Endocytosis of cholera toxin into neuronal GERL.

Cholera toxin linked covalently by glutaraldehyde to horseradish peroxidase was incubated with cultured chicken sympathetic neurons at 4 degrees. Cells were washed and brought to 37 degrees to permit endocytosis of bound toxin on plasma membranes. Massive internalization of the ligand into vesicles and cisterns of the Golgi--endoplasmic reticulum--lysosome (GERL) system was demonstrated by the cytochemical reaction for the enzyme. Surface binding and subsequent endocytosis of the cholera toxin--enzyme conjugate was inhibited when conjugate and monosialoganglioside (GM1) were simultaneously applied to cells at 4 degrees. Cholera toxin is not toxic to neurons at the levels used. These results indicate that GERL is the primary site of endocytosis of presumed complexes of cholera toxin with its plasma membrane receptor (GM1 ganglioside-containing moieties). It is suggested that, in neurons, plasma-membrane bound ligands are taken up primarily into GERL.