Membrane Tension in Negatively Charged Lipid Bilayers in a Buffer under Osmotic Pressure.

Osmotic pressure (Π) induces membrane tension in cell membranes and the lipid bilayers of vesicles and plays an important role in the functions and physical properties of these membranes. We recently developed a method to determine quantitatively the membrane tension of giant unilamellar vesicles (GUVs) under Π and applied it to GUVs comprising electrically neutral dioleoylphosphatidylcholine (DOPC). Here, we examined the effect of Π on GUVs composed of DOPC and negatively charged dioleoylphosphatidylglycerol (DOPG) in a buffer containing a physiological concentration of ions. First, we examined the rate constant, kr, for constant tension (σex)-induced rupture of DOPG/DOPC (4/6)-GUVs under Π and obtained the dependence of kr on σex in GUVs for various values of Π. Comparing this dependence in the absence of Π provided values for membrane tension due to Π, σosm, which agree with the theoretical values within the experimental error. The values of σosm for DOPG/DOPC-GUVs were smaller than those for DOPC-GUVs under the same Π. Two factors, that is, the solute concentration in a GUV suspension and the elastic modulus of the GUV membrane, can reasonably explain this difference based on the theory of σosm. We also examined the effect of Π on the rate constant, kFF, for the transbilayer movement of lipid molecules in single GUVs. The values of kFF increased with increasing Π, indicating that kFF increased with σosm. This result supports the existence of prepores in stretched lipid bilayers. Based on these results, we discuss the membrane tension of DOPG/DOPC-GUVs under Π.

Effects of Mechanical Properties of Lipid Bilayers on the Entry of Cell-Penetrating Peptides into Single Vesicles.

The translocation of cell-penetrating peptides (CPPs) through plasma membranes of living cells is an important physiological phenomenon in biomembranes. To reveal the mechanism underlying the translocation of a CPP, transportan 10 (TP10), through lipid bilayers, we examined the effects of the mechanical properties of lipid bilayers on the entry of carboxyfluorescein (CF)-labeled TP10 (CF-TP10) into a giant unilamellar vesicle (GUV) using the single GUV method. First, we examined the effect of lateral tension in membranes on the entry of CF-TP10 into single GUVs comprising a mixture of dioleoylphosphatidylglycerol (DOPG) and dioleoylphosphatidylcholine (DOPC) (2/8). CF-TP10 entered the GUV lumen before the membrane permeation of Alexa Fluor 647 hydrazide (AF647) from the GUV and thus before pore formation in the membrane. The fraction of entry of CF-TP10 before pore formation and the rate of membrane rupture increased with tension. The CF-TP10-induced fractional change in the membrane area increased continuously with time until membrane rupture, but it increased more slowly than did the CF-TP10 concentration in the GUV membrane. A high mole fraction of cholesterol inhibited the entry of CF-TP10 into single GUVs by suppressing the translocation of CF-TP10 from the external to the internal monolayer, although higher concentrations of CF-TP10 induced the formation of pores through which CF-TP10 rapidly translocated. Suppression of the translocation of CF-TP10 by cholesterol can be reasonably explained by the large line tension of a prepore. We discussed the role of mechanical properties in membranes on the entry of CF-TP10 into single GUVs and proposed a hypothesis of the mechanism that CF-TP10 translocates across a bilayer through transient hydrophilic prepores in the membrane.

Experimental Estimation of Membrane Tension Induced by Osmotic Pressure.

Osmotic pressure (Π) induces the stretching of plasma membranes of cells or lipid membranes of vesicles, which plays various roles in physiological functions. However, there have been no experimental estimations of the membrane tension of vesicles upon exposure to Π. In this report, we estimated experimentally the lateral tension of the membranes of giant unilamellar vesicles (GUVs) when they were transferred into a hypotonic solution. First, we investigated the effect of Π on the rate constant, kp, of constant-tension (σex)-induced rupture of dioleoylphosphatidylcholine (DOPC)-GUVs using the method developed by us recently. We obtained the σex dependence of kp in GUVs under Π and by comparing this result with that in the absence of Π, we estimated the tension of the membrane due to Π at the swelling equilibrium, σosmeq. Next, we measured the volume change of DOPC-GUVs under small Π. The experimentally obtained values of σosmeq and the volume change agreed with their theoretical values within the limits of the experimental errors. Finally, we investigated the characteristics of the Π-induced pore formation in GUVs. The σosmeq corresponding to the threshold Π at which pore formation is induced is similar to the threshold tension of the σex-induced rupture. The time course of the radius change of GUVs in the Π-induced pore formation depends on the total membrane tension, σt; for small σt, the radius increased with time to an equilibrium one, which remained constant for a long time until pore formation, but for large σt, the radius increased with time and pore formation occurred before the swelling equilibrium was reached. Based on these results, we discussed the σosmeq and the Π-induced pore formation in lipid membranes.

Effects of Lipid Composition on the Entry of Cell-Penetrating Peptide Oligoarginine into Single Vesicles.

The cell-penetrating peptide R9, an oligoarginine comprising nine arginines, has been used to transport biological cargos into cells. However, the mechanisms underlying its translocation across membranes remain unclear. In this report, we investigated the entry of carboxyfluorescein (CF)-labeled R9 (CF-R9) into single giant unilamellar vesicles (GUVs) of various lipid compositions and the CF-R9-induced leakage of a fluorescent probe, Alexa Fluor 647 hydrazide (AF647), using a method developed recently by us. First, we investigated the interaction of CF-R9 with dioleoylphosphatidylglycerol (DOPG)/dioleoylphosphatidylcholine (DOPC) GUVs containing AF647 and small DOPG/DOPC vesicles. The fluorescence intensity of the GUV membrane due to CF-R9 (i.e., the rim intensity) increased with time to a steady-state value, and then the fluorescence intensity of the membranes of the small vesicles in the GUV lumen increased without leakage of AF647. This result indicates that CF-R9 entered the GUV lumen from the outside by translocating across the lipid membrane without forming pores through which AF647 could leak. The fraction of entry of CF-R9 at 6 min in the absence of pore formation, Pentry (6 min), increased with an increase in CF-R9 concentration, but the CF-R9 concentration in the lumen was low. We obtained similar results for dilauroyl-PG (DLPG)/ditridecanoyl-PC (DTPC) (2/8) GUVs. The values of Pentry (6 min) of CF-R9 for DLPG/DTPC (2/8) GUVs were larger than those obtained with DOPG/DOPC (2/8) GUVs at the same CF-R9 concentrations. In contrast, a high concentration of CF-R9 induced pores in DLPG/DTPC (4/6) GUVs through which CF-R9 entered the GUV lumen, so the CF-R9 concentration in the lumen was higher. However, CF-R9 could not enter DOPG/DOPC/cholesterol (2/6/4) GUVs. Analysis of the rim intensity showed that CF-R9 was located only in the outer monolayer of the DOPG/DOPC/cholesterol (2/6/4) GUVs. On the basis of analyses of these results, we discuss the elementary processes by which CF-R9 enters GUVs of various lipid compositions.