The many faces of lipid rafts

Aspects of Biological Replication and Evolution Independent of the Central Dogma: Insights from Protein-Free Vesicular Transformations and Protein-Mediated Membrane Remodeling

Biological membrane remodeling is central to living systems. In spite of serving as “containers” of whole-living systems and functioning as dynamic compartments within living systems, biological membranes still find a “blue collar” treatment compared to the “white collar” nucleic acids and proteins in biology. This may be attributable to the fact that scientific literature on biological membrane remodeling is only 50 years old compared to ~ 150 years of literature on proteins and a little less than 100 years on nucleic acids. However, recently, evidence for symbiotic origins of eukaryotic cells from data only on biological membranes was reported. This, coupled with appreciation of reproducible amphiphilic self-assemblies in aqueous environments (mimicking replication), has already initiated discussions on origins of life beyond nucleic acids and proteins. This work presents a comprehensive compilation and meta-analyses of data on self-assembly and vesicular transformations in biological membranes—starting from model membranes to establishment of Influenza Hemagglutinin-mediated membrane fusion as a prototypical remodeling system to a thorough comparison between enveloped mammalian viruses and cellular vesicles. We show that viral membrane fusion proteins, in addition to obeying “stoichiometry-driven protein folding”, have tighter compositional constraints on their amino acid occurrences than general-structured proteins, regardless of type/class. From the perspective of vesicular assemblies and biological membrane remodeling (with and without proteins) we find that cellular vesicles are quite different from viruses. Finally, we propose that in addition to pre-existing thermodynamic frameworks, kinetic considerations in de novo formation of metastable membrane structures with available “third-party” constituents (including proteins) were not only crucial for origins of life but also continue to offer morphological replication and/or functional mechanisms in modern life forms, independent of the central dogma.

Melting transitions in biomembranes

We investigated melting transitions in native biological membranes containing their membrane proteins. The membranes originated from E. coli, B. subtilis, lung surfactant and nerve tissue from the spinal cord of several mammals. For some preparations, we studied the pressure, pH and ionic strength dependence of the transition. For porcine spine, we compared the transition of the native membrane to that of the extracted lipids. All preparations displayed melting transitions of 10-20° below physiological or growth temperature, independent of the organism of origin and the respective cell type. We found that the position of the transitions in E. coli membranes depends on the growth temperature. We discuss these findings in the context of the thermodynamic theory of membrane fluctuations close to transition that predicts largely altered elastic constants, an increase in fluctuation lifetime and in membrane permeability. We also discuss how to distinguish lipid melting from protein unfolding transitions. Since the feature of a transition slightly below physiological temperature is conserved even when growth conditions change, we conclude that the transitions are likely to be of major biological importance for the survival and the function of the cell.

Influence of the membrane environment on cholesterol transfer

Cholesterol, an essential component in biological membranes, is highly unevenly distributed within the cell, with most localized in the plasma membrane while only a small fraction is found in the endoplasmic reticulum, where it is synthesized. Cellular membranes differ in lipid composition and protein content, and these differences can exist across their leaflets too. This thermodynamic landscape that cellular membranes impose on cholesterol is expected to modulate its transport. To uncover the role the membrane environment has on cholesterol inter- and intra-membrane movement, we used time-resolved small angle neutron scattering to study the passive movement of cholesterol between and within membranes with varying degrees of saturation content. We found that cholesterol moves systematically slower as the degree of saturation in the membranes increases, from a palmitoyl oleyl phosphotidylcholine membrane, which is unsaturated, to a dipalmitoylphosphatidylcholine (DPPC) membrane, which is fully saturated. Additionally, we found that the energetic barrier to move cholesterol in these phosphatidylcholine membranes is independent of their relative lipid composition and remains constant for both flip-flop and exchange at ∼100 kJ/mol. Further, by replacing DPPC with the saturated lipid palmitoylsphingomyelin, an abundant saturated lipid of the outer leaflet of the plasma membrane, we found the rates decreased by a factor of two. This finding is in stark contrast with recent molecular dynamic simulations that predict a dramatic slow-down of seven orders of magnitude for cholesterol flipping in membranes with a similar phosphocholine and SM lipid composition.

Exchangeable Mimics of DPPC and DPPG Exhibiting Similar Nearest-Neighbor Interactions in Fluid Bilayers

The interactions between an exchangeable mimic of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), Phos(±), with an exchangeable mimic of cholesterol, Chol, have been analyzed in fluid bilayers by means of nearest-neighbor recognition measurements. These interactions have been found to be very similar to those of an exchangeable mimic of 1,2-dipalmitoyl-sn-glycerol-3-phospho-(1'rac-glycerol) (DPPG), Phos(-), interacting with Chol. Thus, both phospholipids have a similar preference for becoming nearest-neighbors of Chol in the liquid-ordered (l0) phase, and both mix, ideally, with Chol in the liquid-disordered (ld) phase. These findings, together with the almost negligible screening effects found for the latter, provide strong evidence that electrostatic forces play a minor role in the preference that both phospholipids have in becoming a favored nearest-neighbor of Chol. They also imply that the main driving force for forming the liquid-ordered phase, and for defining the lateral organization of this phase, is an intrinsic affinity that high-melting lipids and cholesterol have for each other.

Two-photon Laurdan studies of the ternary lipid mixture DOPC:SM:cholesterol reveal a single liquid phase at sphingomyelin:cholesterol ratios lower than 1

The ternary lipid mixture DOPC:eggSM:cholesterol in excess water has been studied in the form of giant unilamellar vesicles using two-photon fluorescence microscopy. Previous publications based on single-photon fluorescence microscopy had reported heterogeneous phase behavior (phase coexistence) in the region of the triangular phase diagram corresponding to SM:cholesterol molar ratios <1. We have examined this region by two-photon microscopy of Laurdan-labeled mixtures and have found that, under our conditions, only a single liquid phase exists. We have shown that macroscopic phase separation in the above region can be artifactually induced by one-photon excitation of the fluorescent probes and ensuing photooxidation and is prevented using two-photon excitation. The main effect of increasing the concentration of cholesterol in mixtures containing 30 mol % SM was to increase the rigidity of the disordered domains. Increasing the concentration of SM in mixtures containing 20 mol % cholesterol gradually augmented the rigidity of the ordered domains, while the disordered domains reached minimal order at a SM:cholesterol 2.25:1 molar ratio, which then increased again. Moreover, the detailed measurement of Laurdan generalized polarization across the whole phase diagram allowed the representation, for both the single- and two-phase regions, of the gradual variation of membrane lateral packing along the diagram, which we found to be governed largely by SM:cholesterol interactions.

Push and pull forces in lipid raft formation: the push can be as important as the pull

Nearest-neighbor recognition measurements have been made using exchangeable mimics of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine in the liquid-ordered (lo) and liquid-disordered (ld) states. In the ld phase, the net interaction between these two lipids is repulsive. In the lo phase, their interactions are neither attractive nor repulsive. These results, together with previous nearest-neighbor measurements, imply that the overall driving force for lipid domain formation in bilayers composed of high-melting lipids, low-melting lipids, and cholesterol, corresponds to a strong pull (attraction) between the high-melting lipids and cholesterol, a significant push (repulsion) between the low-melting and high-melting lipids, and a significant push between the low-melting lipids and cholesterol. In a broader context, these results provide strong support for the notion that repulsive forces play a major role in the formation of lipid rafts.

The structural role of cholesterol in cell membranes: from condensed bilayers to lipid rafts

CONSPECTUS: Defining the two-dimensional structure of cell membranes represents one of the most daunting challenges currently facing chemists, biochemists, and biophysicists. In particular, the time-averaged lateral organization of the lipids and proteins that make up these natural enclosures has yet to be established. As the classic Singer-Nicolson model of cell membranes has evolved over the past 40 years, special attention has focused on the structural role played by cholesterol, a key component that represents ca. 30% of the total lipids that are present. Despite extensive studies with model membranes, two fundamental issues have remained a mystery: (i) the mechanism by which cholesterol condenses low-melting lipids by uncoiling their acyl chains and (ii) the thermodynamics of the interaction between cholesterol and high- and low-melting lipids. The latter bears directly on one of the most popular notions in modern cell biology, that is, the lipid raft hypothesis, whereby cholesterol is thought to combine with high-melting lipids to form "lipid rafts" that float in a "sea" of low-melting lipids. In this Account, we first describe a chemical approach that we have developed in our laboratories that has allowed us to quantify the interactions between exchangeable mimics of cholesterol and low- and high-melting lipids in model membranes. In essence, this "nearest-neighbor recognition" (NNR) method involves the synthesis of dimeric forms of these lipids that contain a disulfide moiety as a linker. By means of thiolate-disulfide interchange reactions, equilibrium mixtures of dimers are then formed. These exchange reactions are initiated either by adding dithiothreitol to a liposomal dispersion to generate a small amount of thiol monomer or by including a small amount of thiol monomer in the liposomes at pH 5.0 and then raising the pH to 7.4. We then show how such NNR measurements have allowed us to distinguish between two very different mechanisms that have been proposed for cholesterol's condensing effect: (i) an umbrella mechanism in which the acyl chains and cholesterol become more tightly packed as cholesterol content increases because they share limited space under phospholipid headgroups and (ii) a template mechanism whereby cholesterol functions as a planar hydrophobic template at the membrane surface, thereby maximizing hydrophobic interactions and the hydrophobic effect. Specifically, our NNR experiments rule out the umbrella mechanism and provide strong support for the template mechanism. Similar NNR measurements have also allowed us to address the question of whether the interactions between low-melting kinked phospholipids and cholesterol can play a significant role in the formation of lipid rafts. Specifically, these NNR measurements have led to our discovery of a new physical principle in the lipids and membranes area that must be operating in biological membranes, that is, a "push-pull" mechanism, whereby cholesterol is pushed away from low-melting phospholipids and pulled toward high-melting lipids. Thus, to the extent that lipid rafts play a role in the functioning of cell membranes, low-melting phospholipids must be active participants.

Phase transitions in supported lipid bilayers studied by AFM

We review the capabilities of Atomic Force Microscopy (AFM) in the study of phase transitions in Supported Lipid Bilayers (SLBs). AFM represents a powerful technique to cover the resolution range not available to fluorescence imaging techniques and where spectroscopic data suggest what the relevant lateral scale for domain formation might be. Phase transitions of lipid bilayers involve the formation of domains characterized by different heights with respect to the surrounding phase and are therefore easily identified by AFM in liquid solution once the bilayer is confined to a flat surface. Even if not endowed with high time resolution, AFM allows light to be shed on some aspects related to lipid phase transitions in the case of both a single lipid component and lipid mixtures containing sterols also. We discuss here the obtained results in light of the peculiarities of supported lipid bilayer model systems.