Self-promoted cellular uptake of peptide/DNA transfection complexes.

The designed alpha-helical amphipathic peptide LAH4 assembles several properties, which makes it an interesting candidate as a gene-delivery vehicle. Besides being short and soluble in aqueous solutions, LAH4 presents cationic residues, which allow for efficient complexation of DNA. In addition, this peptide is poorly hemolytic at neutral pH, while it is able to destabilize biological membranes in acidic conditions. In this study, the structure of the peptide/DNA transfection complex was examined by circular dichroism and solid-state nuclear magnetic resonance spectroscopies and the thermodynamics of its formation and disassembly was monitored in a quantitative manner as a function of pH by isothermal titration calorimetry. Notably, the number of peptides within the complex considerably decreases upon acidification of the medium. This observation has direct and important consequences for the mechanism of action because the acidification of the endosome results in high local concentrations of free peptide in this organelle. Thus, these peptides become available to interact with the endosomal membranes and thereby responsible for the delivery of the transfection complex to the cytoplasm. When these data are taken together, they indicate a dual role of the peptide during the transfection process, namely, DNA complexation and membrane permeabilization.