Water permeability of lipid membranes of GUVs and its dependence on actin cytoskeletons inside the GUVs

Response of cells to osmotic pressure and control of water permeability of biomembranes are very important for cells to survive. There have been many studies using cells, but their details remain unknown. In this study, we investigated water permeability of various lipid membranes of GUVs by the micropipet aspiration technique, and then its dependence on actin cytoskeletons inside the GUVs. First, we measured of water permeability of various lipid membranes. When we transferred a single GUV fixed by a micropipet using a small aspiration pressure into a buffer with a higher osmolarity, the volume of the GUV decreased due to the water efflux. We determined the decrease in the GUV volume by the increase in aspiration length in the micropipet. We analyzed quantitatively the time course of volume change of the GUV on the basis of the theoretical equation, and obtain water permeability of the lipid membrane. The average value of water permeability of DOPC membrane in the L_alpha phase was 45 plusmn 4 mum/s. The water permeability depended greatly on kinds of lipid membranes. For example, the water permeability of DPPC/cholesterol (6/4, molar ratio) membrane in a liquid- ordered phase was 1.3 plusmn 0.1 mum/s, which was much smaller than that of DOPC membrane. Second, we investigated the effect of actin filaments on water permeability of lipid membranes. We succeeded in preparing DOPC/DOPG (l/l)-GUVs containing 1 mg/mL actin filaments. The water permeability of the GUV containing the actin filaments was lower than that of DOPC/DOPG (l/l)-GUVs containing 1 mg/mL actin monomers (no polymerization). This result indicates that the actin filaments reduced the water efflux through the GUV when an osmotic pressure was applied on the GUV. It can be explained by the decrease in chemical potential of water induced by the actin filaments under the nonequilibrium condition due to the osmotic pressure. We discuss the mechanism of the effect of the actin filaments on the osmot- - ic response of the GUV.