Cellular uptake mechanism and knockdown activity of siRNA-loaded biodegradable DEAPA-PVA-g-PLGA nanoparticles

Efficient downregulation of gene expression depends on the uptake, intracellular distribution and efficient release of siRNA from their carrier. Therefore, the cellular uptake behavior and mechanism and intracellular localization of siRNA-loaded biodegradable nanoparticles were investigated. A biodegradable polymer, composed of poly(vinyl alcohol) (PVA) modified with diamine moieties and grafted with PLGA, abbreviated as DEAPA-PVA-g-PLGA, was used for the preparation of siRNA-loaded nanoparticles by solvent displacement. Particle sizes and morphology were determined by dynamic light scattering (DLS) and scanning electron microscopy (SEM). The dependence of particle uptake into H1299-EGFP cells (lung cancer cells expressing green fluorescent protein) on both incubation time and temperature was studied by flow cytometry. Inhibition experiments focusing on clathrin- or caveolae-mediated uptake or uptake by macropinocytosis were performed. The intracellular localization was investigated by confocal laser scanning microscopy. The GFP knockdown efficiency was determined in vitro to establish the potential of the nanoparticles for the downregulation of gene expression. Nanoparticles with diameters of 120-180nm were successfully generated. In contrast to the uptake of standard PEI-polyplexes, which increased continuously over a period of 4h, nanoparticle uptake was complete within 2h. A decrease in particle uptake at 4°C (in comparison with 37°C) suggests an active uptake process. Inhibition experiments revealed the predominance of clathrin-mediated uptake for siRNA-loaded nanoparticles. The siRNA-loaded nanoparticles could be clearly located within cells, mainly in intracellular vesicles. Particle uptake could be increased by the addition of lung surfactant to the formulation. Bioactivity in terms of successful GFP knockdown in vitro was demonstrated and could be further optimized by the use of surfactant-modified particles. In conclusion, a high and rapid cellular uptake was shown for siRNA-loaded nanoparticles. Cell internalization is based on an energy-dependent and predominantly clathrin-mediated process. Particle localization in endosomes and lysosomes was demonstrated. Evidence for the efficient delivery of bioactive siRNA and specific GFP knockdown provides a solid basis for the application of DEAPA-PVA-g-PLGA-based particles for gene silencing in vivo.