The activation mechanism of glycoprotein hormone receptors

• Applicant: Dr. Antje Brüser
• Subject Area: Endocrinology, Diabetology, Metabolism ; Biochemistry ; Pharmacology
• Term: from 2015 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 276121400

Glycoprotein hormone receptors (GPHR) are a subfamily within the class of rhodopsin-like G protein-coupled receptors (GPCR) and possess a large extracellular domain. Glycoprotein hormones (GPH) are the main regulators of the pituitary-thyroid and gonadal axes in vertebrates. Selective interactions between the GPH (TSH, LH, hCG, FSH) and their cognate GPHR (TSHR, LHR, FSHR) ensure specificity in GPH signaling. However, autoantibodies and mutations in GPHR can cause endocrine dysfunctions such as Graves' disease, hypo- and hyperthyroidism. The fundamental mechanism how the hormones can activate GPHR and how mutations and autoantibodies can effect receptor function remains unclear. In preliminary work we could show that a short peptide sequence in the C-terminal part of the extracellular N terminus functions as a tethered agonist for all GPHR and is conserved between ortholos and paralogs. Additionally, inactivating mutations in this activating sequence of GPHR displayed no activity in cAMP assays after stimulation with their GPH but could be activated by the agonistic peptide. Modified peptides are not able to activate GPHR but can block GPHR activation induced by GPH. Obviously, GPH and mutations in the extracellular domain may initiate structural changes within the ectodomain of GPHR and the intramolecular agonist is exposed to the 7-transmembrane helix domain triggering G-protein activation. Currently, the molecular mechanism unmasking this tethered agonistic domain is speculative. The aim of this proposal is to understand the mechanics how the tethered agonist is exposed to and how it interacts with the transmembrane domain in order to initiate signaling. Thus, we will utilize mutagenesis studies combined with homology modelling and direct structural analysis. Further, since the peptide agonist is identical between all GPHR but the receptors differ in their signal transduction the small sequence differences between the GPHR must account for the differences in signaling abilities which will be elucidated in this study. As a translational outcome our new data will help to evaluate the pharmacological potential of this activation mechanism. In line with this, the identification of the tethered peptide agonist for GPHR now allows for identification of the agonist binding pocket and a rational ligand design.

• DFG Programme: Research Grants