Recently, many researchers have reported that DNA nanostructures, such as tetrahedral framework nucleic acids (tFNAs), have great potential to be useful tools in clinical and laboratory applications due to their programmable shapes, functional sites, and biological responses. However, finite endocytosis and stability in cells and body fluids compromise the functions of DNA nanostructures as a result of various adverse factors. In this study, we successfully synthesized PEGylated protamine, and tFNAs were adsorbed to it in a proper ratio of nitrogen in protamine to phosphorus in tFNAs (N/P ratio) as the functional complex. Furthermore, we demonstrated that PEGylated protamine-adsorbed tFNAs show a more prominent positive effect on cell viability and proliferation than naked tFNAs do. An increase in endocytosis can be observed in three different tissue-derived cells with the PEG-protamine-tFNA (PPT) complex. The increased endocytic ability is mediated by multiple pathways; moreover, the stimulatory effect of the PPT complex on the endocytic ability is dramatically blocked by the inhibition of the caveola-dependent pathway. Consistently, when tFNAs are stabilized by PEGylated protamine, they often tend to escape from lysosomes and survive for a longer period in biological fluids rather than being rapidly eliminated from the kidneys. The in vitro and in vivo results of our study demonstrate that the PPT complex method is a feasible, potent, and low-cost strategy that improves tFNA biocompatibility, stability, and internalization. This study provides evidence supporting the possibility of implementing PPTs for use in drug delivery, bioimaging, and gene transfection in the future.
Keywords: PEGylated protamine; endocytosis; in vivo stability; lysosomal escape; tFNAs.