Control of Line Tension at Phase-Separated Lipid Domain Boundaries: Monounsaturated Fatty Acids with Different Chain Lengths and Osmotic Pressure

Abstract

Line tension at phase-separated lipid domain boundaries is an important factor that governs the stability of the phase separation. We studied the control of the line tension in lipid membranes composed of dioleoylphosphocholine (DOPC), dipalmitoylphosphocholine (DPPC), and cholesterol (Chol) by the addition of the following three monounsaturated fatty acids (MUFAs) with different chain lengths: palmitoleic acid (PaA), oleic acid (OA), and eicosenoic acid (EiA). In addition, we attempted to alter the line tension by applying osmotic pressure. The phase behavior of the MUFA-containing lipid membranes in the presence and absence of osmotic stress was observed by fluorescence and confocal laser scanning microscopy. The line tension was quantitatively measured from the domain boundary fluctuation by flicker spectroscopy, and the interactions between the lipids and MUFAs were examined by differential scanning calorimetry. PaA and OA, which are shorter MUFAs, decreased the line tension, whereas EiA changed the liquid domain to a solid domain. The osmotic pressure increased the line tension, even in the presence of MUFAs. It may be possible to control the line tension by combining the chemical approach of MUFA addition and the physical approach of applying osmotic pressure.

Keywords: line tension; lipid membrane; monounsaturated fatty acid; osmotic pressure; phase separation.

Figure 1

Phase separation observed by fluorescence microscopy for DOPC/DPPC/MUFA/Chol: (a,e) 40/40/0/20, (b,f) 30/40/10/20 (MUFA = PaA), (c,g) 30/40/10/20 (MUFA = OA), and (d,h) 30/40/10/20 (MUFA = EiA). The upper (a–d) and lower (e–h) rows are the images at ΔC = 0 (without osmotic stress) and 180 mM (with osmotic stress), respectively. Scale bars are 5 μm.

Figure 2

Line tension at the L_o/L_d phase boundary for (a) DOPC/DPPC/PaA/Chol and (b) DOPC/DPPC/OA/Chol. PaA (or OA) = 0%, 10%, 20%, and 30% correspond to DOPC/DPPC/PaA(OA)/Chol = 40/40/0/20, 30/40/10/20, 20/40/20/20, and 10/40/30/20, respectively. The black and red lines represent the results for ΔC = 0 mM (without osmotic stress) and 180 mM (with osmotic stress), respectively.

Figure 3

Phase-separated vesicles observed by confocal laser scanning microscopy for (a) DOPC/DPPC/MUFA/Chol = 40/40/0/20, (b) 30/40/10/20 (MUFA = PaA), (c) 30/40/10/20 (MUFA = OA), and (d) 30/40/10/20 (MUFA = EiA) at ΔC = 0 (without osmotic stress) and 180 mM (with osmotic stress). The red and green colors correspond to Rho-DHPE and NBD-PE, respectively. Scale bars are 10 μm.

Figure 4

DSC thermographs for (a) DPPC/PaA/Chol, (b) DPPC/OA/Chol, and (c) DPPC/EiA/Chol. The black, red, blue, and green lines indicate DPPC/MUFA/Chol ratios of 90/0/10, 85/5/10, 80/10/10, and 75/15/10, respectively.

Figure 5

Variation in the peak temperatures obtained from peak deconvolution for (a) DPPC/PaA/Chol, (b) DPPC/OA/Chol, and (c) DPPC/EiA/Chol. MUFA contents of 0%, 5%, 10%, and 15% correspond to DPPC/MUFA/Chol ratios of 90/0/10, 85/5/10, 80/10/10, and 75/15/10, respectively. The solid and dashed lines indicate the lower- and higher-temperature peaks, respectively. Because we obtained symmetric thermographs at PaA and OA contents of 10% and 15%, we have simply plotted the peak temperatures from the thermographs in these cases.