Relationship between the secondary structure of the peptide based siRNA carrier and effective gene silencing effect on lung epithelial cells

Abstract

Pulmonary delivery of small interfering RNA (siRNA) has potential in treating many lung diseases. KL4 is a synthetic surfactant peptide that was initially designed to imitate the function of surfactant protein B (SP-B). The safety and the cationic nature of the KL4 make it an attractive candidate for siRNA delivery. Previous data showed that KL4 peptide could mediate efficient gene silencing effect of siRNA in human lung epithelial cells without signs of cytotoxicity at concentrations used for transfection. However, one of the problems associated with KL4 is poor aqueous solubility. To overcome this problem, five KL4-modified peptides were introduced by replacing leucine with other less hydrophobic amino acid residues such as valine and alanine. The siRNA binding affinity of these modified peptides was examined using a gel retardation assay, and the transfection efficiency was evaluated on A549 cells using siRNA targeting Glyceraldehyde 3-phosphate dehydrogenase (GAPDH). The secondary structure of the modified peptides was investigated by circular dichroism (CD). The results showed that only the KL4 peptide which adopted an alpha helix structure could transfect the siRNA into the cells and mediate an efficient gene silencing effect.