Mechanisms of cellular uptake and endosomal escape of calcium-siRNA nanocomplexes

Ca²⁺-siRNA nanocomplexes represent a simple yet an effective platform for siRNA delivery into the cell cytoplasm, with subsequent successful siRNA-induced target gene silencing. Herein, we aimed to elucidate the roles played by calcium ions in siRNA nanocomplex formation, cell uptake, and endosomal escape. We investigated whether the replacement of Ca²⁺in the nanocomplex by other bivalent cations would affect their cell entry and subsequent gene silencing. Our results indicate that Mg²⁺ and Ba²⁺ lead to the formation of nanocomplexes of similar physical features (size = 100 nm, surface charge ζ = −8 mV) as the Ca²⁺-siRNA nanocomplexes. Yet, these nanocomplexes were not uptaken by the cells to the same extent as those prepared with Ca²⁺, and siRNA-induced target gene silencing was not obtained. Cell internalization of Ca²⁺⁻-siRNA nanocomplexes, examined by employing chemical inhibitors to clathrin-, caveolin- and dynamin-mediated endocytosis pathways, indicated the involvement of all mechanisms in the process. Inhibition of endosome acidification by bafilomycin completely abolished the siRNA-mediated silencing by Ca²⁺-siRNA nanocomplexes. Collectively, our results indicate that Ca²⁺ promotes cell internalization and rapid endosomal escape, thus leading to the efficient siRNA-induced target gene silencing elicited by the Ca²⁺-siRNA nanocomplexes.