Characterizing an siRNA-Containing Lipid-Nanoparticle Prepared by a Microfluidic Reactor: Small-Angle X-ray Scattering and Cryotransmission Electron Microscopic Studies

A new cationic-lipid/siRNA particle that was designed to deliver siRNA was investigated by the combination of small-angle X-ray scattering (SAXS), asymmetric field flow fractionation coupled with multiangle light scattering, and cryotransmission electron microscopy (cryo-TEM). The particle was prepared through two-step mixing using a microfluidic technique. In the first step, siRNA was premixed with a cationic lipid in an EtOH-rich solution. In the second step, the premixed solution was mixed with other lipids, followed by solvent exchange with water. SAXS showed formation of a siRNA/cationic lipid pair in the first step, and this pair consisted of the major part of the core in the final particle. The relationship between the hydrodynamic radius and the radius of gyration indicated the formation of a densely packed core and PEG-rich shell, confirming a well-known core-shell model. SAXS and cryo-TEM showed that the ordering of the core structure enhanced as the siRNA content increased.

Self and directed assembly: people and molecules

Self-assembly and directed-assembly are two very important aspects of supramolecular chemistry. As a young postgraduate student working in Canada with Tom Fyles my introduction to Supramolecular Chemistry was through the self-assembly of phospholipid membranes to form vesicles for which we were developing unimolecular and self-assembling transporter molecules. The next stage of my development as a scientist was in Japan with Seiji Shinkai where in a “Eureka” moment, the boronic acid templating unit (directed-assembly) of Wulff was combined with photoinduced electron transfer systems pioneered by De Silva. The result was a turn-on fluorescence sensor for saccharides; this simple result has continued to fuel my research to the present day. Throughout my career as well as assembling molecules, I have enjoyed bringing together researchers in order to develop collaborative networks. This is where molecules meet people resulting in assemblies worth more than the individual “molecule” or “researcher”. My role in developing networks with Japan was rewarded by the award of a Daiwa-Adrian Prize in 2013 and I was recently rewarded for developing networks with China with an Inaugural CASE Prize in 2015.

Micelles consisting of choline phosphate-bearing calix[4]arene lipids

We synthesized new calix[4]arene-based lipids, denoted by CPCaLn, bearing the choline phosphate (CP) group which is an inverse phosphoryl choline (PC) structure. Small-angle X-ray scattering and multi-angle light scattering coupled with field flow fractionation showed that these lipids form monodisperse micelles with a fixed aggregation number and diameters of 1.9 and 2.6 nm for lipids bearing C3 and C6 alkyl tails, respectively. Furthermore, when CPCaLn was mixed with the fluorescein isothiocyanate (FITC)-bearing lipids and added to cells, strong fluorescence was observed at 37 °C, but not at 4 °C, indicating that the micelles were taken up by the cells through endocytosis. Recent studies have shown that replacement of polymer-attached PC groups with CP groups markedly promotes cellular uptake, even though the surface charge is neutral. On the basis of the idea, CPCaLn micelles interacted with cells in the same way, suggesting that the micelles bearing CP groups are expected to use as carriers in the drug delivery system.

The role of the helper lipid dioleoylphosphatidylethanolamine (DOPE) for DNA transfection cooperating with a cationic lipid bearing ethylenediamine

Gene therapy is expected to treat various incurable diseases including viral infections, autoimmune disorders, and cancers. Cationic lipids (CL) have been used as carriers of therapeutic DNAs for gene therapy because they can form a complex with DNA and such a complex can be incorporated into cells and transport the bound DNA to cytosol. The CL/DNA complexes are called lipoplexes and categorized as a non-viral vector. Lipoplexes are often prepared by adding a neutral phospholipid dioleoylphosphatidylethanolamine (DOPE) to CL in order to enhance transfection. However, the role of DOPE is not fully understood. We synthesized a new CL having an ethylenediamine cationic head group, denoted by DA, and found that addition of DOPE to DA achieved a good efficiency, almost in the similar level of commonly used transfection reagent Lipofectamine 2000 (Invitrogen). The composition of DA:DOPE=1:1 showed the highest efficiency. This lipoplex showed structural transition when pH was changed from 7 to 4, corresponding pH lowering in late endosome, while DOPE itself showed structural transition at more basic pH around 8. The present data showed that the DOPE/DA composition determines the structural transition pH and choosing a suitable pH, i.e., a suitable composition, is essential to increase the transfection efficiency.