Characterization of degradable polyelectrolyte multilayers fabricated using DNA and a fluorescently-labeled poly(β-amino ester): shedding light on the role of the cationic polymer in promoting surface-mediated gene delivery.

Polyelectrolyte multilayers (PEMs) fabricated from cationic polymers and DNA have been investigated broadly as materials for surface-mediated DNA delivery. One attractive aspect of this "multilayered" approach is the potential to exploit the presence of cationic polymer "layers" in these films to deliver DNA to cells more effectively. Past studies demonstrate that these films can promote transgene expression in vitro and in vivo, but significant questions remain regarding roles that the cationic polymers could play in promoting the internalization and processing of DNA. Here, we report physicochemical and in vitro cell-based characterization of DNA-containing PEMs fabricated using fluorescently end-labeled derivatives of a degradable polycation (polymer 1) used in past studies of surface-mediated transfection. This approach permitted simultaneous characterization of polymer and DNA in solution and in cells using fluorescence-based techniques, and provided information about the locations and behaviors of polymer 1 that could not be obtained using other methods. LSCM and flow cytometry experiments revealed that polymer 1 and DNA released from film-coated objects were both internalized extensively by cells and that they were colocalized to a significant extent inside cells (e.g., ~58% of DNA was colocalized with polymer). Fluorescence anisotropy measurements of solutions containing partially eroded films were also consistent with the presence of aggregates of polymer 1 and DNA in solution (e.g., after release from surfaces, but prior to internalization by cells). Our results support the view that polymer 1, which is incorporated into these materials as "layers" rather than as part of optimized, preformed "polyplexes", can act to promote or enhance surface-mediated DNA delivery. More broadly, our results suggest opportunities to improve the delivery properties of DNA-containing PEMs by incorporation of additional "layers" of other conventional cationic polymers designed to address specific intracellular barriers to transfection, such as endosomal escape, more effectively.