Discovery of antibodies against PTH1R, a class B GPCR: characterisation of their binding and impact on PTH1R function

Abstract

Parathyroid hormone receptor 1 (PTH1R) belongs to the Secretin or Class B family of the G-protein coupled receptor (GPCR) superfamily and natively binds parathyroid hormone (PTH) and parathyroid hormone related peptide (PTHrP). Ligand binding to PTH1R is a two-step process, which first involves binding to a large extracellular domain (ECD) and then to the extracellular loops (ECL). This in turn causes activation of the receptor by inducing conformational changes in the transmembrane domains. PTH1R plays a crucial role in bone metabolism and calcium homeostasis, signalling mainly through Gs and Gq/11 G-proteins. Here, we used phage display technology and immunisation followed by B-cell screening and isolation, to discover PTH1R-specific antibodies (mAbs) against the ECD and the ECL respectively. Antibody binding to the receptor was confirmed using ELISA, flow-cytometry and confocal microscopy. Intracellular cyclic AMP (cAMP) assay revealed that antibody binding had no effect on G-protein mediated signalling by the PTH1R. However, β-arrestin recruitment was down-regulated by the ECD specific antibody indicating that this antibody acts as a biased antagonist. These studies also highlighted that the ECL specific antibody acted as a neutral allosteric ligand (NAL). Affinity measurements by surface plasmon resonance using full-length receptor, solubilised in styrene-maleic acid copolymer (SMA), revealed binding affinities (KD) of ~ 4 nM and ~0.9 nM for ECD and ECL specific antibodies respectively. Epitope mapping using hydrogen-deuterium exchange-mass spectrometry (HDX-MS) indicated that the ECD antibody binding site had some overlap with the known PTH binding region of the ECD. HDX-MS also indicated that the ECL specific mAb bound to ECL-1. To the best of our knowledge, the ECD binding antibody is the first PTH1R antibody showing antagonism of β-arrestin recruitment. The ECL specific antibody, although not exhibiting function in the range assays tested, is anticipated to be a useful tool for PTH1R structural studies using cryo-electron microscopy (cryo-EM) and/or X-ray crystallography.