Recent Experiments Support a Microemulsion Origin of Plasma Membrane Domains: Dependence of Domain Size on Physical Parameters

Visualizing lipid membrane structure with cryo-EM: past, present, and future

The development of electron cryomicroscopy (cryo-EM) has evolved immensely in the last several decades and is now well-established in the analysis of protein structure both in isolation and in their cellular context. This review focuses on the history and application of cryo-EM to the analysis of membrane architecture. Parallels between the levels of organization of protein structure are useful in organizing the discussion of the unique parameters that influence membrane structure and function. Importantly, the timescales of lipid motion in bilayers with respect to the timescales of sample vitrification is discussed and reveals what types of membrane structure can be reliably extracted in cryo-EM images of vitrified samples. Appreciating these limitations, a review of the application of cryo-EM to examine the lateral organization of ordered and disordered domains in reconstituted and biologically derived membranes is provided. Finally, a brief outlook for further development and application of cryo-EM to the analysis of membrane architecture is provided.

Systematic analysis of curvature-dependent lipid dynamics in a stochastic 3D membrane model

To minimize the free energy of the system, lipid membranes display curvature-dependent rearrangements at the local and global scale. The optimal membrane shape is generally approximated by averaging the curvature preference of individual lipids across the whole surface. Potential stress due to imperfections in lipid packing caused by local lipid inhomogeneities, however, is frequently neglected. Here, we developed a stochastic 3D membrane model to investigate the relevance of this parameter for shape-dependent lipid and membrane dynamics. A systematic analysis of the discretized Helfrich type Hamiltonian indicates that stress-energy arising from imperfections in packing is analogous to van der Waals interactions, jointly determining membrane shape and localization of curvature-sensitive lipids based on their relative strengths. Insights from this work can be used to characterize natural and design synthetic agents for membrane-shape changes.

Enthalpic and entropic contributions to interleaflet coupling drive domain registration and antiregistration in biological membrane

Biological membrane is a complex self-assembly of lipids, sterols, and proteins organized as a fluid bilayer of two closely stacked lipid leaflets. Differential molecular interactions among its diverse constituents give rise to heterogeneities in the membrane lateral organization. Under certain conditions, heterogeneities in the two leaflets can be spatially synchronized and exist as registered domains across the bilayer. Several contrasting theories behind mechanisms that induce registration of nanoscale domains have been suggested. Following a recent study showing the effect of position of lipid tail unsaturation on domain registration behavior, we decided to develop an analytical theory to elucidate the driving forces that create and maintain domain registry across leaflets. Towards this, we formulated a Hamiltonian for a stacked lattice system where site variables capture the lipid molecular properties such as the position of unsaturation and various other interactions that could drive phase separation and interleaflet coupling. We solve the Hamiltonian using Monte Carlo simulations and create a complete phase diagram that reports the presence or absence of registered domains as a function of various Hamiltonian parameters. We find that the interleaflet coupling should be described as a competing enthalpic contribution due to interaction of lipid tail termini, primarily due to saturated-saturated interactions, and an interleaflet entropic contribution from overlap of unsaturated tail termini. A higher position of unsaturation is seen to provide weaker interleaflet coupling. Thermodynamically stable nanodomains could also be observed for certain points in the parameter space in our bilayer model, which were further verified by carrying out extended Monte Carlo simulations. These persistent noncoalescing registered nanodomains close to the lower end of the accepted nanodomain size range also point towards a possible "nanoscale" emulsion description of lateral heterogeneities in biological membrane leaflets.

Special Issue: Dynamics and Nano-Organization in Plasma Membranes

Cell membranes develop extraordinarily complex lipids and proteins geared to perform functions required by cells [...].

On the small size of liquid-disordered + liquid-ordered nanodomains

Four-component phase diagrams reveal that Liquid-disordered + liquid-ordered (Ld + Lo) nanodomains are exclusively found adjacent to a three-phase region, and so cannot be a one-phase microemulsion. Of importance for understanding biological membranes, a small change in lipid bilayer composition can change the size of these coexisting phase domains hundreds of fold, between tens of nanometers and microns. Nanodomain diameter, measured from small angle neutron scattering, is in the range 15-35 nm, consistent with stabilization by repulsive dipole fields. Ld/Lo line tension controls the Ld + Lo domain size transition. Other than size, chemical and physical properties of the phase domains do not seem to change during the transition. Unfavorable lipid-lipid pairwise interactions, rather than phase thickness mismatch, seem to be the main reason for Ld + Lo immiscibility. Pairwise interactions of cholesterol-phospholipid seem to be favorable, whereas pairwise interactions of high-melting phospholipid with low-melting phospholipid are unfavorable. Measured Ld/Lo line tension, like the phase separation, is created mainly by unfavorable lipid-lipid pairwise interactions. Lipid dipole-dipole repulsion opposes these unfavorable lipid-lipid pairwise interactions and thus, in a sense, is the reason that nanodomains form. Bilayer physical and chemical properties measured from macroscopic domains of coexisting Ld + Lo phases should be good approximations for the properties of coexisting nanoscopic domains.

Spatial extent of a single lipid's influence on bilayer mechanics

To what spatial extent does a single lipid affect the mechanical properties of the membrane that surrounds it? The lipid composition of a membrane determines its mechanical properties. The shapes available to the membrane depend on its compositional material properties, and therefore, the lipid environment. Because each individual lipid species' chemistry is different, it is important to know its range of influence on membrane mechanical properties. This is defined herein as the lipid's mechanical extent. Here, a lipid's mechanical extent is determined by quantifying lipid redistribution and the average curvature that lipid species experience on fluctuating membrane surfaces. A surprising finding is that, unlike unsaturated lipids, saturated lipids have a complicated, nonlocal effect on the surrounding surface, with the interaction strength maximal at a finite length-scale. The methodology provides the means to substantially enrich curvature-energy models of membrane structures, quantifying what was previously only conjecture.