Intracellular transport of ovalbumin afterin vivo endocytosis in rainbow trout liver

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

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 alpha-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 degrees C in a serum free medium. Cod AEC endocytosed mannose-terminated glycoproteins (125I-alpha-mannosidase, 125I-invertase, 125I-mannan, 125I-ovalbumin and unlabelled lysosomal alpha-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.

The effect of temperature on intracellular transport of asialo‐glycoproteins in rainbow trout liver

The intracellular pathway of a endocytosed glycoprotein has been studied in rainbow trout (Oncorhynchus mykiss) liver by means of differential and isopycnic gradient centrifugation. When 125I‐tyramine‐cellobiose‐labelled asialoorosomucoid was injected intravenously, the protein was removed rapidly from the blood by hepatic endocytosis. The tracer was localized initially in a small, slowly sedimenting organelle (endosome) and transferred to the denser lysosomes where degradation took place. This process took several hours at the acclimated temperature of 10 ° C. The transport could be accelerated markedly or retarded by transfer of fish to higher or lower temperatures respectively. The intracellular transport steps were more susceptible to changing temperature than the internalization process. In particular, the degradation was temperature sensitive. These results may have implications for the effects of changing temperature on immune function in fishes. The impairment of transport upon a rapid temperature decrease, will reduce the rate of antigen processing and presentation in these animals.

Hepatic uptake and intracellular processing of LDL in rainbow trout

The process of receptor-mediated endocytosis in poikilothermic vertebrates such as salmonid fish have not been subjected to much research, compared to the detailed studies done in mammalian systems. We have investigated the hepatic uptake and intracellular processing of low-density lipoprotein (LDL) in rainbow trout liver. After intravenous injection of the [125I]tyramine-cellobiose ([125I]TC) -labelled lipoprotein, the liver was perfused and cells isolated or fractionated by differential centrifugation and isopycnic centrifugation in Nycodenz gradients. We found that LDL was mainly endocytosed by parenchymal cells of the liver. Cell fractionation experiments showed that LDL was localized sequentially in three groups of organelles of increasing density. Initially, LDL was localized in small, slowly sedimenting endosomes before being transferred to denser endosomes (prelysosomes) and finally to dense lysosomes. The lysosomes were identified by three lysosomal marker enzymes. Degradation products formed from [125I]TC-labelled LDL could also be detected in prelysosomal vesicles. In vitro experiments with cultured trout hepatocytes demonstrated that intracellular processing of [125I]TC-LDL in these cells could be suppressed by endocytic and lysosomal inhibitors. The catabolism of LDL in rainbow trout therefore follows the endocytic-lysosomal pathway described for many macromolecules in mammalian cells.