Transitions Between Distinct Compaction Regimes in Complexes of Multivalent Cationic Lipids And DNA
Cationic lipids (CLs) have found widespread use as nonviral gene carriers (vectors), including applications in clinical trials of gene therapy. However, their observed transfection efficiencies (TEs) are inferior to those of viral vectors, providing a strong incentive for a detailed understanding of CL-DNA complex behavior. In recent systematic studies employing monovalent as well as newly synthesized multivalent lipids (MVLs), the membrane cationic charge density has been identified as a key parameter governing the TE of lamellar CL-DNA complexes. In this work, we use x-ray scattering and molecular simulations to investigate the structural properties of complexes containing MVLs. At low mole fraction of neutral lipids (NLs), {phi}{sub NL}, the complexes show dramatic DNA compaction, down to essentially close-packed DNA arrays with a DNA interaxial spacing d{sub DNA} = 25{angstrom}. A gradual increase in {phi}{sub NL} does not lead to a continuous increase in d{sub DNA} as observed for DNA complexes of monovalent CLs. Instead, distinct spacing regimes exist, with sharp transitions between the regimes. Three packing states have been identified: (1), close packed, (2), condensed, but not close packed, with d{sub DNA} = 27-28{angstrom}, and (3), an expanded state, where d{sub DNA} increases gradually with {phi}{sub NL}. Based on our experimental and computational results, we conclude that the DNA condensation is mediated by the multivalent cationic lipids, which assemble between the negatively charged DNA rods. Quite remarkably, the computational results show that the less tightly packed structure in regime 2 is thermodynamically more stable than the close packed structure in regime 1. Accordingly, the constant DNA spacing observed in regime 2 is attributed to lateral phase coexistence between this stable CL-DNA complex and neutral membranes. This finding may explain the reduced TE measured for such complexes: transfection involves endosomal escape and disassembly of the complex, and these processes are inhibited by the high thermodynamic stability. Our results, which demonstrate the existence of an inverse correlation between the stability and transfection activity of lamellar CL-DNA complexes are, therefore, consistent with a recently proposed model of cellular entry.
- Research Organization:
- SLAC National Accelerator Lab., Menlo Park, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC02-76SF00515
- OSTI ID:
- 953148
- Report Number(s):
- SLAC-REPRINT-2009-136; BIOJAU; TRN: US200914%%347
- Journal Information:
- Biophys. J. 95:836,2008, Vol. 95, Issue 2; ISSN 0006-3495
- Country of Publication:
- United States
- Language:
- English
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