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

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

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

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.

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

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