Multicomponent structure of the thyrotropin receptor: Relationship to Graves' disease

Abstract

The thyrotropin receptor is proposed to contain both a glycoprotein and a ganglioside component. Monoclonal antibodies have been developed against soluble thyroid TSH receptor preparations and using Graves' lymphocytes. These show that initial recognition of thyrotropin requires the glycoprotein component, but that monoclonal antibodies to this component block thyrotropin function (“blocking antibodies”) rather than mimic thyrotropin. Monoclonal antibodies which stimulate thyroid activity in cultured cell systems (cAMP increase) or mouse bioassays, all interact with gangliosides. Using monoclonal antibodies to the glycoprotein component of the thyrotropin receptor, we show that two protein bands, molecular weights 18,000–23,000 and 50,000–55,000, are precipitated from detergent-solubilized preparations. Using a crosslinking procedure with 125I-labeled thyrotropin, we show that thyrotropin binding is related to the disappearance of the 18,000–23,000 molecular weight band on sodium dodecylsulfate gels and the appearance of a 30,000–33,000 molecular weight thyrotropin-membrane component complex. Higher molecular weight thyrotropin-membrane complexes of 75,000–80,000 and 250,000 are visualized when binding studies are performed at pH 7.4 in physiologic medium; larger amounts of the 30,000–33,000 complex are evident at pH 6.0 in a low salt medium. It is thus proposed that the glycoprotein component of the thyrotropin receptor is composed of two subunits with apparent molecular weights of 18,000–23,000 and 50,000–55,000; that the 18,000–23,000 subunit interacts with thyrotropin; and that different receptor subunits can exist depending on in vitro binding conditions. Using monoclonal-stimulating antibodies or natural autoimmune IgG preparations from patients' sera, we show that stimulating antibodies exhibit species-specific binding to human thyroid ganglioside preparations. The data unequivocally establish the pluritopic nature of the immunoglobulins in Graves' disease; demonstrate their effector potential in the pathogenesis of the disorder; and relate individual components or determinants of the thyrotropin receptor structure with specific autoimmune immunoglobulins.