Internalization of Lectins in Neuronal GERL

Conjugates of ricin agglutinin and phytohemagglutinin with horseradish peroxidase (HRP) were used for a cytochemical study of internalization of their plasma membrane "receptors" in cultured isolated mouse dorsal root ganglion neurons. Labeling of cells with lectin-HRP was done at 4°C, and internalization was performed at 37°C in a culture medium free of lectin-HRP. 15-30 min after incubation at 37°C, lectin-HRP-receptor complexes were seen in vesicles or tubules located near the plasma membrane. After 1-3 h at 37°C, lectin-HRP-receptor complexes accumulated in vesicles and tubules corresponding to acid phosphatase-rich vesicles and tubules (GERL) at the trans aspect of the Golgi apparatus. A few coated vesicles and probably some dense bodies contained HRP after 3-6 h of incubation at 37°C. Soluble HRP was not endocytosed under the conditions of this experiment or when it was present in the incubation medium at 37°C. Internalization of lectin-HRP-receptor conjugates was decreased or inhibited by mitochondrial respiration inhibitors but not by cytochalasin B or colchicine. These studies indicate that lectin-labeled plasma membrane moieties of neurons are endocytosed primarily in elements of GERL.