Morphological changes of giant liposomes caused by cytoskeletal proteins, surfactants, or optical tweezers

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1)Liposomes are micro-compartments made of lipid bilayer membranes with characteristics quite similar to those of biological membranes. To form artificial cell-like structures, we generated liposomes that contained subunit proteins of cytoskeletons : actin, and its associating proteins. Spherical liposomes were transformed into disk-or dumbbell-shapes by the polymerization of encapsulated actin. Bipolar-or cell-like-shaped liposomes were developed by the assembly of encapsulated actin and its crosslinking proteins, fascin, -actinin, or filamin, by mechanical forces generated by the organization of internal actin-networks. Dynamic processes of morphological transformations of liposomes were visualized by high intensity dark-field light microscopy. The differences in morphology among transformed liposomes indicate that actin-crosslinking proteins determine liposome shape by organizing their specific actin networks. Morphological analysis reveals that the crosslinking manner, i.e. distance and angular flexibility between adjacent crosslinked actin filaments, is essential for the morphogenesis.2)Topological changes, such as fusion and division of membrane vesicles, play an essential role in cellular activities. To investigate the mechanism of these processes, we visualized the liposomes undergoing topological transformation induced by interactions between liposomal membranes and fusogenic peptides derived from influenza hemagglutinin, melittin, or surfactants with direct, real-time observation using high-intensity dark-field microscopy. As the results, a variety of novel topological transformations were found, including the opening-up of liposomes, the direct expulsion of inner vesicles, intermittent quakes, and inside-out topological inversion. These novel findings reveal that the lipid bilayer itself possesses the ability to undergo topological transformation.