Delivery of small interfering RNA (siRNA) for silencing of aberrantly expressed genes is a promising therapy for the treatment of various genetic disorders. Polymeric carriers have been used in the design of efficient delivery systems to generate nanoscale siRNA polyplexes. Despite the great amount of research pursued on siRNA therapeutics, the underlying mechanisms of polyplex dissociation in cytosol are still unclear. The fate of siRNA polyplexes during intracellular stages of delivery and how the endogenous molecules may affect the integrity of polyplexes remains to be explored. In this study, we have focused on miRNA-21 as a representative anionic endogenous molecule and performed gel electrophoresis mobility shift assays, particle size and zeta (zeta)-potential analyses, and a series of all-atom molecular dynamics simulations to elucidate the effect of miRNA on siRNA-PEI polyplexes. We report a slightly better binding to PEI by miRNA than that of siRNA, and speculated that miRNA may disrupt the integrity of preformed siRNA-PEI polyplexes. In contrast to our initial speculation, however, introduction of miRNA to a preformed siRNA-PEI polyplex revealed formation of a miRNA layer surrounding the polyplex through interactions with PEI. The resulting structure is a ternary siRNA-PEI-miRNA complex, where the experimentally determined zeta-potential was found to decrease as a function of miRNA added.
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