7 cells mutants of the TSH receptor in which variable portions of the amino-terminal ectodomain are replaced by a 19-residue tag from bovine rhodopsin. Whereas none of the rhodopsin-tagged truncated mutants could be activated by saturating concentrations of TSH, the constructs with the shortest amino-terminal extension displayed increased constitutive activity toward the cAMP pathway, when compared with the wild-type holoreceptor. The shortest truncated construct was strongly activated by the introduction of mutations in transmembrane segment VI (D633A), or in the third intracellular loop (A623I) of the receptor. The magnitude of the stimulation was similar to that observed when the same mutations were introduced in the intact wild-type receptor. On the contrary, the shortest truncated construct was unaffected by activating mutations affecting residues of the extracellular loop region (I486F, I568T) or the top of transmembrane segment VII (del658-661). Together, our results are compatible with a model in which activation of the cAMP pathway by the TSH receptor involves switching of the ectodomain from a tethered inverse agonist to a true agonist.

Characterization of ThOX proteins as components of the thyroid H(2)O(2)-generating system

We have recently cloned two thyroid-specific cDNAs encoding new members of the NADPH oxidase family. ThOX1 and ThOX2 proteins are colocalized with thyroperoxidase at the apical membrane of human thyroid cells. In the present study we have determined their subcellular localization and maturation in relation to their enzymatic activity. A majority of ThOX proteins accumulated inside the cell and only a small fraction was expressed at the surface. Western blots demonstrated that ThOX's are glycoproteins of 180,000 and 190,000. When totally deglycosylated the molecular weight of both ThOX1 and ThOX2 drops to 160,000. Ca(2+) stimulates the basal H(2)O(2) generation in PC Cl3 cells at a level corresponding to 20% of the leukocyte H(2)O(2) production stimulated by PMA. Nonthyroid cell lines transfected with ThOX1 and ThOX2 show only a single immunoreactive band in Western blot analysis, corresponding to the protein of 180,000. This "immature" protein remains exclusively intracellular and does not present any enzymatic activity. This is not modified by coexpression of thyroperoxidase and p22(Phox). Transfection of ThOX cDNAs into PLB-XCGD cells does not reconstitute their NADPH oxidase activity. We conclude that (1) the thyroid contains some elements of the leukocyte H(2)O(2)-generating system but not all of them; (2) ThOX's are predominantly or exclusively located inside the cell in thyrocytes or in transfected cells, respectively, and as such they are inactive; (3) ThOX's cannot replace gp91(Phox) in the leukocyte; and (4) the thyroid H(2)O(2)-generating system is analogous to the leukocyte system with regard to ThOX's and gp91(Phox) but very different in other aspects. Additional thyroid-specific components are probably required to get complete protein processing and full enzymatic activity in the thyroid.

Tyrosine sulfation is required for agonist recognition by glycoprotein hormone receptors

The glycoprotein hormone receptors (thyrotrophin receptor, TSHr; luteinizing hormone/chorionic gonadotrophin receptor, LH/CGr; follicle-stimulating hormone receptor, FSHr) constitute a subfamily of rhodopsin-like G protein-coupled receptors (GPCRs) with a long N-terminal extracellular extension responsible for high-affinity hormone binding. These ectodomains contain two cysteine clusters flanking nine leucine-rich repeats (LRR), a motif found in several protein families involved in protein-protein interactions. Similar to the situation described recently in CCR5, we demonstrate here that the TSHr, as it is present at the cell surface, is sulfated on tyrosines in a motif located downstream of the C-terminal cysteine cluster. Sulfation of one of the two tyrosines in the motif is mandatory for high-affinity binding of TSH and activation of the receptor. Site-directed mutagenesis experiments indicate that the motif, which is conserved in all members of the glycoprotein hormone receptor family, seems to play a similar role in the LH/CG and FSH receptors.

Characterization of the secretable ectodomain of thyrotropin receptor produced by the recombinant baculovirus system

Thyrotropin receptor (TSHR) is a member of the glycoprotein hormone receptor family and an autoantigen of Graves' disease. Various attempts have been made to obtain a large amount of soluble ectodomain of TSHR in insect or mammalian cells, but most of them failed to secrete the overexpressed ectodomain. In the present study, we observed that about one-third of the ectodomain protein (sTSHR-gp), in which the signal peptide of TSHR was replaced by the baculovirus-encoded glycoprotein 67-signal peptide, was secreted into the culture medium and the remainder stayed within cells in the recombinant baculovirus system. Microsequencing the N-terminal of the purified protein confirmed that the baculovirus signal peptide was cleaved at the expected site. Carbohydrate studies using several glycosidases and lectins revealed that the secreted form of the ectodomain had biantennary carbohydrate, whereas the non-secreted form had high-mannose. Moreover, the secreted form of sTSHR-gp exhibited high-affinity ligand binding, whereas the non-secreted form did not show any significant ligand binding. Regarding the interactions of TSHR ectodomains with anti-TSHR antibodies, both the secreted and non-secreted forms of sTSHR-gp, almost completely neutralized the stimulatory and inhibitory anti-TSHR antibody activities. In conclusion, we succeeded in secreting the ectodomain of TSHR into culture medium, which was capable of binding to TSH and neutralizing anti-TSHR antibody activities.

Purification and characterization of a soluble bioactive amino-terminal extracellular domain of the human thyrotropin receptor

The amino-terminal ectodomain of the human TSH receptor has been expressed at the surface of CHO cells as a glycosylphosphatidylinositol-anchored molecule containing a 10-residue histidine tag close to its C terminus. The soluble ectodomain could be released from the cells by treatment with a glycosylphosphatidylinositol-phospholipase C and purified to apparent homogeneity by cobalt-Sepharose chromatography. Two nanomoles of material was obtained, which was suitable for analysis by mass spectrometry. This allowed the identification of four out of the six potential N-glycosylation sites as being effectively glycosylated. A proportion of the purified soluble ectodomain displayed specific binding of (125)I-labeled TSH, allowing for the first time performance of classical saturation binding experiments. Two classes of high-affinity binding sites were identified: site A, K(d) 0.014 nM; site B, K(d) 0.83 nM. The significance of site A, whose affinity is much higher than for the holoreceptor at the surface of intact cells, remains to be clarified. The purified ectodomain was capable of inhibiting efficiently the thyroid stimulating activity of immunoglobulins from patients with Graves' disease. It allowed computation of the amounts of these immunoglobulins in patient's serum, giving values up to 10 microg/mL. Contrary to all currently available assays, the soluble ectodomain of the TSH receptor purified in a functionally competent conformation allows direct studies of its interactions with TSH and autoantibodies and opens the way to structural studies.

Effects of mutations involving the highly conserved S281HCC motif in the extracellular domain of the thyrotropin (TSH) receptor on TSH binding and constitutive activity

A model has been proposed in which, in the absence of TSH, the extracellular domain of the TSH receptor would exert a silencing effect on the serpentine domain involved in activation of the G(alpha)(s) protein. Mutation of S281 in the ectodomain is supposed to release this constraint, thereby causing receptor activation. This defines S281 and its neighbors as a segment important in intramolecular signal transduction. The functional importance of this segment was explored by site-directed mutagenesis experiments involving S281, as well as the two cysteine residues (C283, C284) present immediately downstream. S281 was mutated to N, T, G, and A in this study, and the functional characteristics of the mutants were compared. We found that S281N, S281T, and S281G display stronger constitutive activity than S281A mutant, suggesting that increase in constitutive activity is related to the extent of disruption of the local structure of the ectodomain. C283 and C284, the two consecutive cysteines that are highly conserved in glycoprotein hormone receptors, were mutated to serine, either alone (S281HSC or S281HCS) or in combination (S281HSS) and were studied in two different TSH receptor backgrounds. The mutated cysteine ectodomains were either linked to a glycosylphosphatidylinositol anchor or the serpentine domain of the wild-type holoreceptor. Glycosylphosphatidylinositol-anchored ectodomain receptors showed good cell surface expression in CHO cells, but only S281HCS was able to bind TSH specifically, illustrating the importance of C283, or the putative disulphide bond, in maintaining the conformation of the ligand binding site. In contrast, cysteine mutants on an extracellular domain-holoreceptor background displayed severely impaired membrane targeting and were poorly expressed in COS cells. However, basal cAMP production, normalized to expression at the plasma membrane, indicated significant increase in constitutive activity of all three mutants, compared with the wild-type receptor. Altogether, these findings support a model in which the ectodomain would act as a silencer of the basal activity of the serpentine portion of the receptor.

A conserved Asn in transmembrane helix 7 is an on/off switch in the activation of the thyrotropin receptor

The thyrotropin (TSH) receptor is an interesting model to study G protein-coupled receptor activation as many point mutations can significantly increase its basal activity. Here, we identified a molecular interaction between Asp(633) in transmembrane helix 6 (TM6) and Asn(674) in TM7 of the TSHr that is crucial to maintain the inactive state through conformational constraint of the Asn. We show that these residues are perfectly conserved in the glycohormone receptor family, except in one case, where they are exchanged, suggesting a direct interaction. Molecular modeling of the TSHr, based on the high resolution structure of rhodopsin, strongly favors this hypothesis. Our approach combining site-directed mutagenesis with molecular modeling shows that mutations disrupting this interaction, like the D633A mutation in TM6, lead to high constitutive activation. The strongly activating N674D (TM7) mutation, which in our modeling breaks the TM6-TM7 link, is reverted to wild type-like behavior by an additional D633N mutation (TM6), which would restore this link. Moreover, we show that the Asn of TM7 (conserved in most G protein-coupled receptors) is mandatory for ligand-induced cAMP accumulation, suggesting an active role of this residue in activation. In the TSHr, the conformation of this Asn residue of TM7 would be constrained, in the inactive state, by its Asp partner in TM6.

Role of complex asparagine-linked oligosaccharides in the expression of a functional thyrotropin receptor

To evaluate the functional role of complex asparagine-linked oligosaccharides of the human thyrotropin receptor (TSHR), a Chinese hamster ovary cell line (JP09) and a K562 cell line (K562-TSHR) expressing this receptor were treated with deoxymannojirimycin (dMM), a mannosidase I inhibitor. dMM blocks the formation of complex-type structures and leads to the formation of high-mannose-type structures. Treatment of cells with dMM led to a decrease in the number of thyrotropin (TSH)-binding sites at the cell surface. Detection of the TSHR at the cell surface using a monoclonal antibody directed against the A subunit showed that this decrease was not due to a decrease in the number of TSHRs expressed at the cell surface. However the recognition of TSHR by a monoclonal antibody directed against the C peptide was greatly decreased. On immunoblotting, after deglycosylation using peptide N-glycanase F, the A subunit was visualized as a doublet (36 and 41 kDa). In control cells the species of higher molecular mass was more abundant whereas after dMM treatment the species of lower molecular mass became more abundant. This difference in molecular mass between the two peptides is compatible with the removal of the C peptide. In conclusion, the results show that inhibition of complex-type structure formation leads to (i) an incapacity for TSHR to bind TSH, without affecting its intracellular transport and (ii) an increase of TSHR susceptibility to proteases that remove the C peptide. We then hypothesized that removal of the C peptide could contribute to the formation of a non-functional TSHR.