Receptor-mediated endocytosis of ricin in rat liver endothelial cells. An immunocytochemical study.

The endocytic pathway of ricin in sinusoidal liver endothelial cells (EC) was traced by means of immunocytochemical labeling of ultrathin cryosections. Ricin, a highly mannosylated glycoprotein, is internalized mainly by receptor-mediated endocytosis via the mannose receptor in the EC. Labeling of specimens fixed at different time points after injection of ligand showed that several subcellular compartments are involved in processing of endocytosed ricin. One minute after injection ricin is seen in coated pits, coated vesicles and cisternal-shaped endosomes. After 6 min, the ligand associates with electron-dense, spherical vesicles and electron-lucent vesicles, presumably representing late endosomes. In the same time period we observed labeling in the vicinity of the Golgi stack. At later time points, ricin is increasingly localized in lysosomes. Both late endosomes and lysosomes showed labeling for Igp120, the lysosomal membrane glycoprotein. To compare uptake of ricin with another mannosylated ligand, we coinjected ricin and mannosylated colloidal gold particles (Man-Aun). Man-Au20, injected 24 h before fixation as a marker for late endocytic compartments, was found in two distinct compartments, presumably representing late endosomes and lysosomes. The distribution of ricin and Man-Au10, the latter injected 15 min before fixation, in early endosomes was strikingly different, indicating that the structure of this compartment is important in the process of sorting of ligand and receptor.