Measurement of receptor endocytosis and recycling.

Receptor trafficking is essential to the delivery of nutrients and to the proper regulation of signaling pathways in mammalian cells. Numerous transmembrane receptors undergo clathrin-mediated endocytosis, followed by sorting in the early endosome. The low-density lipoprotein (LDL) receptor-related protein (LRP) is a multiligand endocytic receptor and a member of the LDL receptor family. At the cell surface, it binds to and continuously internalizes numerous ligands including lipoproteins, proteases, protease inhibitors, growth factors, and beta-amyloid precursor protein via clathrin-mediated endocytosis. Its rapid endocytosis rate allows efficient clearance of extracellular and transmembrane ligands. Once internalized into the early or sorting endosome, LRP ligands are delivered to lysosomes to be degraded, whereas LRP is efficiently recycled to the plasma membrane. Herein, the authors describe quantitative methods to measure the endocytosis and recycling capacity of receptors, using LRP as a model receptor.