Functional effects of monoclonal antibodies to the purified amino-terminal extracellular domain of the human Ca²⁺ receptor

We generated three functionally unique monoclonal antibodies to the purified human CaR extracellular domain. Flow cytometry studies of chimeric receptors localized their epitopes to lobe 2 of the VFT domain. These results lead us to propose a mechanism for the functional effects of these antibodies. Introduction: The human Ca²⁺ receptor (CaR), which plays a central role in the regulation of [Ca²⁺]₀ homeostasis, has a distinctively large extracellular domain that consists of a bilobed Venus flytrap (VFT) domain, involved in agonist binding, and a cysteine-rich domain. Functional antibodies that specifically bind to this domain would have therapeutic potential and could be used as a tool to gain insights into receptor activation as well. Materials and Methods: We generated three monoclonal antibodies (mAbs), 7F8, 5C8, and 1A8, to the purified human CaR extracellular domain. Functional characterization of these antibodies included Ca²⁺ stimulation of phosphoinositide hydrolysis to examine effects of intact or protease digested antibodies on sensitivity of the receptor to extracellular Ca²⁺ and flow cytometry assay of binding of the antibodies to HEK-293 cells expressing chimeric receptors to map antibody epitopes. Results: We found these mAbs specifically recognize native but not denatured human CaR or homologous native Fugu CaR. Sensitivity of the human CaR to extracellular calcium was increased by binding of 5C8 but decreased by binding of 1A8. A chimeric receptor FCFCF, with lobe 2 region of the human CaR VFT domain in the Fugu CaR backbone, bound all three mAbs, and the sensitivity of this chimeric CaR to extracellular Ca²⁺ was also increased by binding of 5C8 and decreased by binding of 1A8. Conclusions: The epitopes of these mAbs reside in the lobe 2 region of the human CaR VFT domain. 5C8 might activate the receptor by facilitating closure and/or rotation of the VFT domains on agonist binding, whereas 1A8 might inhibit the receptor by impeding such agonist-induced conformational changes. Recombinant antibodies with antigen binding domains of 5C8 and 1A8 could be useful in the treatment of hyperparathyroidism and osteoporosis, respectively.