Determination of bilayer membrane bending stiffness by tether formation from giant, thin-walled vesicles

Abstract

The curvature elastic modulus (bending stiffness) of stearoyloleoyl phosphatidylcholine (SOPC) bilayer membrane is determined from membrane tether formation experiments. R. E. Waugh and R. M. Hochmuth 1987. Biophys. J. 52:391-400) have shown that the radius of a bilayer cylinder (tether) is inversely related to the force supported along its axis. The coefficient that relates the axial force on the tether to the tether radius is the membrane bending stiffness. Thus, the bending stiffness can be calculated directly from measurements of the tether radius as a function of force. Giant (10-50-microns diam) thin-walled vesicles were aspirated into a micropipette and a tether was pulled out of the surface by gravitational forces on small glass beads that had adhered to the vesicle surface. Because the vesicle keeps constant surface area and volume, formation of the tether requires displacement of material from the projection of the vesicle in the pipette. Tethers can be made to grow longer or shorter or to maintain equilibrium by adjusting the aspiration pressure in the micropipette at constant tether force. The ratio of the change in the length of the tether to the change in the projection length is proportional to the ratio of the pipette radius to the tether radius. Thus, knowing the density and diameter of the glass beads and measuring the displacement of the projection as a function of tether length, independent determinations of the force on the tether and the tether radius were obtained. The bending stiffness for an SOPC bilayer obtained from these data is approximately 2.0 x 10(-12) dyn cm, for tether radii in the range of 20-100 nm. An equilibrium relationship between pressure and tether force is derived which closely matches experimental observation.

Hydrodynamic narrowing of tubes extruded from cells

“…We follow the thermodynamic analysis of tether formation proposed by Waugh and coworkers (7,13,14), Evans and Yeung (15), and Derényi and coworkers (16) for lipidic bilayers and extended to cell membranes by Sheetz and coworkers (10, 12). As shown in Fig.…”

“…Membranous tails also are observed to be left by migrating cells in culture dishes (3). Tethers can be extracted from synthetic vesicles or living cells by the application of an external point force [using a fluid drag (4,5) or pipette-tweezer system (6,7)]. In the case of living cells, where the lipidic membrane is coupled to a cytoskeleton, tethers can be used as membrane sensors to measure the membrane-cytoskeleton adhesion energy W 0 (8)(9)(10)(11)(12).…”

A novel method for measuring the bending rigidity of model lipid membranes by simulating tethers

“…[8][9][10][11] The second approach is typically implemented by measuring the force needed to pull nanoscale bilayer tubes ͑tethers͒ from vesicles. [12][13][14][15] mation of a tube involves the creation of a high curvature, the work to pull a tether is basically done against bending energy; hence the modulus can be determined from it.…”

“…Recently, however, Cuvelier et al 15 devised a clever setup involving two tethers which avoids such complications. Even though the above analysis is standard in the tether literature, [12][13][14][15] it is still only approximate. Notice that this time we have neglected thermal fluctuations altogether.…”

Shape behavior of lipid vesicles as the basis of some cellular processes

“…The importance of the nonlocal bending term for the shape behavior of phospholipid vesicles has been proven in tether-pulling experiments (Bo and Waugh, 1989;Waugh et al, 1992). In those experiments, which were designed to measure membrane elastic parameters, a thin cylindrical tether was pulled out of an aspirated vesicle.…”