Critical and off-critical miscibility transitions in model extracellular and cytoplasmic myelin lipid monolayers

Abstract

Monolayers based on the composition of the cytoplasmic (CYT) or extracellular (EXT) sides of the myelin bilayer form coexisting immiscible liquid phases similar to the liquid-ordered/liquid-disordered phases in phospholipid/cholesterol monolayers. Increasing the temperature or surface pressure causes the two liquid phases to mix, although in significantly different fashion for the CYT and EXT monolayers. The cerebroside-rich EXT monolayer is near a critical composition and the domains undergo coalescence and a circle-to-stripe transition along with significant roughening of the domain boundaries before mixing. The phase transition in the cerebroside-free cytoplasmic side occurs abruptly without domain coalescence; hence, the cytoplasmic monolayer is not near a critical composition, although the domains exhibit shape instabilities within 1-2 mN/m of the transition. The change in mixing pressure decreases significantly with temperature for the EXT monolayer, with dΠ(crit)/dT ∼ 1.5 mN/m/°C, but the mixing pressure of the CYT monolayer varies little with temperature. This is due to the differences in the nonideality of cholesterol interactions with cerebrosides (EXT) relative to phospholipids (CYT). EXT monolayers based on the composition of white matter from marmosets with experimental allergic encephalomyelitis (EAE), an animal model of multiple sclerosis, remain phase-separated at higher surface pressures than control, while EAE CYT monolayers are similar to control. Myelin basic protein, when added to the CYT monolayer, increases lipid miscibility in CYT monolayers; likely done by altering the dipole density difference between the two phases.

Figure 1

Distributions of domain areas of model monolayers of compositions reflecting (a) control cytoplasmic (CYT), (b) control extracellular (EXT), (c) EAE CYT, and (d) EAE EXT lipid mixtures at liftoff (Π ∼ 0 mN/m). The average domain area and area fraction of the dark, fluorescent dye-excluding domains size was analyzed from five randomly selected frames of movies taken during compression of the film (see also the Supporting Material). The area fraction of the dark domains (f_avg) in the EXT monolayers is approximately twice that of the CYT lipid mixtures. In analogy to other lipid/cholesterol mixtures, the dark discrete domains are liquid-ordered (L_o) phase in which the saturated lipids and cholesterol are concentrated. The bright, continuous phase is the liquid-disordered (L_d) phase in which the remaining cholesterol, unsaturated and charged lipids reside.

Figure 2

Fluorescence images of healthy (control) and diseased (EAE), inner (CYT) and outer (EXT) myelin monolayers containing 1 wt % TR-DHPE on a MOPS buffer subphase at T ≈ 20°C and pH ≈ 7.2. All of four model myelin monolayers show a continuous bright L_d phase-separating discrete, dark L_o phase domains. During compression, the two phases become homogeneous at the miscibility pressure, Π_crit (see Fig. 6). The large stripes present in control EXT monolayers at Π ≈ 30 mN/m indicate proximity to a critical composition (see Fig. 4).

Figure 3

Fluorescence images of control and EAE, inner (CYT) and outer (EXT) myelin monolayers containing 1 wt % TR-DHPE on a MOPS buffer subphase at T ≈ 37°C and pH ≈ 7.2. The EXT monolayers show a dramatic decrease in the miscibility transition pressure as the temperature increases, the EAE monolayers are in a single homogeneous phase for Π = 10, and the EAE EXT monolayer is in a single phase by Π = 20 (see Fig. 6). For the control and EAE CYT monolayers, the miscibility transition remains between 20 and 30 mN/m. All of the monolayers collapse by ejecting small bilayer fragments at Π ∼ 40 mN/m.

Figure 4

Control EXT lipid mixtures show evidence of being near a liquid-liquid miscibility critical composition. Well below the miscibility transition of 34–37 mN/m, the domains at 27.9 mN/m undergo fluctuations toward elliptical and stripe shapes. The domains begin to coalesce. As the surface pressure increases toward the transition, the domains extend into elongated stripes and become more interconnected. The domains appear to speckle in the movies (see Movie S1 in the Supporting Material). At 34.0 mN/m, the domain boundaries begin to fluctuate and become fuzzy. Finally the domains mix, although due to the slow diffusion near critical points, it takes a long time for the bright- and dark-phase domains to mix and contrast in the images is observed up to surface pressures of 42 mN/m (see Movie S1).

Figure 5

In the control CYT monolayers, the domains do not coalesce or change shape until the surface pressure is within ∼1 mN/m of the miscibility transition. At 23.1 mN/m the domains are round and remain well dispersed. However, at 23.6 mN/m, polygonal shape instabilities predicted by theory (25), consistent with a decrease in the line tension between phases, set in. (Upper yellow arrow) Here, a small domain has elongated into an elliptical shape (two-sided polygon). (Lower red arrow) Here, the domain changed to a different shape (from circular to a five-sided polygon). By 23.8 mN/m, this domain at the lower red arrow has evolved further (into a five-pointed star). (Upper blue arrow) Here, an even smaller domain displays four branches. This is because larger domains can accommodate higher-order instabilities than the smaller domains (see Eq. 4). By 24.2 mN/m, mixing has occurred and all but one domain (red-arrow domain) have faded away, limited by the finite diffusion of the fluorescent lipid dye (see Movie S2).

Figure 6

Miscibility pressure versus temperature phase diagrams for the (a) EXT and (b) CYT monolayers. The X is an estimate of the approximate surface pressure (∼30 mN/m) of a bilayer in vivo according to Demel et al. (33). Both control and EAE EXT monolayers should be single phase at 37°C, although EAE EXT monolayers are much closer to phase separation than control EXT monolayers under physiological conditions. On the other hand, both control and EAE CYT monolayers are close to their phase-separation pressure at 37°C and 30 mN/m; the mixing-demixing boundaries for CYT monolayers have less temperature dependence and also show less variation between EAE and control lipid compositions.

Figure 7

Miscibility transition pressures (Π_crit) as a function of the relative amount of MBP to PS (−) in the CYT monolayer at room temperature. The absolute amount of MBP added to the subphase is shown on the top of the graph. The miscibility transition pressure first decreases and then increases as the mole ratio of MBP/PS increases. The minimum in Π_crit occurs at a mole ratio of MBP (+20) to PS (−1) equal to 0.12 (or 1 MPB to 8 PS), which is also close to the MBP concentration that provides the greatest adhesion between CYT bilayers (9). The increase in the miscibility pressure for higher ratios may be due to charge reversal due to excess MPB at the interface inducing a positive charge in the domains and restoring the and repulsive electrostatic interactions. The adhesion between CYT bilayers also decreases with increased MBP concentration above the optimal concentration (9).