Abstract
The mechanical properties of cell-sized giant unilamellar liposomes were studied by manipulating polystyrene beads encapsulated within the liposomes using double-beam laser tweezers. Mechanical forces were applied to the liposomes from within by moving the beads away from each other, which caused the liposomes to elongate. Subsequently, a tubular membrane projection was generated in the tip at either end of the liposome, or the bead moved out from the laser trap. The force required for liposome transformation reached maximum strength just before formation of the projection or the moving out of the bead. By employing this manipulation system, we investigated the effects of membrane lipid compositions and environment solutions on the mechanical properties. With increasing content of acidic phospholipids, such as phosphatidylglycerol or phosphatidic acid, a larger strength of force was required for the liposome transformation. Liposomes prepared with a synthetic dimyristoylphosphatidylcholine, which has uniform hydrocarbon chains, were transformed easily compared with liposomes prepared using natural phosphatidylcholine. Surprisingly, bovine serum albumin or fetuin (soluble proteins that do not bind to membranes) decreased liposomal membrane rigidity, whereas the same concentration of sucrose showed no particular effect. These results show that the mechanical properties of liposomes depend on their lipid composition and environment.
Highlights
Liposomes, which are artificial membrane vesicles, are a useful model to study how the shape and morphology of lipid bilayer membranes are controlled by external factors, such as physical parameters and chemical or biological agents
This study succeeded in quantifying the mechanical properties of cell-sized giant unilamellar liposomes by manipulating beads encapsulated within them using laser tweezers
The force required to deform liposomes takes a different strength in the range from a few pN up to 30 pN depending on the lipid composition and solution conditions, which are comparable to forces that can be generated by polymerizing cytoskeletons and molecular motors in living cells
Summary
Liposomes, which are artificial membrane vesicles, are a useful model to study how the shape and morphology of lipid bilayer membranes are controlled by external factors, such as physical parameters and chemical or biological agents. A tubular membrane projection was generated in the tip at either end (Figure 1b), or in some cases, the beads moved out from the laser trap without developing a tubular projection, probably because the repulsive force exerted on the beads exceeded the trapping force of the laser (Figure 1c) This process is similar to the liposomal transformation caused by the elongation of encapsulated cytoskeletons. Once the membrane tube developed, a decreased and constant force (about 4 pN) was required for further tube elongation or shortening In the latter case, the force required for the membrane transformation reached a maximum strength (about 10–20 pN) just before the bead moved out from the laser trap. Possible mechanisms by which the internal and external factors modulate the membrane transformability are discussed
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