In vivo imaging of ligand receptor binding with Gaussia luciferase complementation

New methods for analyzing ligand-receptor binding under physiologic conditions in cell-based assays and living animals are much needed for the testing and validation of candidate therapeutic agents targeting ligand-receptor binding. Here, Kathryn Luker and her colleagues developed a molecular imaging assay for ligand-receptor binding based on Gaussia luciferase complementation, focusing on chemokine CXCL12 and its chemokine receptors CXCR4 and CXCR7, although the assay could be applied more broadly to any receptor with a protein ligand. Studies of ligand-receptor binding and the development of receptor antagonists would benefit greatly from imaging techniques that translate directly from cell-based assays to living animals. We used Gaussia luciferase protein fragment complementation to quantify the binding of chemokine (C-X-C motif) ligand 12 (CXCL12) to chemokine (C-X-C motif) receptor 4 (CXCR4) and CXCR7. Studies established that small-molecule inhibitors of CXCR4 or CXCR7 specifically blocked CXCL12 binding in cell-based assays and revealed differences in kinetics of inhibiting chemokine binding to each receptor. Bioluminescence imaging showed CXCL12-CXCR7 binding in primary and metastatic tumors in a mouse model of breast cancer. We used this imaging technique to quantify drug-mediated inhibition of CXCL12-CXCR4 binding in living mice. We expect this imaging technology to advance research in areas such as ligand-receptor interactions and the development of new therapeutic agents in cell-based assays and small animals.