Ole G. Mouritsen

Lipid rafts: At a crossroad between cell biology and physics

Membrane lateral heterogeneity is accepted as a requirement for the function of biological membranes and the notion of lipid rafts gives specificity to this broad concept. However, the lipid raft field is now at a technical impasse because the physical tools to study biological membranes as a liquid that is ordered in space and time are still being developed. This has lead to a disconnection between the concept of lipid rafts as derived from biochemical and biophysical assays and their existence in the cell. Here, we compare the concept of lipid rafts as it has emerged from the study of synthetic membranes with the reality of lateral heterogeneity in biological membranes. Further application of existing tools and the development of new tools are needed to understand the dynamic heterogeneity of biological membranes.

Physical properties of the fluid lipid-bilayer component of cell membranes : a perspective

The motivation for this review arises from the conviction that, as a result of the mass of experimental data and observations collected in recent years, the study of the physical properties of membranes is now entering a new stage of development. More and more, experiments are being designed to answer specific, detailed questions about membranes which will lead to a quantitative understanding of the way in which the physical properties of membranes are related to and influence their biological function.

Mattress model of lipid-protein interactions in membranes.

A thermodynamic model is proposed for describing phase diagrams of mixtures of lipid bilayers and amphiphilic proteins or polypeptides in water solution. The basic geometrical variables of the model are the thickness of the hydrophobic region of the lipid bilayer and the length of the hydrophobic region of the proteins. The model incorporates the elastic properties of the lipid bilayer and the proteins, as well as indirect and direct lipid-protein interactions expressed in terms of the geometrical variables. The concept of mismatch of the hydrophobic regions of the lipids and proteins is an important ingredient of the model. The general phase behavior is calculated using simple real solution theory. The phase behavior turns out to be quite rich and is used to discuss previous experiments on planar aggregations of proteins in phospholipid bilayers and to propose a systematic study of synthetic amphiphilic polypeptides in bilayers of different thicknesses. The model is used to interpret the influence of the lipid-protein interaction on calorimetric measurements and on local orientational order as determined by deuterium nuclear magnetic resonance.

What's so special about cholesterol?

Cholesterol (or other higher sterols such as ergosterol and phytosterols) is universally present in large amounts (20–40 mol%) in eukaryotic plasma membranes, whereas it is universally absent in the membranes of prokaryotes. Cholesterol has a unique ability to increase lipid order in fluid membranes while maintaining fluidity and diffusion rates. Cholesterol imparts low permeability barriers to lipid membranes and provides for large mechanical coherence. A short topical review is given of these special properties of cholesterol in relation to the structure of membranes, with results drawn from a variety of theoretical and experimental studies. Particular focus is put on cholesterols ability to promote a special membrane phase, the liquidordered phase, which is unique for cholesterol (and other higher sterols like ergosterol) and absent in membranes containing the cholesterol precursor lanosterol. Cholesterols role in the formation of special membrane domains and so-called rafts is discussed.

Dynamical order and disorder in lipid bilayers

Various order and disorder phenomena in lipid bilayers are considered as they arise due to the very many-particle character of the bilayer. Particular attention is paid to dynamically maintained order in terms of lateral density- and compositional fluctuations that lead to dynamic heterogeneity, local structure, and lipid-domain formation on length scales of 10-1000 A. The influence of cholesterol and various drugs on the local structure is described. A discussion is presented of the possible role played by lipid order and disorder phenomena for the functional dynamics of membranes.

Theoretical analysis of protein organization in lipid membranes.

The fundamental physical principles of the lateral organization of trans-membrane proteins and peptides as well as peripheral membrane proteins and enzymes are considered from the point of view of the lipid-bilayer membrane, its structure, dynamics, and cooperative phenomena. Based on a variety of theoretical considerations and model calculations, the nature of lipid-protein interactions is considered both for a single protein and an assembly of proteins that can lead to aggregation and protein crystallization in the plane of the membrane. Phenomena discussed include lipid sorting and selectivity at protein surfaces, protein-lipid phase equilibria, lipid-mediated protein-protein interactions, wetting and capillary condensation as means of protein organization, mechanisms of two-dimensional protein crystallization, as well as non-equilibrium organization of active proteins in membranes. The theoretical findings are compared with a variety of experimental data.

Relationships between lipid membrane area, hydrophobic thickness, and acyl-chain orientational order. The effects of cholesterol.

A microscopic interaction model for a fully hydrated lipid bilayer membrane containing cholesterol is used to calculate, as a function of temperature and composition, the membrane area, the membrane hydrophobic thickness, and the average acyl-chain orientational order parameter, S. The order parameter, S, is related to the first moment, M1, of the quadrupolar magnetic resonance spectrum which can be measured for lipids with perdeuterated chains. On the basis of these model calculations as well as recent experimental measurements of M1 using magnetic resonance and of membrane area using micromechanical measurements, a discussion of the possible relationships between membrane area, hydrophobic thickness, and moments of nuclear magnetic resonance spectra is presented. It is pointed out that S under certain circumstances may be useful for estimating the hydrophobic membrane thickness. This is particularly advantageous for multicomponent membranes where structural data are difficult to obtain by using diffraction techniques. The usefulness of the suggested relationships is demonstrated for cholesterol-containing bilayers.

From Lanosterol to Cholesterol: Structural Evolution and Differential Effects on Lipid Bilayers

Cholesterol is an important molecular component of the plasma membranes of mammalian cells. Its precursor in the sterol biosynthetic pathway, lanosterol, has been argued by Konrad Bloch (Bloch, K. 1965. Science. 150:19-28; 1983. CRC Crit. Rev. Biochem. 14:47-92; 1994. Blonds in Venetian Paintings, the Nine-Banded Armadillo, and Other Essays in Biochemistry. Yale University Press, New Haven, CT.) to also be a precursor in the molecular evolution of cholesterol. We present a comparative study of the effects of cholesterol and lanosterol on molecular conformational order and phase equilibria of lipid-bilayer membranes. By using deuterium NMR spectroscopy on multilamellar lipid-sterol systems in combination with Monte Carlo simulations of microscopic models of lipid-sterol interactions, we demonstrate that the evolution in the molecular chemistry from lanosterol to cholesterol is manifested in the model lipid-sterol membranes by an increase in the ability of the sterols to promote and stabilize a particular membrane phase, the liquid-ordered phase, and to induce collective order in the acyl-chain conformations of lipid molecules. We also discuss the biological relevance of our results, in particular in the context of membrane domains and rafts.

Computer Studies of Phase Transitions and Critical Phenomena

1. Introduction.- 2. Computer Methods in the Study of Phase Transitions and Critical Phenomena.- 2.1 Statistical Mechanics and Phase Transitions.- 2.1.1 Modern theories of phase transitions and critical phenomena.- 2.1.2 Statistical mechanics, order parameters, fluctuations, critical exponents, scaling, and universality.- 2.2 Numerical Simulation Techniques.- 2.2.1 Monte Carlo methods.- 2.2.2 A Monte Carlo importance-sampling method.- 2.2.3 A realization of a Monte Carlo method.- 2.2.4 General limitations of the Monte Carlo method.- 2.2.5 Broken ergodicity.- 2.2.6 Distribution functions.- 2.2.7 Coarse-graining techniques and criteria of convergence.- 2.2.8 Finite-size effects.- 2.2.9 Determining the nature of a phase transition.- 2.2.10 Computational details.- 2.2.11 General advantages of the Monte Carlo method: Applications.- 2.3 Exact Configurational Counting and Series Expansions.- 2.3.1 A general approach.- 2.3.2 The moment method.- 2.3.3 Principles of the calculation.- 2.3.4 Step 1. Determination of all distinct graphs and their multiplicities.- 2.3.5 Step 2. Embedding of connected graphs into a lattice.- 2.3.6 General correlation function series.- 2.3.7 Capabilities and limitations of a general approach.- 3. Monte Carlo Pure-model Calculations.- 3.1 Critical Behavior of the Three-dimensional Ising Model.- 3.1.1 The Ising model and its order parameter.- 3.1.2 Numerical evidence of a phase transition in the Ising model on a diamond lattice.- 3.1.3 Finite-size scaling analysis and critical behavior.- 3.1.4 Are Monte Carlo techniques practicable in the study of critical phenomena?.- 3.2 Phase Behavior of Ising Models with Multi-spin Interactions.- 3.2.1 Higher-order exchange in magnetic systems.- 3.2.2 Ising models with multi-spin interactions.- 3.2.3 First-order phase transitions of Ising models with pure multi-spin interactions.- 3.2.4 Universality and tricritical behavior of Ising models with two- and four-spin interactions: Pair interactions as a symmetry-breaking field.- 3.3 Thermodynamics of One-dimensional Heisenberg Models.- 3.3.1 One-dimensional magnetic models.- 3.3.2 The anisotropic Heisenberg model in a magnetic field.- 3.3.3 Comparison with theoretical calculations on a continuum model.- 3.3.4 A model ofthe linear magnet CsNiF3?.- 4. Testing Modern Theories of Critical Phenomena.- 4.1 Fluctuation-induced First-order Phase Transitions.- 4.1.1 The role of fixed points in the renormalization group theory.- 4.1.2 Motivation for computer studies of fluctuation-induced first-order phase transitions.- 4.1.3 Phase transitions in antiferromagnets with order Parameters of dimension n=6 and n=3.- 4.1.4 Crossover from first-order to continuous transitions in a symmetry-breaking field.- 4.1.5 Fluctuation-induced first-order phase transitions in Ising models with competing interactions.- 4.2 Critical Phenomena at Marginal Dimensionality.- 4.2.1 The role of a marginal spatial dimension.- 4.2.2 Computer experiments of hypercubic Ising models: ?A romance of many dimensions?.- 4.2.3 Susceptibility and critical isotherm of the four-dimensional Ising model.- 4.2.4 Conclusions on critical behavior in marginal dimensions.- 4.3 Basic Assumptions of Critical Correlation Theories.- 4.3.1 Review of a critical correlation theory.- 4.3.2 Testing the basic assumption by Monte Carlo calculations.- 5. Numerical Experiments.- 5.1 Phase Transitions in Lipid Bilayers and Biological Membranes.- 5.1.1 What are biological membranes and what do they do?.- 5.1.2 Lipid bilayers are model membranes.- 5.1.3 Phase behavior of lipid bilayers.- 5.1.4 Back to biology: Are phase transitions at all relevant to the biological functions of the membrane?.- 5.1.5 Theories of lipid bilayer phase transitions.- 5.1.6 Computer simulations of lipid bilayers.- 5.1.7 Multi-state models of lipid bilayers.- 5.1.8 Computer simulations of the q-state models for the gel-fluid phase transition.- 5.1.9 Computer Simulation of the phase behavior of lipid bilayers with ?impurities?: cholesterol, proteins, and Polypeptides.- 5.1.10 Have Computer studies provided any new insight into the properties of biological membranes?.- 5.2 Nuclear Dipolar Magnetic Ordering and Phase Transitions.- 5.2.1 Nuclear dipolar magnetic ordering.- 5.2.2 The secular dipolar Hamiltonian.- 5.2.3 Perspectives in studies of nuclear dipolar magnetic ordering.- 5.2.4 Motivation for a numerical Simulation study of nuclear dipolar magnetic ordering.- 5.2.5 Monte Carlo studies of systems with truncated classical secular dipolar interactions.- 5.2.6 Nature of the spin structures: ?Permanent? structures or the devils staircase?.- 5.2.7 Double-layered spin structures in CaF2-like systems: Continuous transitions and critical behavior.- 5.2.8 Multi-layered spin structures in CaF2-like systems: Firstorder phase transitions.- 5.2.9 Can series expansions provide information on the nature of the phase transitions?.- 5.2.10 Nuclear antiferrimagnetic susceptibilities of systems with two spin species: LiF and LiH.- 5.3 Phase Transitions of Adsorbed Monolayers.- 5.3.1 Two-dimensional phases of molecules adsorbed on solid surfaces.- 5.3.2 N2 physisorbed on graphite: The anisotropic-planar rotor model.- 5.3.3 The Heisenberg model with cubic anisotropy.- 5.3.4 Fluctuation-induced first-order phase transition in the anisotropic-planar rotor model.- 5.3.5 Comparison with experiments on N2 physisorbed on graphite.- 5.3.6 Phase behavior on the anisotropic-planar rotor model with vacancies.- 5.3.7 Physical realizations of the anisotropic-planar rotor model with vacancies.- 5.4 Kinetics of Growth.- 5.4.1 Growth.- 5.4.2 Computer Simulation of domain-growth kinetics.- 5.4.3 Domain-growth kinetics of herringbonephases.- 5.4.4 Domain-growth kinetics of pinwheel phases.- 5.4.5 Kinetics of growth and critical phenomena.

Passive ion permeability of lipid membranes modelled via lipid-domain interfacial area

A microscopic interaction model of the gel-to-fluid chain-melting phase transition of fully hydrated lipid bilayer membranes is used as a basis for modelling the temperature dependence of passive transmembrane permeability of small ions, e.g. Na+. Computer simulation of the model shows that the phase transition is accompanied by strong lateral density fluctuations which manifest themselves in the formation of inhomogeneous equilibrium structures of coexisting gel and fluid domains. The interfaces of these domains are found to be dominated by intermediate lipid-chain conformations. The interfacial area is shown to have a pronounced peak at the phase transition. By imposing a simple model for ion diffusion through membranes which assigns a high relative permeation rate to the domain interfaces, the interfacial area is then identified as a membrane property which has the proper temperature variation to account for the peculiar experimental observation of a strongly enhanced passive ion permeability at the phase transition. The excellent agreement with the experimental data for Na+-permeation, taken together with recent experimental results for the phase transition kinetics, provides new insight into the microphysical mechanisms of reversible electric breakdown. This insight indicates that there is no need for aqueous pore-formation to explain the experimental observation of a dramatic increase in ion conductance subsequent to electric pulses.