Enzyme-based visualization of receptor-ligand binding in tissues

New methods of elucidating the ligand-binding activity of receptors could improve our understanding of receptor function, key events they control, and their presence in normal and pathological states. We describe a method for visualizing receptor-ligand binding in cells and tissues that substitutes fluorescein for radioactive labels, and detects receptor bound, fluoresceinated ligand with an antifluorescein/horseradish peroxidase amplification system. Receptor-bound ligand is then visualized by light microscopy against a standard hemotoxylin-stained background of cell structure. Quantitative versions of the assay provide an apparent dissociation constant and number of receptors per cell at saturation in cell or tissue specimens. Receptors examined include the folate receptor, bombesin peptide-binding receptors, the epidermal growth factor receptor, the neuropeptide Y receptor, the asialoglycoprotein receptor, and RGD peptide-binding integrins. Using fluoresceinated versions of molecules, we show the method can visualize and quantitate receptor-bound ligands in cell culture monolayers and animal tissue specimens. Ligand binding to receptors present in tissues was visualized in normal and pathological samples of human tissue microarrays. The enzyme-amplified detection of receptor-bound fluoresceinated ligand is a simple and nonradioactive-based method that provides information on the receptor activity in tissue specimens.