High affinity binding of thyrotropin (TSH) and thyroid-stimulating autoantibody for the TSH receptor extracellular domain.

There have been some controversial data as to whether the extracellular domain of the thyrotropin receptor (TSHR) is sufficient for constituting the high affinity binding site(s) for TSH and thyroid-stimulating antibody (TSAb). The present study was, therefore, designed to further evaluate the functional significance of the TSHR extracellular domain. The new chimeric receptor (designated TSHEX-LHTMR) consisting of the human (h) TSHR extracellular domain and the rat lutropin/choriogonadotropin (LH/CG) receptor transmembrane region was constructed, stably expressed in Chinese hamster ovary cells, and tested for its abilities to bind TSH and hCG and to increase intracellular cAMP production in response to hormone and TSAb stimulation. The binding affinity for TSH and the ability to produce cAMP in response to TSH stimulation in the chimeric receptor TSHEX-LHTMR was comparable to those in the wild-type (wt) TSHR (Kd = approximately 0.3 nM and EC50 = approximately 3 nM). The TSHEX-LHTMR and the wt-TSHR also demonstrated similar TSAb activity. However, the TSHEX-LHTMR, unlike the wt-LH/CGR, did not bind hCG or respond to hCG stimulation. These results demonstrate that the functional properties of the TSHR are not affected by the replacement of the receptor transmembrane region with the corresponding region of the LH/CGR, suggesting, together with other previous reports, that the TSHR extracellular domain appears to be of primary importance for the high affinity binding for TSH and TSAb, although the TSHR transmembrane region can contribute to high affinity binding and also bioactivity for TSH and TSAb.