Covalent crosslinking of thyrotropin to thyroid plasma membrane receptors: subunit composition of the thyrotropin receptor

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Withdrawn: TSH Receptor Cleavage Into Subunits and Shedding of the A-Subunit; A Molecular and Clinical Perspective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

An oxidation resistant radioligand for corticotropin-releasing factor receptors

The methionine residues in Tyr-corticotropin-releasing factor (CRF) and Tyr-sauvagine radioligands are subject to oxidation, which renders them biologically inactive. Therefore [Tyr(0,) Gln(1,) Leu(17)]sauvagine (YQLS), in which the methionine was replaced with leucine was synthesized and labeled with (125)Iodine using chloramine-T. Mass spectroscopy revealed that chloramine-T-treatment did not oxidize YQLS. (125)I-YQLS bound with high affinity to cells expressing the murine CRF receptor 1 (CRFR1), CRF receptor 2 (CRFR2), and the mouse brain regions known to express both CRF receptors. (125)I-YQLS chemically cross-linked to CRFR1. In conclusion, (125)I-YQLS is oxidation-resistant, high affinity radioligand that can be chemically cross-linked to the CRF receptors.

Immunoprecipitation isolates multiple TSH receptor forms from human thyroid tissue

Attempts to purify the thyroid-stimulating hormone receptor (TSHR) have been complicated by its susceptibility to proteolysis and its low level of expression in thyrocytes and many transfected cells. Controversy exists over its size and structure. Multiple, single-polypeptide forms of the TSHR (230, 187, and 95-100 kDa) have been recently identified in immunoblots of crude plasma membranes prepared from COS-7 cells transfected with rat or human cDNA, but the relationship of these receptor species to the TSHR in human thyroid tissue has been heretofore unknown. We have developed a technique for immunoprecipitation of the TSHR which employed IgG purified from a polyclonal rabbit antiserum to TSHR residues 352-366. We have used immunoprecipitation to isolate the previously characterized 95-100 kDa TSH-holoreceptor, 187 kDa intermediate, and 230 kDa precursor forms of the TSHR from plasma membrane prepared from transfected COS-7 cells and human thyroid tissue. The presence of all three forms was not altered by the addition of reducing agent to the sample buffer, demonstrating the single polypeptide structure of the TSHR. This is, to our knowledge, the first report of the purification from transfected COS-7 cells of these TSHR species identified previously only in immunoblots of crude plasma membrane, and the first report of the identification by any means of these TSHR forms in human thyroid tissue. The isolation of TSHR from human thyroid tissue requires confirmation by direct means, but promises to make the receptor available in a soluble form for studies of its structure and function.

Single subunit structure of the human thyrotropin receptor

We have produced rabbit antibody against a synthetic peptide corresponding to N-terminal region of the extracellular domain of human thyrotropin receptor (hTSH-R) (N peptide, aminoacid residues 29-57). Western blot analysis revealed that N-peptide antibody recognized recombinant hTSH-R stably expressing in CHO-K1 cells as a mol. wt. about 104 kDa regardless in the presence or absence of disulfide-reducing agent. The band was not detected in untransfected CHO-K1 cells and no band was also stained by the antibody absorbed with N-peptide. In a reducing condition, the antibody also bound the rat receptor from FRTL5 cells as the same molecular size (104 kDa). These results clearly indicate that TSH-R is composed of a single subunit and that two subunit model for the TSH-R may reflect artifactual proteolytic cleavage of the receptor during membrane preparation.

Binding of thyrotropin to selected Mycoplasma species: detection of serum antibodies against a specific Mycoplasma membrane antigen in patients with autoimmune thyroid disease

Radiolabeled human (hTSH) and bovine (bTSH) thyroid stimulating hormone was shown to bind to five species of Mycoplasma, the wall-less prokaryotes. The maximum binding capacity of 125I-bTSH to these five species was about 7.9 x 10(-13) moles-1.4 x 10(-12) moles for 50-100 micrograms protein with dissociation constants of approximately 1.7 to 2.2 x 10(-7)M. Approximately 50% of the 125I-bTSH binding was displaced by excess, unlabeled bTSH or hTSH, but labeled bTSH was not effectively displaced by growth hormone, LH, FSH, prolactin, or the beta subunit of hTSH, FSH and LH. Antisera prepared against Mycoplasma gallisepticum and Mycoplasma pneumoniae bound to human thyroid membranes and guinea pig fat cells, suggesting that receptors on human thyroid tissues and on Mycoplasma cells may have similarities in antigenicity. These findings were substantiated by the occurrence of TSH binding to Mycoplasma antisera. Further, sera from three of six patients with Graves' disease containing antibodies to thyroid tissues also reacted to a 108 Kd polypeptide of Mycoplasma gallisepticum.

Further studies on the covalent crosslinking of thyrotropin to its receptor: evidence that both the alpha and beta subunits of thyrotropin are crosslinked to the receptor

Highly purified alpha- and beta-subunits of thyrotropin were individually radioiodinated and, subsequently, recombined with their unlabeled complementary subunits. This procedure resulted in the formation of [125I]thyrotropin(TSH) hybrid molecules which were labeled on only one hormone subunit. Characterization of the binding properties of these two hybrid molecules demonstrated that both yielded nonlinear Scatchard plots with Kd and Bmax values similar to those obtained with radioiodinated native TSH and that both were capable of interaction with the high- and low-affinity binding components of the TSH receptor. The recombined [125I]TSH molecules were then crosslinked to the TSH receptor using disuccinimidyl suberate. Following electrophoresis and autoradiography, two labeled TSH-receptor complexes with Mr of 68,000 and 80,000 were observed. These two complexes exhibited hormone specificity and electrophoretic mobility identical to those previously observed using native [125I]TSH. Crosslinking with increasing concentrations of disuccinimidyl suberate suggested that the formation of the 68,000 and 80,000 complexes was sequential with the 68,000 appearing before the 80,000. Furthermore, the two bands were labeled regardless of which TSH subunit of the hybrid TSH was radioiodinated. These data strongly suggest that the 68,000 and 80,000 TSH-receptor complexes are the result of crosslinking to the TSH alpha-beta dimer and not to one subunit in the case of the 68,000 complex and to the TSH alpha-beta dimer in the case of the 80,000 complex, as had been hypothesized previously.