Mannose-receptor-mediated clearance of lysosomal α-mannosidase in scavenger endothelium of cod endocardium

Abstract

Mannose-receptor-mediated clearance of circulating glycoproteins was studied in Atlantic cod ( Gadus morhua) . Distribution studies with radioiodinated and fluorescently labelled ligands showed that cod liver lysosomal α-mannosidase and yeast invertase were rapidly eliminated from blood via a mannose specific pathway in liver parenchymal cells and endocardial endothelial cells of atrium and ventricle. Asialo–orosomucoid, a galactose-terminated glycoprotein, was cleared by liver only. In vitro studies were performed with primary cultures of atrial-endocardial endothelial cells (AEC), incubated at 12°C in a serum free medium. Cod AEC endocytosed mannose-terminated glycoproteins ( 125I-α-mannosidase, 125I-invertase, 125I-mannan, 125I-ovalbumin and unlabelled lysosomal α-mannosidase), whereas 125I-asialo–orosomucoid was not recognised. Uptake of radiolabelled mannose-terminated ligands was inhibited 80–100% in the presence of excess amounts of mannan, invertase, d-mannose, l-fucose or EGTA. Our results suggest that the cod endocardial endothelial cells express a specific Ca 2+-dependent mannose receptor, analogous to the mannose receptor on mammalian macrophages and liver sinusoidal endothelial cells.