The Role of Ganglioside GM1 in Cellular Internalization Mechanisms of Poly(amidoamine) Dendrimers

Generation 7 (G7) poly (amidoamine) (PAMAM) dendrimers with amine, acetamide, and carboxylate end groups were prepared to investigate polymer/cell membrane interactions in vitro. G7 PAMAM dendrimers were used in this study because higher-generation of dendrimers are more effective in permeabilization of cell plasma membranes and in the formation of nanoscale holes in supported lipid bilayers than smaller, lower-generation dendrimers. Dendrimer-based conjugates were characterized by H-1 NMR, UV/vis spectroscopy, GPC, HPLC, and CE. Positively charged amine-terminated G7 dendrimers (G7-NH2) were observed to internalize into KB, Rat2, and C6 cells at a 200 nM concentration. By way of contrast, neither negatively charged G7 carboxylate-terminated dendrimers (G7-COOH) nor neutral acetamide-terminated G7 dendrimers (G7-Ac) associated with the cell plasma membrane or internalized under similar conditions. A series of in vitro experiments employing endocytic markers cholera toxin subunit B (CTB), transferrin, and GM(1)-pyrenc were performed to further investigate mechanisms of dendrimer internalization into cells. G7-NH2 dendrimers colocalized with CTB; however, experiments with C6 cells indicated that internalization of G7-NH2 was not ganglioside GM, dependent. The G7/CTB colocalization was thus ascribed to an artifact of direct interaction between the two species. The presence of GM(1) in the membrane also had no effect upon XTT assays of cell viability or lactate dehydrogenase (LDH) assays of membrane permeability.