Forum 03/23/2006

“Structure-function relationship studies of novel multivalent cationic lipids for gene delivery”

Alexandra Zidovska – UC Santa Barbara, Materials Department

Newly designed multivalent lipids ranging in head group charge from 4+ to 16+ have been synthesized and investigated as DNA delivery vectors. These dendritic lipids (DLs) allow a controlled study of the relationship between membrane charge density and transfection efficiency (TE). An earlier report from our group described that TE of different cationic lipids of charge 1+ to 5+ follows a common, bell shaped curve as a function of membrane charge density [1]. To further probe this universal behavior, the dendritic lipids with higher valence were designed in order to reach higher values of. Structural studies using x-ray diffraction reveal new phases, where cylindrical micelles of DLs form a hexagonal lattice which holds together a continuous DNA network, described as HIC [2] (Fig.1).  The new hexagonal phase is highly transfecting in the regime where the TE of lamellar complexes follows a decrease in the bell curve (i.e. for higher  values), demonstrating the importance of understanding both the function and structure of lipid mixtures in the context of improved non-viral gene delivery. A surprising dependence of efficiency on the charge ratio DNA:DL was also observed. Extensive small angle x-ray scattering studies have revealed a rich phase diagram of micelles made from DL/DOPC mixtures. As shown by cryo-TEM the extreme difference in curvature between DLs and DOPC results in distinct micellar morphologies as a function of DL/DOPC ratios.