Crystal structure of a ligand-free stable TSH receptor leucine-rich repeat domain.

The crystal structures of the thyroid-stimulating hormone receptor (TSHR) leucine-rich repeat domain (amino acids 22-260; TSHR260) in complex with a stimulating human monoclonal autoantibody (M22TM) and in complex with a blocking human autoantibody (K1-70™) have been solved. However, attempts to purify and crystallise free TSHR260, that is not bound to an autoantibody, have been unsuccessful due to the poor stability of free TSHR260. We now describe a TSHR260 mutant that has been stabilised by the introduction of six mutations (H63C, R112P, D143P, D151E, V169R and I253R) to form TSHR260-JMG55TM, which is approximately 900 times more thermostable than wild-type TSHR260. These six mutations did not affect the binding of human TSHR monoclonal autoantibodies or patient serum TSHR autoantibodies to the TSHR260. Furthermore, the response of full-length TSHR to stimulation by TSH or human TSHR monoclonal autoantibodies was not affected by the six mutations. Thermostable TSHR260-JMG55TM has been purified and crystallised without ligand and the structure solved at 2.83 Å resolution. This is the first reported structure of a glycoprotein hormone receptor crystallised without ligand. The unbound TSHR260-JMG55TM structure and the M22 and K1-70 bound TSHR260 structures are remarkably similar except for small changes in side chain conformations. This suggests that neither the mutations nor the binding of M22TM or K1-70TM change the rigid leucine-rich repeat domain structure of TSHR260. The solved TSHR260-JMG55TM structure provides a rationale as to why the six mutations have a thermostabilising effect and provides helpful guidelines for thermostabilisation strategies of other soluble protein domains.

3 Screen islet cell autoantibody ELISA: A sensitive and specific ELISA for the combined measurement of autoantibodies to GAD65, to IA-2 and to ZnT8.

3 Screen, a new ELISA for the combined measurement of autoantibodies to GAD65, to IA-2 and to ZnT8, has been developed and evaluated. In the assay serum samples were incubated (overnight; 2-8°C) in ELISA plate wells coated with GAD65, IA-2 and ZnT8 followed by a wash step and incubation with biotinylated GAD65, IA-2 and ZnT8 (1h; 2-8°C,). The assay was completed by addition of streptavidin-peroxidase and tetramethylbenzidine. Samples tested in the 3 Screen were also analysed in ELISAs and radiobinding assays for the three individual autoantibodies. 129/132 (98%) samples from newly diagnosed T1DM children and 1/100 non-diabetic children controls were positive in 3 Screen. There was good agreement between 3 Screen and the individual autoantibody assays. Dilution of positive samples showed good linearity characteristics. In the 2015 Islet Autoantibody Standardization Program 3 Screen achieved 94% sensitivity, 95.6% specificity and 0.948 area under curve by ROC analysis. 3 Screen provides a simple and sensitive method for combined measurement of three major diabetes associated autoantibodies in a single sample. The assay should be a useful tool for large scale population screening for individuals at risk of developing T1DM.