Endocytosis mediated by the mannose receptor in liver endothelial cells. An immunocytochemical study

Immunocytochemical labeling of ultrathin cryosections from rat liver showed that mannose-terminated glycoproteins are removed rapidly from the blood stream mainly by the sinusoidal endothelial cells. The mannose-terminated glycoprotein ovalbumin was injected intravenously into rats 1 min, 6 min, and 24 min before perfusion fixation of the liver. Several minor and at least three major subcellular compartments were shown to be involved in the endocytic process. One minute after injection, ovalbumin was found at the cell surface, in coated pits, in coated vesicles, in tubular structures, and bound to the membrane of large early endosomes of which some showed a cisternal structure. After 6 min, ovalbumin was found in the lumen of large electron-lucent late endosomes and after 24 min in electron-dense structures, presumably lysosomes. The early endosomes have an ultrastructure which, together with the labeling pattern, indicates that this compartment has the same function as the CURL identified in parenchymal liver cells. The results are in accordance with recent biochemical findings indicating that ovalbumin endocytosed by endothelial cells is found sequentially in three different subcellular fractions depending on the time between injection and cooling for fractionation (G. M. Kindberg, T. Berg: Intracellular transport of endocytosed mannose terminated glycoproteins in rat liver endothelial cells. In: E. Wisse, D. L. Knook, K. Decker (eds.): Cells of the Hepatic Sinusoid. Vol. 2. pp. 120-124. Kupffer Cell Foundation. Rijswijk The Netherlands 1989).