Recombinant human TSH receptor expressed in E. coli

Characterization of the secretable ectodomain of thyrotropin receptor produced by the recombinant baculovirus system

Thyrotropin receptor (TSHR) is a member of the glycoprotein hormone receptor family and an autoantigen of Graves' disease. Various attempts have been made to obtain a large amount of soluble ectodomain of TSHR in insect or mammalian cells, but most of them failed to secrete the overexpressed ectodomain. In the present study, we observed that about one-third of the ectodomain protein (sTSHR-gp), in which the signal peptide of TSHR was replaced by the baculovirus-encoded glycoprotein 67-signal peptide, was secreted into the culture medium and the remainder stayed within cells in the recombinant baculovirus system. Microsequencing the N-terminal of the purified protein confirmed that the baculovirus signal peptide was cleaved at the expected site. Carbohydrate studies using several glycosidases and lectins revealed that the secreted form of the ectodomain had biantennary carbohydrate, whereas the non-secreted form had high-mannose. Moreover, the secreted form of sTSHR-gp exhibited high-affinity ligand binding, whereas the non-secreted form did not show any significant ligand binding. Regarding the interactions of TSHR ectodomains with anti-TSHR antibodies, both the secreted and non-secreted forms of sTSHR-gp, almost completely neutralized the stimulatory and inhibitory anti-TSHR antibody activities. In conclusion, we succeeded in secreting the ectodomain of TSHR into culture medium, which was capable of binding to TSH and neutralizing anti-TSHR antibody activities.

Different bioactivities of human thyrotropin receptors with different signal peptides

For investigation of the mechanism and pathogenesis of Graves' disease, availability of a large amount of functional human thyrotropin receptor (TSHR) capable of recognition by Graves' autoantibodies is essential. Many attempts have been made to produce the extracellular domain of TSH receptor (TSHRE) in a baculovirus expression system. However, the receptor is expressed as an insoluble form and the refolded protein is often not recognized by the autoantibodies. In this study, we found that the TSHRE expressed with its own signal peptide (VL3-RE) in insect cells is retained inside of the cells and found in both soluble and insoluble fractions in equal proportion. The signal peptide is not removed. The receptor in the soluble fraction is not recognized by either TSH or Graves' autoantibodies. The TSHRE with an insect-specific mellitin signal peptide (Mel-RE) is also retained inside of the cell and found in both the soluble and insoluble fractions in equal proportion. However, the signal peptide is removed and the receptor is recognized by the Graves' autoantibodies but not by TSH. Also, the amount of Mel-RE expressed was 5-10-fold higher than VL3-RE. The two receptor preparations apparently have the same degree of glycosylation as evidenced by the same increased mass (approximately 15 kDa) due to glycosylation. However, the two receptors have different affinity for an anion-exchange resin and different pI. Deglycosylated receptors have the same pI. This suggests that the composition of sugars may be different. Taken together, the results suggest that the two receptors are modified and folded differently by different pathways due to the presence of different signal peptides. Use of an insect-specific signal peptide is recommended for expression of TSHR that is recognized by Graves' autoantibodies in a baculovirus system.