Critical Phenomena in Plasma Membrane Organization and Function

Abstract

Lateral organization in the plane of the plasma membrane is an important driver of biological processes. The past dozen years have seen increasing experimental support for the notion that lipid organization plays an important role in modulating this heterogeneity. Various biophysical mechanisms rooted in the concept of liquid-liquid phase separation have been proposed to explain diverse experimental observations of heterogeneity in model and cell membranes with distinct but overlapping applicability. In this review, we focus on the evidence for and the consequences of the hypothesis that the plasma membrane is poised near an equilibrium miscibility critical point. Critical phenomena explain certain features of the heterogeneity observed in cells and model systems but also go beyond heterogeneity to predict other interesting phenomena, including responses to perturbations in membrane composition.

Keywords: cell membrane; critical composition fluctuations; lipid rafts; membrane microdomains; thermodynamics.

Figure 1

Phase diagram of lipid mixtures. (a) Phase diagrams of three component mixtures (C1, C2, and C3) are conventionally drawn on an equilateral triangle. The three vertices are pure mixtures of each lipid component, points along the edges are binary mixtures, and points within the triangle contain all three components. Compositions can be read by measuring the perpendicular distance to each edge and adding the resulting percentages, which always sum to 100%. Two examples are shown. (b) A qualitative phase diagram for ternary lipid mixtures of high melting temperature (T_m) lipids, low T_m lipids, and cholesterol. Thick red lines indicate the boundaries of liquid–solid (S_o) coexistence, and the thick blue line represents the boundary of liquid-liquid coexistence of the liquid-disordered (L_d) and liquid-ordered (L_o) phases. Points along this boundary also indicate the composition of coexisting phases, and the specific compositions in coexistence are indicated by blue and red shaded areas within binary coexistence regions. The purple triangle represents compositions that exhibit all three phases in coexistence. The compositions of the three phases are indicated by the three vertices of the triangle. The L_d–L_o coexistence region terminates at a miscibility critical point along the high cholesterol edge that is indicated by an orange star.

Figure 2

Phase diagrams of ternary lipid mixtures exhibit the same overall topology. Curves on the phase diagrams indicate phase boundaries. In particular, the high cholesterol phase boundary defines the boundary of the liquid-liquid miscibility gap in each case. Sample points and/or deduced tie-line endpoints are indicated in some cases, and some diagrams label single phases as L_α or L_d, L_o, and L_β or S_o with abbreviations given below. (a) DPhPC/ DPPC/Chol by fluorescence microscopy at 16°C. Panel a adapted from Reference with permission. (b) DOPC/DPPC/Chol by deuterium NMR spectroscopy. Panel b adapted from Reference with permission. (c) DOPC/DSPC/Chol by fluorescence microscopy and FRET. Panel c adapted from Reference with permission. (d) DPC/PSM/Chol by FRET, neutron scattering, and DSC. Panel d adapted from Reference with permission. (e) POPC/PSM/Chol by EPR spectroscopy. Panel e adapted from Reference with permission. (f) DOPC/eggSM/Chol by atomic force microscopy at 28°C. Panel f adapted from Reference with permission. Abbreviations: BSM, brain sphingomyelin; Chol, cholesterol; DOPC, 1,2-dioleoyl-sn-glycero-3-phosphocholine; DPC, dodecylphosphocholine; DPhPC, diphytanoyl phosphatidylcholine; DPPC, dipalmitoylphosphatidylcholine; DSC, differential scanning calorimetry; DSPC, 1,2-distearoyl-sn-glycero-3-phosphocholine; eggSM, egg sphingomyelin; EPR, electron paramagnetic resonance; FRET, Förster resonance energy transfer; L_α, lamellar liquid crystalline phase; L_β, a lamellar gen phase; L_d, liquid-disordered phase; L_o, liquid-ordered phase; NMR, nuclear magnetic resonance; POPC,1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine; PSM, palmitoylsphingomyelin; S_o, solid or gel phase.

Figure 3

Critical systems exhibit universal features that extend beyond the two-phase region. (a) Simulation snapshots of two lattice models. (Top) An Ising model at various reduced temperatures t and compositions m, and (middle) a three-component model with interactions designed to reproduce typical phase diagrams of ternary mixtures of Cholesterol (dark blue), a low melting temperature (T_m) lipid (yellow) and a high T_m lipid (cyan) at fixed temperature and various compositions. (Bottom) Model parameters used for (i)-(vi) in each model are indicated on the respective phase diagram. Compositions of the three-component model are chosen so that the effective reduced temperature t and effective magnetization m of the model match the t and m of the corresponding Ising model. Miscibility gaps, tie-lines, and the directions of t and m are shown on each schematic phase diagram. The regions that correspond to liquid-ordered (L_o) and liquid-disordered (L_d) mixtures in ternary lipid mixtures are also indicated. (b) Measurements of the divergence of correlation length ξ versus t as the critical point is approached from the one-phase region. (Top) In the two models from panel a. For the three-component model, t is the cholesterol (Chol) content normalized by the Chol content at the critical point. (Middle) In GUVs and giant plasma membrane vesicles (GPMVs) with varying temperature. Data from Reference . In each case, the expected 2D Ising power law ξ ∝ t⁻¹ is observed although the proportionality constant differs, resulting in each data set being plotted with a different y-axis scale. (Bottom) Representative GUV and GPMV images near the critical point.

Figure 4

Four functional mechanisms primarily driven by liquid-ordered (L_o)–liquid-disordered (L_d) phase partitioning in a near-critical membrane. Membrane proteins and other components are characterized by how they partition into L_o–L_d domains, as shown here schematically. The combination of the partitioning of various components confers to the systems particular properties that can be used to drive biological function. a) Critical Casimir forces yield an effective attraction between components with like order preference (blue-blue circles), and an effective repulsion between components that prefer opposite lipid order (blue-magenta circles). The strength of these interactions increases rapidly when t is reduced andξ becomes large. (b) Clustering a protein that prefers L_o lipids (green circle) induces a distinct L_o domain due to the high susceptibility of the membrane near the critical point. As a result, other Lo-preferring proteins (blue circles) are recruited to the cluster, and L_d -preferring proteins (magenta circles) are excluded. Similarly, an L_d domain can be stabilized by clustering an L_d-preferring component. (c) The white shape is a large protein with two conformations, pentagon or star. One conformation (pentagon) has no order preference, and the other (star) prefers an L_o environment. The star conformation becomes more likely as t is decreased, because there are large patches of L_o membrane that satisfy its preferred boundary condition. (d) Changes in t also result in changes in the chemical potentials of some membrane components, including components (red ovals) that can bind to a membrane protein (orange squares) as an allosteric modulator. Therefore, a change in t can induce differences in binding-site occupancy and the resulting distribution of protein states.