The role of ganglioside GM ₁ 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 ¹ H NMR, UV/vis spectroscopy, GPC, HPLC, and CE. Positively charged amine-terminated G7 dendrimers (G7-NH ₂ ) 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 -pyrene were performed to further investigate mechanisms of dendrimer internalization into cells. G7-NH ₂ dendrimers colocalized with CTB; however, experiments with C6 cells indicated that internalization of G7-NH ₂ was not ganglioside GM ₁ 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.