Applications of Synthetic Polymer Discoidal Lipid Nanoparticles to Biomedical Research

Liposomes are artificially prepared vesicular lipid nanoparticles with a bilayer structure, resembling cell membrane. Their ability to encapsulate various molecules along with excellent biocompatibility makes them ideal delivery vehicles for pharmaceuticals. They can also serve as platforms for membrane proteins to elucidate the structure and function in lipid membranes. Nascent high-density lipoproteins are discoidal lipid nanoparticles with a bilayer structure, which can be reconstituted with their constituents. Such reconstituted nanoparticles, nanodisks, were originally generated in terms of elucidation for mechanisms of lipoprotein metabolisms. At the same time, like liposomes, nanodisks have been developed as delivery vehicles and platforms for membrane proteins in structural biology. From a developmental background, apolipoproteins, their analogs, or fragment peptides were initially used as scaffolding molecules to wrap around the edge of the disk-shaped lipid bilayer. Since the discovery that styrene-maleic acid copolymers produce nanodisks instead of apolipoproteins, variously modified or novel polymers have been synthesized to broaden the applications of polymer nanodisks. This review provides an overview of the types of synthetic polymers used to produce nanodisks, and the biomedical applications of nanodisks to the developments of delivery vehicles and to the structural studies of membrane proteins.