Endocytosis and compartmentation of peroxidases and cationized ferritin in neuroblastoma cells

To investigate further the selectivity of retrograde axonal transport of horseradish peroxidase (HRP) isoenzymes previously observed in the rat central visual pathways, cultured mouse neuroblastoma cells (clone N18) were examined for selectivity of endocytosis of peroxidases and cationized ferritin in vitro. Differentiating N18 cells were incubated with proteins in various timing and pulse-chase experiments, and examined for the ultrastructural localization of endocytosed protein using cytochemical techniques. Semi-quantitative morphometry was performed on some of the samples. Major findings were as follows. (1) The protein was internalized into vesicles (coated and uncoated), short tubules and occasional small cup-shaped bodies within the first few minutes, and transferred via tubules and uncoated vesicles to secondary lysosome-like organelles (vacuoles, multivesicular bodies and dense bodies) from 5 to 15 min, with dense bodies representing the final site of protein accumulation. All the proteins tested were endocytosed through the same pathways in the soma and neurite of the cell. (2) There was no indication of a net orthograde or retrograde neuritic transport proteins. (3) There were induced increases in both vesicle and tubule formation as a result of protein endocytosis, with HRP isoenzyme C showing the greatest effect. (4) The apparent rate of internalization of HRP isoenzyme C into vesicles during the initial 5 min was significantly greater than for other peroxidases amd cationized ferritim. (5) Relatively more tubules than vesicles were involved in the uptake of protein as endocytosis was prolonged from 5 min to 8 h, with HRP isoenzyme C showing the largest effect. (6) There were indications of preferential compartmentation of endocytosed protein into certain organelles.