Tetanus toxin binding to mouse spinal cord cells: an evaluation of the role of gangliosides in toxin internalization.

Studies on the binding of 125I-labelled tetanus toxin to mouse spinal cord cell cultures revealed two classes of toxin acceptor, one a high-affinity protease-sensitive site, the other a lower affinity protease-resistant site. The latter was shown to be sialidase-sensitive and may represent binding to gangliosides. Tetanus toxin internalization by spinal cord cells is rapid and coated pits have been implicated in the process (Parton et al. J. Neurochem., 49 (1987) 1057-1068). To investigate the role of gangliosides in the internalization process, trisialoganglioside was incorporated into the plasma membrane of Balb/c 3T3 cells which lack endogenous toxin-binding activity. 125I-labelled tetanus toxin bound specifically to ganglioside treated cells at 4 degrees C. On warming cells to 37 degrees C, a proportion of bound toxin was degraded indicating that some internalization of toxin had occurred. The mechanism of internalization was studied using tetanus toxin adsorbed to colloidal gold. At 4 degrees C, toxin-gold was concentrated in non-coated cell surface membrane invaginations. On warming cells to 37 degrees C, toxin gold was internalized via non-coated vesicles and accumulated within multivesicular bodies and lysosomes. The relative roles of coated and non-coated vesicular uptake systems in the mechanism of action of tetanus toxin are discussed.