Calorimetric study of the interaction of binary DMTAP/DOTAP cationic liposomes with plasmid DNA

Abstract

Cationic liposomes have been suggested as possible agents for nonviral gene transfer. The interaction of plasmid DNA (pDNA) with dispersions of stable unilamellar cationic liposomes based on the binary lipid system 1,2-dimyristoyl-3-trimethyl-ammonium-propane (DMTAP):1,2-dioleoyl-3-trimethyl-ammonium-propane (DOTAP) has been studied by using isothermal titration calorimetry (ITC), high-precision differential scanning calorimetry (DSC), dynamic light scattering (DLS), and circular dichroism (CD). Systematic calorimetric and DLS exploration of the DMTAP:DOTAP binary system reveals that single-bilayer liposomes are stable at the 4:1 molar ratio, exhibiting the main lipid-phase transition temperature at ~25.3°C, and a total enthalpy change δH = 8.5 ± 0.4 kcal/mol. The interaction of pDNA with unilamellar DMTAP:DOTAP vesicles was investigated by ITC experiments, which clearly distinguished endothermic binding between the phosphate and the ammonium groups from exothermic processes, driven by slow kinetics, corresponding to interliposomal, DNA-triggered aggregation that leads to the formation of large multilamellar liposome/pDNA assemblies. Lipid-added-to-pDNA and pDNA-added-to-lipid experiments have been carried out in order to systematically explore the interaction mechanisms. Complex ITC profiles are revealed, which may be linked to packing rearrangements of the pDNA molecules bound at the outer liposomal surface, possibly due to binding to more than one liposome or due to p-DNA-enhanced heterogeneity in the local lipid concentration. DNA-mediated aggregation effects are detected at high [ammonium]/[phosphate] molar ratios in the case of lipid-added-to-pDNA interactions and at relatively low [phosphate]/[ammonium] molar ratios in the case of pDNA-added-to-lipid.

Keywords::

• Cationic liposomes
• DMTAP
• DOTAP
• large unilamellar vesicles
• plasmid DNA
• lipoplexes
• ITC
• DSC

Acknowledgments

SG and GN acknowledge support from the Graduate Fellowship Program of NCSR “Demokritos.” The research has been partially supported by Grants from the General Secretiat of Research and Technology of Greece, Program “Excellence in Research.” Helpful discussions with CM Paleos and D Tsiourvas are acknowledged. The BRCT BRCA1 plasmid was kindly provided by AN Monteiro.

Declaration of interest: The authors report no conflict of interest. The authors alone are responsible for the content and writing of this paper.