We previously found that the 19 amino-acid fusogenic peptide named S19 from human syncytin 1, a protein involved in cell membrane fusion in placenta formation, enhances the efficiency of the intracellular delivery of TAT-fused proteins. Although there is concern about the low efficiency of endosomal escape after uptake into cells, several approaches have been shown to enhance the efficiency of endosomal escape of the TAT peptide. The first and most commonly used CPP is a cationic peptide TAT derived from HIV1 that binds to the cell surface by electrostatic interaction and is taken up into cells by endocytosis. In recent years, membrane-active peptides such as cell-penetrating peptides (CPPs) have been widely used for the intracellular delivery of various biomacromolecules, such as proteins and nucleic acids, as well as nanomaterials for biomedical and nanobiotechnological applications. These results indicated that S19 and TAT-fused siRNA-binding proteins, especially Ago2, should be useful for the rapid and efficient delivery of siRNA without the addition of any endosome-disrupting agent. In addition, we found that the smallest RISC delivery induced faster knockdown than traditional siRNA lipofection, probably due to the decreased time required for RISC formation in the cytoplasm. Thus, we next prepared fusion proteins in which the S19 and TAT peptides were fused with Ago2 instead of RBD and found that the efficiencies of siRNA delivery and knockdown obtained using TAT-S19-Ago2 were higher than those using TAT-Ago2. Because siRNA must dissociate from RBD and bind to Argonaute 2 (Ago2) to form the RNA-induced silencing complex (RISC) after the protein/siRNA complex is delivered into the cytoplasm, a dilemma arises: stronger binding between RBD and siRNA increases intracellular uptake but makes RISC formation more difficult. We found that the intracellular uptake of the RBD-S19-TAT/siRNA complex was increased compared to that of the RBD-TAT/siRNA complex, and the expression level of the target mRNA was decreased. ![]() ![]() We prepared fusion proteins in which the S19 and TAT peptides were fused to the viral RNA-binding domains (RBDs) as carrier proteins, added the RBD-S19-TAT/siRNA complex to human cultured cells, and investigated the cytoplasmic delivery of the complex and the knockdown efficiency of target genes. Here, to overcome this problem, we adopted the membrane penetration-enhancing peptide S19 from human syncytin 1 previously identified in our laboratory. Although protein-based methods using cell-penetrating peptides such as TAT have been expected to provide an alternative approach to siRNA delivery, the low efficiency of endosomal escape of siRNA/protein complexes taken up into cells by endocytosis remains a problem.
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