Development of a Cell-Based Assay to Assess Binding of the proNGF Prodomain to Sortilin

Sortilin was first identified based on its activity as part of intracellular protein sorting machinery. Recently, it was discovered that sortilin also acts as a cell surface receptor for the propeptide form of nerve growth factor (proNGF), progranulin, and neurotensin. The interaction of sortilin to these neurotrophic ligands is linked to diseases of the nervous system that lead to neurodegeneration and neuropathic pain. Blocking of the interaction of sortilin to these ligands may prevent or slow the progress of these nervous system disorders. In vitro screening assays for blocking compounds or peptides are part of the standard set of tools for drug discovery. However, assays for sortilin biology are not readily available to determine if the selected blocking agent inhibits sortilin activity on the surface of cells. We have developed a sortilin specific cell based assay to identify compounds that specifically block interaction between sortilin and proNGF prodomain. The assay system records both the presence of sortilin on the cell surface and the interaction with the pro domain of NGF. Fluorescent images of the sortilin expressing cells are analyzed for the presence of pro domain of NGF. Sortilin-positive and sortilin-negative cells within one well are concomitantly and automatically analyzed. Sortilin-pro domain interaction can be blocked dose dependently by neurotensin and synthetic compounds. The assay will facilitate the discovery of entities interfering with the binding of sortilin to the NGF pro domain. This assay can be modified to screen for inhibitors of the binding of ligands to other complex cell surface receptors.