A quantitative electron microscopic study of the intracellular localization of wheat germ agglutinin in retinal neurons

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Abstract
Previous work has established that, following endocytosis, wheat germ agglutinin, like a number of other plasma membrane bound ligands, is transported to the Golgi apparatus-complex. Previous studies that provided qualitative information about the intracellular distribution of internalized wheat germ agglutinin used techniques that precluded any quantitative conclusions about the relative magnitude of the labeling of endosomes, lysosomes, and the Golgi apparatus-complex. Using quantitative ultrastructural autoradiography, this study compares the time course and relative magnitude of labeling of various intracellular compartments to the labeling in the Golgi area. Fifteen minutes after intraocular injection, wheat germ agglutinin is confined to the inner surface of the retina and the immediate subsurface neuropil with little labeling of the retinal ganglion cell perikarya. Thirty minutes after injection, the plasma membrane (6.97 ± 1.17), endosomes (10.27 ± 3.98), smooth vesicles and tubules (1.94 ± 1.66), and lysosomes (2.42 ± 1.21) of the retinal ganglion cells are labeled, while the Golgi apparatus-complex is not labeled (0.29 ± 0.25). (Figures in parentheses are the calculated relative radioactive source density ± the standard deviation.) The relative labeling density of the plasma membrane and endosomes decreases somewhat during the next 90 minutes (plasma membrane, 4.76 ± 0.67; endosomes, 7.23 ± 2.02), while the labeling density of smooth vesicles and tubules and of lysosomes rises (smooth vesicles and tubules, 5.56 ± 0.94; lysosomes, 7.76 ± 1.56). The Golgi apparatus-complex, which is unlabeled at 30 minutes, is weakly labeled at 2 hours (1.26 ± 0.28). The fact that the lysosomes are already labeled at 30 minutes while the Golgi apparatus-complex is unlabeled at that time indicates that the transport of wheat germ agglutinin to the Golgi apparatus-complex is a relatively late phenomenon, and suggests that the bulk of the wheat germ agglutinin destined for lysosomes does not pass through the Golgi apparatus-complex.

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