Soluble ecto-domain mutant of thyrotropin (TSH) receptor incapable of binding TSH neutralizes the action of thyroid-stimulating antibodies from Graves' patients

Testosterone downregulates angiotensin II type-2 receptor via androgen receptor-mediated ERK1/2 MAP kinase pathway in rat aorta

Introduction:

Blood pressure is lower in females than males. Angiotensin II type-2 receptor (AT₂R) induces vasodilation. This study determined whether sex differences in vascular AT₂R expression occur and if androgens exert control on AT₂R expression in the vasculature.

Methods:

AT₂Rs in the aorta of male and female Sprague-Dawley rats were examined following alteration in androgen levels by gonadectomy or hormone supplementation.

Results:

AT₂R mRNA and protein expression levels were lower in the aortas of males than females. In males, testosterone withdrawal by castration significantly elevated AT₂R mRNA and protein levels and testosterone replacement restored them. In females, increasing androgen levels decreased AT₂R mRNA and protein expression and this was attenuated by androgen receptor blocker flutamide. Ex vivo, dihydrotestosterone downregulated AT₂R in endothelium-intact but not endothelium-denuded aorta. Dihydrotestosterone-induced AT₂R downregulation in isolated aorta was blocked by an androgen receptor antagonist. Furthermore, blockade of ERK1/2 but not p38 MAP kinase or TGFβ signaling with specific inhibitors abolished dihydrotestosterone-induced AT₂R downregulation.

Conclusion:

Androgens downregulate AT₂R expression levels in aorta, in vivo and ex vivo. The androgen receptor-mediated ERK1/2 MAP kinase-signaling pathway may be a key mechanism by which testosterone downregulates AT₂R expression, implicating androgens’ contributing role to gender differences in vascular AT₂R expression.

Differential requirement of signal transducer and activator of transcription-4 (Stat4) and Stat6 in a thyrotropin receptor-289-adenovirus-induced model of Graves' hyperthyroidism

T helper type 1 (Th1) and Th2 cells have critical roles in the development of cell-mediated and humoral immune responses, respectively. This division of function predicts that Th1 cells mediate inflammatory diseases and Th2 cells promote antibody (Ab)-mediated autoimmunity. Our previous studies using HEK-293 cells expressing the extracellular domain of the TSH receptor (TSHR) showed that Stat4-/- mice, which lack Th1 cells, are susceptible, whereas Stat6-/- mice, which lack Th2 cells, are resistant to the induction of Graves' hyperthyroidism. To investigate the role of Stat4 and Stat6 genes in other murine models of hyperthyroidism, we injected wild-type BALB/c, Stat4-/-, and Stat6-/- mice with an adenovirus expressing amino acid residues 1-289 of TSHR (TSHR-289-ad or 289-ad). The viral system induces a much stronger immune response with much more rapid onset of disease. Our results showed that 56% of wild-type, 75% of Stat4-/-, and 39% of Stat6-/- mice developed hyperthyroidism. Hyperthyroid mice exhibited thyroid stimulatory Abs. The Stat4-/- mice developed a higher incidence and greater severity of hyperthyroidism compared with wild-type and Stat6-/- mice. BALB/c and Stat4-/- mice showed significantly higher TSHR Abs of the IgG1 subclass and IL-4 compared with Stat6-/- mice. In contrast, Stat6-/- mice had predominantly the IgG2a subclass of TSHR Ab and produced significantly higher amounts of IFN-gamma than BALB/c and Stat4-/- mice. All hyperthyroid mice showed enlarged thyroid glands with hyperactivity. These results suggest that in the TSHR-289-ad model, the Th2 cells are more efficient in mediating disease, but in the absence of Th2 cells, Th1 cells may still initiate a reduced incidence of Graves' hyperthyroidism.

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.

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.

Induction of Experimental Autoimmune Graves’ Disease in BALB/c Mice

We immunized BALB/c mice with M12 cells (H-2d) expressing either mouse (mM12 cells) or human thyrotropin receptor (TSHR) (hM12 cells). Immunized mice developed autoantibodies to native TSHR by day 90 and, by day 180, showed considerable stimulatory Ab activity as measured by their ability to enhance cAMP production (ranging from 6.52 to 20.83 pmol/ml in different treatment groups relative to 1.83 pmol/ml for controls) by TSHR-expressing Chinese hamster ovary cells. These mice developed severe hyperthyroidism with significant elevations in both tetraiodothyronine and triiodothyronine hormones. Tetraiodothyronine levels in different experimental groups ranged from a mean of 8.66–12.4 μg/dl, relative to 4.8 μg/dl in controls. Similarly, mean triiodothyronine values ranged from 156.18 to 195.13 ng/dl, relative to 34.99 ng/dl for controls. Next, we immunized BALB/c mice with a soluble extracellular domain of human TSHR (TBP), or TBP expressed on human embryonic kidney cells (293 cells) (293-TBP cells). These mice showed severe hyperthyroidism in a manner very similar to that described above for mice immunized with the mouse TSHR or human TSHR, and exhibited significant weight loss, with average weight for treatment groups ranging from 20.6 to 21.67 g, while controls weighed 24.2 g. Early after onset of the disease, histopathological examination of thyroids showed enlargement of colloids and thinning of epithelial cells without inflammation. However, later during disease, focal necrosis and lymphocytic infiltration were apparent. Our results showed that conformationally intact ectodomain of TSHR is sufficient for disease induction. Availability of a reproducible model in which 100% of the animals develop disease should facilitate studies aimed at understanding the molecular pathogenesis of Graves’ disease.

Identification of an important thyrotrophin binding site on the human thyrotrophin receptor using monoclonal antibodies

A thyrotrophin (TSH) binding site has been identified on the extracellular domain of the human thyrotrophin receptor (hTSHR) using monoclonal antibodies that recognise the native hTSHR. These antibodies were produced by immunising BALB/c mice with denatured recombinant material, selected by their reaction with recombinant hTSHR expressed on heterologous cell lines using flow cytofluorimetric analysis, and characterised by immunoblotting and immunoprecipitation. The epitopes the monoclonal antibodies recognise were determined using multiple overlapping synthetic peptides. All of the antibodies reacted with epitopes within the region 335-390; these epitopes must be accessible on the external surface of the native hTSHR. None of the antibodies stimulated cAMP production of recombinant hTSHR cell lines. The epitopes of two antibodies (residues 337-342 and 355-358) are in the small peptide thought to be removed by proteolytic processing of hTSHR. A further five different antibodies (determined from their variable region sequences) all reacted with residues 381-384 emphasising the immunogenicity of this region. The functional importance of residues 381-384 as a TSH binding site was shown by the fact that some of these monoclonal antibodies caused inhibition of radiolabelled TSH binding of 80-90% at 1 microg/ml and greater than 50% inhibition at 0.1 microg/ml (0.65 nM--i.e. comparable in effectiveness with TSH itself). Residues 381-384 may form part of the target regions recognised by inhibitory autoantibodies found in Graves' disease.

Recombinant growth differentiation factor-9 (GDF-9) enhances growth and differentiation of cultured early ovarian follicles

Transgenic mice with deletion of the GDF-9 (growth differentiation factor-9) gene are characterized by the arrest of ovarian follicle development at the primary stage. Based on the hypothesis that GDF-9 is important for early follicle development, we isolated rat GDF-9 complementary DNA (cDNA) and generated recombinant GDF-9 protein to study its physiological role. Using bacteria-derived GDF-9-glutathione S-transferase (GST) fusion protein, specific antibodies to the mature form of GDF-9 was generated. Immunohistochemical staining of ovarian sections indicated the localization of GDF-9 protein in the oocyte of primary, secondary and preantral follicles, whereas immunoblotting demonstrated the secretion of GDF-9 by mammalian cells transfected with GDF-9 cDNAs. Recombinant GDF-9 was shown to be an N-glycosylated protein capable of stimulating early follicle development. Growth of preantral follicles isolated from immature rats was enhanced by treatment with either GDF-9 or FSH whereas the combined treatment showed an additive effect. In addition, treatment with GDF-9, like forskolin, also stimulated inhibin-alpha content in explants of neonatal ovaries. In contrast, the stimulatory effects of GDF-9 were not mimicked by amino-terminal tagged GDF-9 that was apparently not bioactive. Thus, the present study demonstrates the important role of GDF-9 in early follicle growth and differentiation. The availability of recombinant bioactive GDF-9 allows future studies on the physiological role of GDF-9 in ovarian development in vivo.

Role of asparagine-linked oligosaccharides in protein folding, membrane targeting, and thyrotropin and autoantibody binding of the human thyrotropin receptor

The amino-terminal ectodomain of thyrotropin (TSH) receptor (TSHR) is heavily glycosylated with asparagine-linked (N-linked) oligosaccharides. The present studies were designed to evaluate how acquisition and processing of N-linked oligosaccharides play a role in the functional maturation of human TSHR. A glycosylation inhibitor tunicamycin, which inhibits the first step of N-linked glycosylation (acquisition of N-linked oligosaccharides), and a series of mutant Chinese hamster ovary (CHO)-Lec cells defective in the different steps of glycosylation processing were used. Inhibition of acquisition of N-linked oligosaccharides by tunicamycin treatment in CHO cells stably expressing TSHR produced nonglycosylated TSHR, which was totally nonfunctional. In contrast, all of the TSHRs synthesized in mutant CHO-Lec1, 2, and 8 cells (mannose-rich, sialic acid-deficient, and galactose-deficient oligosaccharides, respectively) bound TSH and produced cAMP in response to TSH with an affinity and an EC50 similar to those in TSHR expressed in parental CHO cells (CHO-TSHR; sialylated oligosaccharides). However, Lec1-TSHR and Lec2-TSHR were not efficiently expressed on the cell surface, whereas the expression levels of Lec8-TSHR and CHO-TSHR were essentially identical. All of the TSHRs expressed in CHO-Lec cells cleaved into two subunits. Finally, anti-TSHR autoantibodies from Graves' patients interacted with all of the TSHRs harboring different oligosaccharides to a similar extent. These data demonstrate that acquisition and processing of N-linked oligosaccharides of TSHR appear to be essential for correct folding in the endoplasmic reticulum and for cell surface targeting in the Golgi apparatus. We also show that complex type carbohydrates are not crucially involved in the interaction of TSHR with TSH and anti-TSHR autoantibodies.

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

A chimeric cDNA construct encoding the extracellular amino-terminal domain (ECD) of the thyrotropin receptor fused to the signal for addition of glycosylphosphatidylinositol from the Thy-1 gene directs efficient expression of the ECD at the plasma membrane of transfected CHO cells. A cell line (GT14) expressing over 106 receptors/cell was isolated, which allows direct detection, by flow cytometry, of autoantibodies from the majority of patients with Graves' disease or autoimmune idiopathic myxedema. Treatment of GT14 cells with a glycosylphosphatidylinositol-specific phospholipase C (PI-PLC) releases a soluble 80 kDa molecule which neutralizes the autoantibodies from Graves patients. Whereas it does not bind TSH when released from the cells by PI-PLC in free form, the soluble ECD displays clear TSH binding activity when it is released as a complex with a monoclonal antibody recognizing a conformational epitope of the ECD. Our results allow production of bioactive ECD of the thyrotropin receptor in high yield, with possible applications in structural analyses.