Investigation on lipid asymmetry using lipid probes: Comparison between spin-labeled lipids and fluorescent lipids

Sum-frequency vibrational spectroscopy, a tutorial: Applications for the study of lipid membrane structure and dynamics

Planar supported lipid bilayers (PSLBs) are an ideal model for the study of lipid membrane structures and dynamics when using sum-frequency vibrational spectroscopy (SFVS). In this paper, we describe the construction of asymmetric PSLBs and the basic SFVS theory needed to understand and make measurements on these membranes. Several examples are presented, including the determination of phospholipid orientation and measuring phospholipid transmembrane translocation (flip-flop).

Studying lipid flip-flop in asymmetric liposomes using 1H NMR and TR-SANS

The specific spatial and temporal distribution of lipids in membranes play a crucial role in determining the biochemical and biophysical properties of the system. In nature, the asymmetric distribution of lipids is a dynamic process with ATP-dependent lipid transporters maintaining asymmetry, and passive transbilayer diffusion, that is, flip-flop, counteracting it. In this chapter, two probe-free techniques, 1H NMR and time-resolved small angle neutron scattering, are described in detail as methods of investigating lipid flip-flop rates in synthetic liposomes that have been generated with an asymmetric bilayer composition.

Fluorescent Probes and Quenchers in Studies of Protein Folding and Protein-Lipid Interactions

Fluorescence spectroscopy provides numerous methodological tools for structural and functional studies of biological macromolecules and their complexes. All fluorescence-based approaches require either existence of an intrinsic probe or an introduction of an extrinsic one. Moreover, studies of complex systems often require an additional introduction of a specific quencher molecule acting in combination with a fluorophore to provide structural or thermodynamic information. Here, we review the fundamentals and summarize the latest progress in applications of different classes of fluorescent probes and their specific quenchers, aimed at studies of protein folding and protein-membrane interactions. Specifically, we discuss various environment-sensitive dyes, FRET probes, probes for short-distance measurements, and several probe-quencher pairs for studies of membrane penetration of proteins and peptides. The goals of this review are: (a) to familiarize the readership with the general concept that complex biological systems often require both a probe and a quencher to decipher mechanistic details of functioning and (b) to provide example of the immediate applications of the described methods.

A novel missense variant in ATP11C is associated with reduced red blood cell phosphatidylserine flippase activity and mild hereditary hemolytic anemia

Adenosine Triphosphatase (ATPase) Phospholipid Transporting 11C gene (ATP11C) encodes the major phosphatidylserine (PS) flippase in human red blood cells (RBCs). Flippases actively transport phospholipids (e.g., PS) from the outer to the inner leaflet to establish and maintain phospholipid asymmetry of the lipid bilayer of cell membranes. This asymmetry is crucial for survival since externalized PS triggers phagocytosis by splenic macrophages. Here we report on pathophysiological consequences of decreased flippase activity, prompted by a patient with hemolytic anemia and hemizygosity for a novel c.2365C > T p.(Leu789Phe) missense variant in ATP11C. ATP11C protein expression was strongly reduced by 58% in patient-derived RBC ghosts. Furthermore, functional characterization showed only 26% PS flippase activity. These results were confirmed by recombinant mutant ATP11C protein expression in HEK293T cells, which was decreased to 27% compared to wild type, whereas PS-stimulated ATPase activity was decreased by 57%. Patient RBCs showed a mild increase in PS surface exposure when compared to control RBCs, which further increased in the most dense RBCs after RBC storage stress. The increase in PS was not due to higher global membrane content of PS or other phospholipids. In contrast, membrane lipid lateral distribution showed increased abundance of cholesterol-enriched domains in RBC low curvature areas. Finally, more dense RBCs and subtle changes in RBC morphology under flow hint toward alterations in flow behavior of ATP11C-deficient RBCs. Altogether, ATP11C deficiency is the likely cause of hemolytic anemia in our patient, thereby underlining the physiological role and relevance of this flippase in human RBCs.

Inhibitory Effect of Lanthanides on Native Lipid Flip-Flop

The influence of ytterbium ions (Yb³⁺), a commonly used paramagnetic NMR chemical shift reagent, on the physical properties and flip-flop kinetics of dipalmitoylphosphatidylcholine (DPPC) planar supported lipid bilayers (PSLBs) was investigated. Langmuir isotherm studies revealed that Yb³⁺ interacts strongly with the phosphate headgroup of DPPC, evidenced by the increases in shear and compression moduli. Using sum-frequency vibrational spectroscopy, changes in the acyl chain ordering and phase transition temperature were also observed, consistent with Yb³⁺ interacting with the phosphate headgroup of DPPC. The changes in the physical properties of the membrane were also observed to be concentration dependent, with more pronounced modification observed at low (50 μM) Yb³⁺ concentrations compared to 6.5 mM Tb³⁺, suggesting a cross-linking mechanism between adjacent DPPC lipids. Additionally, the changes in membrane packing and phase transition temperatures in the presence of Tris buffer suggested that a putative Yb(Tris)³⁺ complex forms that coordinates to the PC headgroup. The kinetics of DPPC flip-flop in the gel and liquid crystalline (lc) phases were substantially inhibited in the presence of Yb³⁺, regardless of the Yb³⁺ concentration. Analysis of the flip-flop kinetics under the framework of transition state theory revealed that the free energy barrier to flip-flop in both the gel and lc phases was substantial increased over a pure DPPC membrane. In the gel phase, the trend in the free energy barrier appeared to follow the trend in the shear moduli, suggesting that the Yb³⁺-DPPC headgroup interaction was driving the increase in the activation free energy barrier. In the lc phase, activation free energies of DPPC flip-flop in the presence of 50 μM or 6.5 mM Yb³⁺ were found to mirror the free energies of TEMPO-DPPC flip-flop, leading to the conclusion that the strong interaction between Yb³⁺ and the PC headgroup was essentially manifested as a headgroup charge modification. These studies illustrate that the presence of the lanthanide Yb³⁺ results in significant modification to the lipid membrane physical properties and, more importantly, results in a pronounced inhibition of native lipid flip-flop.

Fabrication and electromechanical characterization of freestanding asymmetric membranes

All biological cell membranes maintain an electric transmembrane potential of around 100 mV, due in part to an asymmetric distribution of charged phospholipids across the membrane. This asymmetry is crucial to cell health and physiological processes such as intracell signaling, receptor-mediated endocytosis, and membrane protein function. Experimental artificial membrane systems incorporate essential cell membrane structures, such as the phospholipid bilayer, in a controllable manner in which specific properties and processes can be isolated and examined. Here, we describe an approach to fabricate and characterize planar, freestanding, asymmetric membranes and use it to examine the effect of headgroup charge on membrane stiffness. The approach relies on a thin film balance used to form a freestanding membrane by adsorbing aqueous phase lipid vesicles to an oil-water interface and subsequently thinning the oil to form a bilayer. We validate this lipid-in-aqueous approach by analyzing the thickness and compressibility of symmetric membranes with varying zwitterionic 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and anionic 1,2-dioleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) sodium salt (DOPG) content as compared with previous lipid-in-oil methods. We find that as the concentration of DOPG increases, membranes become thicker and stiffer. Asymmetric membranes are fabricated by controlling the lipid vesicle composition in the aqueous reservoirs on either side of the oil. Membrane compositional asymmetry is qualitatively demonstrated using a fluorescence quenching assay and quantitatively characterized through voltage-dependent capacitance measurements. Stable asymmetric membranes with DOPC on one side and DOPC-DOPG mixtures on the other were created with transmembrane potentials ranging from 15 to 80 mV. Introducing membrane charge asymmetry decreases both the thickness and stiffness in comparison with symmetric membranes with the same overall phospholipid composition. These initial successes demonstrate a viable pathway to quantitatively characterize asymmetric bilayers that can be extended to accommodate more complex membranes and membrane processes in the future.

Enzymatic trans-bilayer lipid transport: Mechanisms, efficiencies, slippage, and membrane curvature

The eukaryotic plasma membrane's lipid composition is found to be ubiquitously asymmetric comparing inner and outer leaflets. This membrane lipid asymmetry plays a crucial role in diverse cellular processes critical for cell survival. A specialized set of transmembrane proteins called translocases, or flippases, have evolved to maintain this membrane lipid asymmetry in an energy-dependent manner. One potential consequence of local variations in membrane lipid asymmetry is membrane remodeling, which is essential for cellular processes such as intracellular trafficking. Recently, there has been a surge in the identification and characterization of flippases, which has significantly advanced the understanding of their functional mechanisms. Furthermore, there are intriguing possibilities for a coupling between membrane curvature and flippase activity. In this review we highlight studies that link membrane shape and remodeling to differential stresses generated by the activity of lipid flippases with an emphasis on data obtained through model membrane systems. We review the common mechanistic models of flippase-mediated lipid flipping and discuss common techniques used to test lipid flippase activity. We then compare the existing data on lipid translocation rates by flippases and conclude with potential future directions for this field.

Lipid flip-flop and desorption from supported lipid bilayers is independent of curvature

Flip-flop of lipids of the lipid bilayer (LBL) constituting the plasma membrane (PM) plays a crucial role in a myriad of events ranging from cellular signaling and regulation of cell shapes to cell homeostasis, membrane asymmetry, phagocytosis, and cell apoptosis. While extensive research has been conducted to probe the lipid flip flop of planar lipid bilayers (LBLs), less is known regarding lipid flip-flop for highly curved, nanoscopic LBL systems despite the vast importance of membrane curvature in defining the morphology of cells and organelles and in maintaining a variety of cellular functions, enabling trafficking, and recruiting and localizing shape-responsive proteins. In this paper, we conduct molecular dynamics (MD) simulations to study the energetics, structure, and configuration of a lipid molecule undergoing flip-flop and desorption in a highly curved LBL, represented as a nanoparticle-supported lipid bilayer (NPSLBL) system. We compare our findings against those of a planar substrate supported lipid bilayer (PSSLBL). Our MD simulation results reveal that despite the vast differences in the curvature and other curvature-dictated properties (e.g., lipid packing fraction, difference in the number of lipids between inner and outer leaflets, etc.) between the NPSLBL and the PSSLBL, the energetics of lipid flip-flop and lipid desorption as well as the configuration of the lipid molecule undergoing lipid flip-flop are very similar for the NPSLBL and the PSSLBL. In other words, our results establish that the curvature of the LBL plays an insignificant role in lipid flip-flop and desorption.

Peptide-Induced Lipid Flip-Flop in Asymmetric Liposomes Measured by Small Angle Neutron Scattering

Despite the prevalence of lipid transbilayer asymmetry in natural plasma membranes, most biomimetic model membranes studied are symmetric. Recent advances have helped to overcome the difficulties in preparing asymmetric liposomes in vitro, allowing for the examination of a larger set of relevant biophysical questions. Here, we investigate the stability of asymmetric bilayers by measuring lipid flip-flop with time-resolved small-angle neutron scattering (SANS). Asymmetric large unilamellar vesicles with inner bilayer leaflets containing predominantly 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and outer leaflets composed mainly of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) displayed slow spontaneous flip-flop at 37 ^◦C (half-time, t_1/2 = 140 h). However, inclusion of peptides, namely, gramicidin, alamethicin, melittin, or pHLIP (i.e., pH-low insertion peptide), accelerated lipid flip-flop. For three of these peptides (i.e., pHLIP, alamethicin, and melittin), each of which was added externally to preformed asymmetric vesicles, we observed a completely scrambled bilayer in less than 2 h. Gramicidin, on the other hand, was preincorporated during the formation of the asymmetric liposomes and showed a time resolvable 8-fold increase in the rate of lipid asymmetry loss. These results point to a membrane surface-related (e.g., adsorption/insertion) event as the primary driver of lipid scrambling in the asymmetric model membranes of this study. We discuss the implications of membrane peptide binding, conformation, and insertion on lipid asymmetry.

Dewetting-induced formation and mechanical properties of synthetic bacterial outer membrane models (GUVs) with controlled inner-leaflet lipid composition

The double-membrane cellular envelope of Gram-negative bacteria enables them to endure harsh environments and represents a barrier to many clinically available antibiotics. The outer membrane (OM) is exposed to the environment and is the first point of contact involved in bacterial processes such as signaling, pathogenesis, and motility. As in the cytoplasmic membrane, the OM in Gram-negative bacteria has a phospholipid-rich inner leaflet and an outer leaflet that is predominantly composed of lipopolysaccharide (LPS). We report on a microfluidic technique for fabricating monodisperse asymmetric giant unilamellar vesicles (GUVs) possessing the Gram-negative bacterial OM lipid composition. Our continuous microfluidic fabrication technique generates 50-150 μm diameter water-in-oil-in-water double emulsions at high-throughput. The water-oil and oil-water interfaces facilitate the self-assembly of phospholipid and LPS molecules to create the inner and outer leaflets of the lipid bilayer, respectively. The double emulsions have ultrathin oil shells, which minimizes the amount of residual organic solvent that remains trapped between the leaflets of the GUV membrane. An extraction process by ethanol and micropipette aspiration of the ultrathin oil shells triggers an adhesive interaction between the two lipid monolayers assembled on the water-oil and oil-water interfaces (i.e., dewetting transition), forcing them to contact and form a lipid bilayer membrane. The effect of different inner-leaflet lipid compositions on the emulsion/vesicle stability and the dewetting transition is investigated. We also report on the values for bending and area expansion moduli of synthetic asymmetric model membranes with lipid composition/architecture that is physiologically relevant to the OM in Pseudomonas aeruginosa bacteria.

Scrambling of natural and fluorescently tagged phosphatidylinositol by reconstituted G protein-coupled receptor and TMEM16 scramblases

Members of the G protein-coupled receptor and TMEM16 (transmembrane protein 16) protein families are phospholipid scramblases that facilitate rapid, bidirectional movement of phospholipids across a membrane bilayer in an ATP-independent manner. On reconstitution into large unilamellar vesicles, these proteins scramble more than 10,000 lipids/protein/s as measured with co-reconstituted fluorescent nitrobenzoxadiazole (NBD)-labeled phospholipids. Although NBD-labeled phospholipids are ubiquitously used as reporters of scramblase activity, it remains unclear whether the NBD modification influences the quantitative outcomes of the scramblase assay. We now report a refined biochemical approach for measuring the activity of scramblase proteins with radiolabeled natural phosphatidylinositol ([3H]PI) and exploiting the hydrolytic activity of bacterial PI-specific phospholipase C (PI-PLC) to detect the transbilayer movement of PI. PI-PLC rapidly hydrolyzed 50% of [3H]PI in large symmetric, unilamellar liposomes, corresponding to the lipid pool in the outer leaflet. On reconstitution of a crude preparation of yeast endoplasmic reticulum scramblase, purified bovine opsin, or purified Nectria haematococca TMEM16, the extent of [3H]PI hydrolysis increased, indicating that [3H]PI from the inner leaflet had been scrambled to the outer leaflet. Using transphosphatidylation, we synthesized acyl-NBD-PI and used it to compare our PI-PLC-based assay with conventional fluorescence-based methods. Our results revealed quantitative differences between the two assays that we attribute to the specific features of the assays themselves rather than to the nature of the phospholipid. In summary, we have developed an assay that measures scrambling of a chemically unmodified phospholipid by a reconstituted scramblase.

Influence of the Membrane Dye R18 and of DMSO on Cell Penetration of Guanidinium-Rich Peptides

A quantitative analysis by confocal fluorescence microscopy of the entry into HEK293 and MCF-7 cells by fluorescein-labeled octaarginine (1) and by three octa-Adp derivatives (2 - 4, octamers of the β-Asp-Arg-dipeptide, derived from the biopolymer cyanophycin) is described, including the effects of the membrane dye R18 and of DMSO on cell penetration.

Cholesterol is enriched in the sphingolipid patches on the substrate near nonpolarized MDCK cells, but not in the sphingolipid domains in their plasma membranes

Information about the distributions of cholesterol and sphingolipids within the plasma membranes of mammalian cells provides insight into the roles of these molecules in membrane function. In this report, high-resolution secondary ion mass spectrometry was used to image the distributions of metabolically incorporated rare isotope-labeled sphingolipids and cholesterol on the surfaces of nonpolarized epithelial cells. Sphingolipid domains that were not enriched with cholesterol were detected in the plasma membranes of subconfluent Madin-Darby canine kidney cells. Surprisingly, cholesterol-enriched sphingolipid patches were observed on the substrate adjacent to these cells. Based on the shapes of these cholesterol-enriched sphingolipid patches on the substrate and their proximity to cellular projections, we hypothesize that they are deposits of membranous particles released by the cell.

Phosphatidylserine Lateral Organization Influences the Interaction of Influenza Virus Matrix Protein 1 with Lipid Membranes

Influenza A virus matrix protein 1 (M1) is an essential component involved in the structural stability of the virus and in the budding of new virions from infected cells. A deeper understanding of the molecular basis of virion formation and the budding process is required in order to devise new therapeutic approaches. We performed a detailed investigation of the interaction between M1 and phosphatidylserine (PS) (i.e., its main binding target at the plasma membrane [PM]), as well as the distribution of PS itself, both in model membranes and in living cells. To this end, we used a combination of techniques, including Förster resonance energy transfer (FRET), confocal microscopy imaging, raster image correlation spectroscopy, and number and brightness (N&B) analysis. Our results show that PS can cluster in segregated regions in the plane of the lipid bilayer, both in model bilayers constituted of PS and phosphatidylcholine and in living cells. The viral protein M1 interacts specifically with PS-enriched domains, and such interaction in turn affects its oligomerization process. Furthermore, M1 can stabilize PS domains, as observed in model membranes. For living cells, the presence of PS clusters is suggested by N&B experiments monitoring the clustering of the PS sensor lactadherin. Also, colocalization between M1 and a fluorescent PS probe suggest that, in infected cells, the matrix protein can specifically bind to the regions of PM in which PS is clustered. Taken together, our observations provide novel evidence regarding the role of PS-rich domains in tuning M1-lipid and M1-M1 interactions at the PM of infected cells.IMPORTANCE Influenza virus particles assemble at the plasma membranes (PM) of infected cells. This process is orchestrated by the matrix protein M1, which interacts with membrane lipids while binding to the other proteins and genetic material of the virus. Despite its importance, the initial step in virus assembly (i.e., M1-lipid interaction) is still not well understood. In this work, we show that phosphatidylserine can form lipid domains in physical models of the inner leaflet of the PM. Furthermore, the spatial organization of PS in the plane of the bilayer modulates M1-M1 interactions. Finally, we show that PS domains appear to be present in the PM of living cells and that M1 seems to display a high affinity for them.

Reconciling Differences between Lipid Transfer in Free-Standing and Solid Supported Membranes: A Time-Resolved Small-Angle Neutron Scattering Study

Maintaining compositional lipid gradients across membranes in animal cells is essential to biological function, but what is the energetic cost to maintain these differences? It has long been recognized that studying the passive movement of lipids in membranes can provide insight into this toll. Confusingly the reported values of inter- and, particularly, intra-lipid transport rates of lipids in membranes show significant differences. To overcome this difficulty, biases introduced by experimental approaches have to be identified. The present study addresses the difference in the reported intramembrane transport rates of dimyristoylphosphatidylcholine (DMPC) on flat solid supports (fast flipping) and in curved free-standing membranes (slow flipping). Two possible scenarios are potentially at play: one is the difference in curvature of the membranes studied and the other the presence (or not) of the support. Using DMPC vesicles and DMPC supported membranes on silica nanoparticles of different radii, we found that an increase in curvature (from a diameter of 30 nm to a diameter of 100 nm) does not change the rates significantly, differing only by factors of order ∼1. Additionally, we found that the exchange rates of DMPC in supported membranes are similar to the ones in vesicles. And as previously reported, we found that the activation energies for exchange on free-standing and supported membranes are similar (84 and 78 kJ/mol, respectively). However, DMPC's flip-flop rates increase significantly when in a supported membrane, surpassing the exchange rates and no longer limiting the exchange process. Although the presence of holes or cracks in supported membranes explains the occurrence of fast lipid flip-flop in many studies, in defect-free supported membranes we find that fast flip-flop is driven by the surface's induced disorder of the bilayer's acyl chain packing as evidenced from their broad melting temperature behavior.

The Ins and Outs of Lipid Flip-Flop

Our current view of cellular membranes centers on the fluid-mosaic model, which envisions the cellular membrane as a "liquidlike" bilayer of lipids, cholesterol, and proteins that freely diffuse in two dimensions. In stark contrast, the exchange of materials between the leaflets of a bilayer was presumed to be prohibited by the large enthalpic barrier associated with translocating hydrophilic materials, such as a charged lipid headgroup, through the hydrophobic membrane core. This static picture with regard to lipid translocation (or "flip-flop" as it is affectionately known) has been a long-held belief in the study of membrane dynamics. The current accepted membrane model invokes specific protein flippase (inward moving), floppase (outward moving), and scramblase (bidirectional) enzymes that assist in the movement of lipids between the leaflets of cellular membranes. The low rate of protein-free lipid flip-flop has also been a cornerstone of our understanding of the bilateral organization of cellular membrane components, specifically the asymmetric distribution of lipid species found in the luminal and extracellular leaflets of the plasma membrane of eukaryotic cells. Much of the previous work contributing to our current understanding of lipid flip-flop has utilized fluorescent- or spin-labeled lipids. However, there is growing evidence that these lipid probes do not accurately convey the dynamics and thermodynamics of native (unlabeled) lipid motion. This Account summarizes our research efforts directed toward developing a deep physical and chemical understanding of protein-free lipid flip-flop in phospholipid membrane models using sum-frequency vibrational spectroscopy (SFVS). Our use of SFVS enables the direct measurement of native lipid flip-flop in model membranes. In particular, we have explored the kinetic rates and activation thermodynamics of lipid translocation as a means of deciphering the underlying chemical and physical directors governing this process. By means of transition state theory, the contributions from enthalpy and entropy on the activation energy barrier to lipid flip-flop have been explored in detail for a variety of lipid species and membrane compositions. Specifically, the effect of lipid structure and packing and the inclusion of cholesterol and transmembrane peptides on the rates and thermodynamics of lipid translocation have been investigated in detail. It is our hope that these studies will provide a new perspective on lipid translocation in biological membranes and the role of lipid flip-flop in generating and maintaining cell membrane lipid asymmetry.

Sphingolipid Organization in the Plasma Membrane and the Mechanisms That Influence It

Sphingolipids are structural components in the plasma membranes of eukaryotic cells. Their metabolism produces bioactive signaling molecules that modulate fundamental cellular processes. The segregation of sphingolipids into distinct membrane domains is likely essential for cellular function. This review presents the early studies of sphingolipid distribution in the plasma membranes of mammalian cells that shaped the most popular current model of plasma membrane organization. The results of traditional imaging studies of sphingolipid distribution in stimulated and resting cells are described. These data are compared with recent results obtained with advanced imaging techniques, including super-resolution fluorescence detection and high-resolution secondary ion mass spectrometry (SIMS). Emphasis is placed on the new insight into the sphingolipid organization within the plasma membrane that has resulted from the direct imaging of stable isotope-labeled lipids in actual cell membranes with high-resolution SIMS. Super-resolution fluorescence techniques have recently revealed the biophysical behaviors of sphingolipids and the unhindered diffusion of cholesterol analogs in the membranes of living cells are ultimately in contrast to the prevailing hypothetical model of plasma membrane organization. High-resolution SIMS studies also conflicted with the prevailing hypothesis, showing sphingolipids are concentrated in micrometer-scale membrane domains, but cholesterol is evenly distributed within the plasma membrane. Reductions in cellular cholesterol decreased the number of sphingolipid domains in the plasma membrane, whereas disruption of the cytoskeleton eliminated them. In addition, hemagglutinin, a transmembrane protein that is thought to be a putative raft marker, did not cluster within sphingolipid-enriched regions in the plasma membrane. Thus, sphingolipid distribution in the plasma membrane is dependent on the cytoskeleton, but not on favorable interactions with cholesterol or hemagglutinin. The alternate views of plasma membrane organization suggested by these findings are discussed.

Assay of Flippase Activity in Proteoliposomes Using Fluorescent Lipid Derivatives

Specific membrane proteins, termed lipid flippases, play a central role in facilitating the movement of lipids across cellular membranes. In this protocol, we describe the reconstitution of ATP-driven lipid flippases in liposomes and the analysis of their in vitro flippase activity based on the use of fluorescent lipid derivatives. Working with purified and reconstituted systems provides a well-defined experimental setup and allows to directly characterize these membrane proteins at the molecular level.

Structural Origins of Cholesterol Accelerated Lipid Flip-Flop Studied by Sum-Frequency Vibrational Spectroscopy

The unique structure of cholesterol and its role in modulating lipid translocation (flip-flop) were examined using sum-frequency vibrational spectroscopy (SFVS). Two structural analogues of cholesterol--cholestanol and cholestene--were examined to explore the influence of ring rigidity and amphiphilicity on controlling distearoylphosphocholine (DSPC) flip-flop. Kinetic rates for DSPC flip-flop were determined as a function of sterol concentration and temperature. All three sterols increased the rate of DSPC flip-flop in a concentration-dependent manner following the order cholestene > cholestanol > cholesterol. Rates of DSPC flip-flop were used to calculate the thermodynamic activation free energy barrier (ΔG(‡)) in the presence of cholesterol, cholestanol, and cholestene. The acyl chain gauche content of DSPC, mean lipid area, and membrane compressibility were correlated to observed trends in ΔG(‡). ΔG(‡) for DSPC flip-flop showed a strong positive correlation with the molar compression modulus (K*) of the membrane, influenced by the type and concentration of the sterol added. Interestingly, cholesterol is distinctive in maintaining invariant membrane compressibility over the range of 2-10 mol %. The results in this study demonstrate that the compression modulus of a membrane plays a significant role in moderating ΔG(‡) and the kinetics of native, protein-free, lipid translocation in membranes.

Lipid-Targeting Peptide Probes for Extracellular Vesicles

Extracellular vesicles released from cells are under intense investigation for their roles in cell-cell communication and cancer progression. However, individual vesicles have been difficult to probe as their small size renders them invisible by conventional light microscopy. However, as a consequence of their small size these vesicles possess highly curved lipid membranes that offer an unconventional target for curvature-sensing probes. In this article, we present a strategy for using peptide-based biosensors to detect highly curved membranes and the negatively charged membrane lipid phosphatidylserine, we delineate several assays used to validate curvature- and lipid-targeting mechanisms, and we explore potential applications in probing extracellular vesicles released from sources such as apoptotic cells, cancer cells, or activated platelets. J. Cell. Physiol. 231: 2327-2332, 2016. © 2016 Wiley Periodicals, Inc.

Cell-Based Lipid Flippase Assay Employing Fluorescent Lipid Derivatives

P-type ATPases in the P4 subfamily (P4-ATPases) are transmembrane proteins unique for eukaryotes that act as lipid flippases, i.e., to translocate phospholipids from the exofacial to the cytofacial monolayer of cellular membranes. While initially characterized as aminophospholipid translocases, studies of individual P4-ATPase family members from fungi, plants, and animals show that P4-ATPases differ in their substrate specificities and mediate transport of a broader range of lipid substrates. Here, we describe an assay based on fluorescent lipid derivatives to monitor and characterize lipid flippase activities in the plasma membrane of cells, using yeast as an example.

Hemagglutinin clusters in the plasma membrane are not enriched with cholesterol and sphingolipids

The clusters of the influenza envelope protein, hemagglutinin, within the plasma membrane are hypothesized to be enriched with cholesterol and sphingolipids. Here, we directly tested this hypothesis by using high-resolution secondary ion mass spectrometry to image the distributions of antibody-labeled hemagglutinin and isotope-labeled cholesterol and sphingolipids in the plasma membranes of fibroblast cells that stably express hemagglutinin. We found that the hemagglutinin clusters were neither enriched with cholesterol nor colocalized with sphingolipid domains. Thus, hemagglutinin clustering and localization in the plasma membrane is not controlled by cohesive interactions between hemagglutinin and liquid-ordered domains enriched with cholesterol and sphingolipids, or from specific binding interactions between hemagglutinin, cholesterol, and/or the majority of sphingolipid species in the plasma membrane.

Effects of Nanoparticle Morphology and Acyl Chain Length on Spontaneous Lipid Transfer Rates

We report on studies of lipid transfer rates between different morphology nanoparticles and lipids with different length acyl chains. The lipid transfer rate of dimyristoylphosphatidylcholine (di-C14, DMPC) in discoidal "bicelles" (0.156 h(-1)) is 2 orders of magnitude greater than that of DMPC vesicles (ULVs) (1.1 × 10(-3) h(-1)). For both bicellar and ULV morphologies, increasing the acyl chain length by two carbons [going from di-C14 DMPC to di-C16, dipalmitoylphosphatidylcholine (DPPC)] causes lipid transfer rates to decrease by more than 2 orders of magnitude. Results from small angle neutron scattering (SANS), differential scanning calorimetry (DSC), and fluorescence correlation spectroscopy (FCS) are in good agreement. The present studies highlight the importance of lipid dynamic processes taking place in different morphology biomimetic membranes.

Pore-forming toxins: Properties, diversity, and uses as tools to image sphingomyelin and ceramide phosphoethanolamine

Pore-forming toxins (PFTs) represent a unique class of highly specific lipid-binding proteins. The cytotoxicity of these compounds has been overcome through crystallographic structure and mutation studies, facilitating the development of non-toxic lipid probes. As a consequence, non-toxic PFTs have been utilized as highly specific probes to visualize the diversity and dynamics of lipid nanostructures in living and fixed cells. This review is focused on the application of PFTs and their non-toxic analogs as tools to visualize sphingomyelin and ceramide phosphoethanolamine, two major phosphosphingolipids in mammalian and insect cells, respectively. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

Interaction of fluorescent phospholipids with cyclodextrins

Fluorescent analogs of phospholipids are often employed to investigate the structure and dynamics of lipids in membranes. Some of those studies have used cyclodextrins e.g., to modulate the lipid phase. However, the role of the fluorescence moiety of analogs for the interaction between cyclodextrins and fluorescent lipids has not been investigated so far in detail. Therefore, in the present study the interaction of various fluorescent phospholipid analogs with methylated α-, β- and γ- cyclodextrins was investigated. The analogs differed in their structure, in the length of the fatty acyl chain, in the position of the fluorescence group, and in the attached fluorescence moiety (7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) or dipyrrometheneboron difluoride (BODIPY)). In aqueous buffer, cyclodextrins bind fluorescent lipids disturbing the organization of the analogs. When incorporated into lipid vesicles, analogs are selectively extracted from the membrane upon addition of cyclodextrins. The results show that the interaction of cyclodextrins with fluorescent phospholipids depends on the cyclodextrin species, the fluorescence moiety and the phospholipid structure. The presented data should be of interest for studies using fluorescent phospholipids and cyclodextrins, since the interaction between the fluorescence group and the cyclodextrin may interfere with the process(es) under study.

Impact of the β-Lactam Resistance Modifier (-)-Epicatechin Gallate on the Non-Random Distribution of Phospholipids across the Cytoplasmic Membrane of Staphylococcus aureus

The polyphenol (−)-epicatechin gallate (ECg) inserts into the cytoplasmic membrane (CM) of methicillin-resistant Staphylococcus aureus (MRSA) and reversibly abrogates resistance to β-lactam antibiotics. ECg elicits an increase in MRSA cell size and induces thickened cell walls. As ECg partially delocalizes penicillin-binding protein PBP2 from the septal division site, reduces PBP2 and PBP2a complexation and induces CM remodelling, we examined the impact of ECg membrane intercalation on phospholipid distribution across the CM and determined if ECg affects the equatorial, orthogonal mode of division. The major phospholipids of the staphylococcal CM, lysylphosphatidylglycerol (LPG), phosphatidylglycerol (PG), and cardiolipin (CL), were distributed in highly asymmetric fashion; 95%–97% of LPG was associated with the inner leaflet whereas PG (~90%) and CL (~80%) were found predominantly in the outer leaflet. ECg elicited small, significant changes in LPG distribution. Atomic force microscopy established that ECg-exposed cells divided in similar fashion to control bacteria, with a thickened band of encircling peptidoglycan representing the most recent plane of cell division, less distinct ribs indicative of previous sites of orthogonal division and concentric rings and “knobbles” representing stages of peptidoglycan remodelling during the cell cycle. Preservation of staphylococcal membrane lipid asymmetry and mode of division in sequential orthogonal planes appear key features of ECg-induced stress.

How perifosine affects liposome-encapsulated drug delivery across a cell barrier

BACKGROUND

The development of efficient drug delivery systems to transport therapeutics across barrier-forming cells remains a challenge. Recently it was shown that liposomes containing perifosine, a synthetic analog of lysophosphatidylcholine, efficiently deliver liposome encapsulated content across barrier-forming cells.

METHODS

To elucidate the mechanism of the delivery, fluorescent and spin labeled analog of perifosine were synthesized and their transport from liposomes to the barrier-forming MDKC cells was measured.

RESULTS & CONCLUSION

Perifosine analogs are rapidly transported from liposomes into cell membranes. The total amount of perifosine accumulated in plasma membranes seems to be the most important factor in efficient transepithelial transport of liposome-encapsulated substances. Lysolipid-containing liposomal formulations seem to be promising candidates as drug delivery systems in general.

Stretch-activated pore of the antimicrobial peptide, magainin 2

Antimicrobial peptide magainin 2 forms pores in lipid membranes and induces membrane permeation of the cellular contents. Although this permeation is likely the main cause of its bactericidal activity, the mechanism of pore formation remains poorly understood. We therefore investigated in detail the interaction of magainin 2 with lipid membranes using single giant unilamellar vesicles (GUVs). The binding of magainin 2 to the lipid membrane of GUVs increased the fractional change in the area of the membrane, δ, which was proportional to the surface concentration of magainin 2, X. This indicates that the rate constant of the magainin 2-induced two-state transition from the intact state to the pore state greatly increased with an increase in δ. The tension of a lipid membrane following aspiration of a GUV also activated magainin 2-induced pore formation. To reveal the location of magainin 2, the interaction of carboxyfluorescein (CF)-labeled magainin 2 (CF-magainin 2) with single GUVs containing a water-soluble fluorescent probe, AF647, was investigated using confocal microscopy. In the absence of tension due to aspiration, after the interaction of magainin 2 the fluorescence intensity of the GUV rim due to CF-magainin 2 increased rapidly to a steady value, which remained constant for a long time, and at 4-32 s before the start of leakage of AF647 the rim intensity began to increase rapidly to another steady value. In contrast, in the presence of the tension, no increase in rim intensity just before the start of leakage was observed. These results indicate that magainin 2 cannot translocate from the outer to the inner monolayer until just before pore formation. Based on these results, we conclude that a magainin 2-induced pore is a stretch-activated pore and the stretch of the inner monolayer is a main driving force of the pore formation.

Multiple bonds for the lipid interest

Polyene lipids and alkyne lipids allow study of lipid organization, dynamics and metabolism. Both types of lipids contain multiple bonds as the essential functional group, leading to minimal disturbance of the hydrophobic properties on which the characteristic behavior of lipids is based. Polyene lipids can directly be traced due to their intrinsic fluorescence, while alkyne lipids need the copper-catalyzed click reaction to an azido-reporter for detection. This review describes recent developments in synthesis and application of both types of lipid analogs with emphasis on metabolic tracing and microscopy imaging. This article is part of a Special Issue entitled Tools to study lipid functions.

Non-linear microscopy of smooth muscle cells in artificial extracellular matrices made of cellulose

Non-linear microscopy has been used to characterize bovine smooth muscle cells and their proliferation, migration, and differentiation in hydrogel cellulose scaffolds, toward the development of fully functional blood vessel implants. The extracellular matrix (ECM) composed of cellulose and endogenous collagen fibers was imaged using Second Harmonic Generation (SHG) microscopy and the cell morphology by Coherent Anti-Stokes Raman Scattering (CARS) microscopy. Images prove that cells adhere on the cellulose scaffold without additional surface modification and that both contractile and proliferating phenotypes are developed. This work shows that non-linear microscopy contributes with unique insights in cell interactions with (artificial) ECM components and has the potential to become an established characterization method in tissue engineering.

Fluorinated colloidal gold immunolabels for imaging select proteins in parallel with lipids using high-resolution secondary ion mass spectrometry

The local abundance of specific lipid species near a membrane protein is hypothesized to influence the protein's activity. The ability to simultaneously image the distributions of specific protein and lipid species in the cell membrane would facilitate testing these hypotheses. Recent advances in imaging the distribution of cell membrane lipids with mass spectrometry have created the desire for membrane protein probes that can be simultaneously imaged with isotope labeled lipids. Such probes would enable conclusive tests to determine whether specific proteins colocalize with particular lipid species. Here, we describe the development of fluorine-functionalized colloidal gold immunolabels that facilitate the detection and imaging of specific proteins in parallel with lipids in the plasma membrane using high-resolution SIMS performed with a NanoSIMS. First, we developed a method to functionalize colloidal gold nanoparticles with a partially fluorinated mixed monolayer that permitted NanoSIMS detection and rendered the functionalized nanoparticles dispersible in aqueous buffer. Then, to allow for selective protein labeling, we attached the fluorinated colloidal gold nanoparticles to the nonbinding portion of antibodies. By combining these functionalized immunolabels with metabolic incorporation of stable isotopes, we demonstrate that influenza hemagglutinin and cellular lipids can be imaged in parallel using NanoSIMS. These labels enable a general approach to simultaneously imaging specific proteins and lipids with high sensitivity and lateral resolution, which may be used to evaluate predictions of protein colocalization with specific lipid species.

Reconstitution of glucosylceramide flip-flop across endoplasmic reticulum: implications for mechanism of glycosphingolipid biosynthesis

Most glycosphingolipids are synthesized by the sequential addition of monosaccharides to glucosylceramide (GlcCer) in the lumen of the Golgi apparatus. Because GlcCer is synthesized on the cytoplasmic face of Golgi membranes, it must be flipped to the non-cytoplasmic face by a lipid flippase in order to nucleate glycosphingolipid synthesis. Halter et al. (Halter, D., Neumann, S., van Dijk, S. M., Wolthoorn, J., de Mazière, A. M., Vieira, O. V., Mattjus, P., Klumperman, J., van Meer, G., and Sprong, H. (2007) Pre- and post-Golgi translocation of glucosylceramide in glycosphingolipid synthesis. J. Cell Biol. 179, 101-115) proposed that this essential flipping step is accomplished via a complex trafficking itinerary; GlcCer is moved from the cytoplasmic face of the Golgi to the endoplasmic reticulum (ER) by FAPP2, a cytoplasmic lipid transfer protein, flipped across the ER membrane, then delivered to the lumen of the Golgi complex by vesicular transport. We now report biochemical reconstitution studies to analyze GlcCer flipping at the ER. Using proteoliposomes reconstituted from Triton X-100-solubilized rat liver ER membrane proteins, we demonstrate rapid (t(½) < 20 s), ATP-independent flip-flop of N-(6-((7-nitro-2-1,3-benzoxadiazol-4-yl)amino)hexanoyl)-D-glucosyl-β1-1'-sphingosine, a fluorescent GlcCer analog. Further studies involving protein modification, biochemical fractionation, and analyses of flip-flop in proteoliposomes reconstituted with ER membrane proteins from yeast indicate that GlcCer translocation is facilitated by well characterized ER phospholipid flippases that remain to be identified at the molecular level. By reason of their abundance and membrane bending activity, we considered that the ER reticulons and the related Yop1 protein could function as phospholipid-GlcCer flippases. Direct tests showed that these proteins have no flippase activity.

Evaluation of fluorescent phosphatidylserine substrates for the aminophospholipid flippase in mammalian cells

A series of fluorescent phosphatidylserine and phosphatidylcholine derivatives were prepared and evaluated by cell microscopy for ability to translocate across mammalian plasma membranes via the putative aminophospholipid flippase. Phosphatidylserine derivatives, with either a neutral 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) or a coumarin fluorophore appended to the 2-acyl chain, entered the cytosol of all three cell lines tested and control experiments showed that the translocation was due to flippase activity. In contrast, a phosphatidylserine conjugate containing a charged and polar carboxyfluorescein was not translocated and remained in the cell plasma membrane. The phosphatidylserine-coumarin derivative exhibits bright fluorescence and higher photostability than the NBD analogues, and thus is a promising new fluorescent probe for extended-imaging studies of flippase action in living cells using laser confocal microscopes.

Flipping and flopping--lipids on the move

The rapid movement of polar lipids from one membrane leaflet to the other is facilitated by lipid flippases or translocases. Although their activity was first observed over 30 years ago, the structures, physiological roles, and molecular mechanisms of this group of proteins remain enigmatic. Lipid flippases maintain membrane lipid asymmetry, and in eukaryotes they are also intimately involved in membrane budding and vesicle trafficking. The ATP-dependent flippases are members of well-characterized protein families, whose other members transport nonlipid substrates across cell membranes. The P(4)-type ATPases carry out the inward translocation of phospholipids, and various ABC transporters are involved in outward lipid movement. The ATP-independent flippases move lipid substrates in both directions between membrane leaflets. With only a few exceptions, the molecular identity of these proteins is still unknown, despite their involvement in key biosynthetic pathways in both bacteria and eukaryotes. This review provides an overview of the different classes of flippases, and summarizes recent progress in their identification and functional characterization. The possible mechanisms of action of lipid flippases are discussed, and future directions explored.

Fluorescence spectroscopy and molecular dynamics simulations in studies on the mechanism of membrane destabilization by antimicrobial peptides

Since their initial discovery, 30 years ago, antimicrobial peptides (AMPs) have been intensely investigated as a possible solution to the increasing problem of drug-resistant bacteria. The interaction of antimicrobial peptides with the cellular membrane of bacteria is the key step of their mechanism of action. Fluorescence spectroscopy can provide several structural details on peptide-membrane systems, such as partition free energy, aggregation state, peptide position and orientation in the bilayer, and the effects of the peptides on the membrane order. However, these "low-resolution" structural data are hardly sufficient to define the structural requirements for the pore formation process. Molecular dynamics simulations, on the other hand, provide atomic-level information on the structure and dynamics of the peptide-membrane system, but they need to be validated experimentally. In this review we summarize the information that can be obtained by both approaches, highlighting their versatility and complementarity, suggesting that their synergistic application could lead to a new level of insight into the mechanism of membrane destabilization by AMPs.

Probes for studying cholesterol binding and cell biology

Cholesterol is a multifunctional lipid in eukaryotic cells. It regulates the physical state of the phospholipid bilayer, is crucially involved in the formation of membrane microdomains, affects the activity of many membrane proteins, and is the precursor for steroid hormones and bile acids. Thus, cholesterol plays a profound role in the physiology and pathophysiology of eukaryotic cells. The cholesterol molecule has achieved evolutionary perfection to fulfill its different functions in membrane organization. Here, we review basic approaches to explore the interaction of cholesterol with proteins, with a particular focus on the high diversity of fluorescent and photoreactive cholesterol probes available today.

Free energy and entropy of activation for phospholipid flip-flop in planar supported lipid bilayers

Basic transition state theory is used to describe the activation thermodynamics for phospholipid flip-flop in planar-supported lipid bilayers (PSLBs) prepared by the Langmuir-Blodgett/Langmuir-Schaeffer method. The kinetics of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) flip-flop were determined as a function of temperature and lateral surface pressure using sum-frequency vibrational spectroscopy (SFVS). From the temperature and lateral pressure dependent DSPC flip-flop kinetics, a complete description of the activation thermodynamics for flip-flop in the gel state, including free energy of activation (DeltaG(++)), area of activation (Deltaa(++)), and entropy of activation (DeltaS(++)), was obtained. The free energy barrier for flip-flop of DSPC was determined to be DeltaG(++) = 105 +/- 2 kJ/mol at 40 degrees C at a deposition surface pressure of 30 mN/m. The free energy barrier was found to consist of large opposing entropic and enthalpic contributions. The influence of alkyl chain length on the activation thermodynamics of flip-flop was also investigated. Decreasing the alkyl chain length led to a decrease in DeltaG(++) due primarily to an increase in DeltaS(++). The values obtained here are compared to previous studies investigating flip-flop by vesicle based methods.

Lipid transfer mediated by a recombinant pro-sterol carrier protein 2 for the accurate preparation of asymmetrical membrane vesicles requires a narrow vesicle size distribution: a free-flow electrophoresis study

We applied protein-mediated lipid transfer using recombinant His-tagged pro-sterol carrier protein 2 (pro-SCP2) to prepare asymmetrical membrane vesicles (AMV) featuring an unequal transmembrane distribution of the negative phospholipid egg-phosphatidylglycerol (EPG). Pure egg-phosphatidylcholine (EPC) vesicles were used as the acceptor and EPC:EPG 90:10 mol % vesicles as the donor populations. The changes in surface charge during EPG transfer were used to quantify the degree of asymmetry by free-flow electrophoresis (FFE). The relative deflection in FFE correlated with EPG content in the outer monolayer (x(EPG)). The initial transfer rates and first order rate constants for the transfer process were determined. The addition of pro-SCP2 at a molar protein-to-lipid ratio R(P/L) of (15-20) x 10(-5) accelerated the EPG transfer to half-times of between 2 and 3 h. Thus, the transmembrane redistribution of EPG by flip-flop, which reduces the degree of asymmetry and occurs at half-times of tens of hours, was minimized during the transfer process. We investigated the influence of membrane curvature on the transfer rate using 50 and 100 nm vesicles with very low size distribution widths (RSD of 13-17%). Transfer occurred with a 55.7% higher initial rate between the smaller vesicles. The use of equally sized acceptor and donor populations of such narrow size distributions was shown to be important for the preparation of AMV with a uniform degree of asymmetry. Under these conditions, AMV were obtained after less than 3 h by preparative FFE separation. In the case of the acceptor vesicles, EPG transfer increased x(EPG) to 3 mol %, whereas it was reduced to 6 mol % in the donor vesicles.

Cholesterol-protein interaction: methods and cholesterol reporter molecules

Cholesterol is a major constituent of the plasma membrane in eukaryotic cells. It regulates the physical state of the phospholipid bilayer and is crucially involved in the formation of membrane microdomains. Cholesterol also affects the activity of several membrane proteins, and is the precursor for steroid hormones and bile acids. Here, methods are described that are used to explore the binding and/or interaction of proteins to cholesterol. For this purpose, a variety of cholesterol probes bearing radio-, spin-, photoaffinity- or fluorescent labels are currently available. Examples of proven cholesterol binding molecules are polyene compounds, cholesterol-dependent cytolysins, enzymes accepting cholesterol as substrate, and proteins with cholesterol binding motifs. Main topics of this report are the localization of candidate membrane proteins in cholesterol-rich microdomains, the issue of specificity of cholesterol- protein interactions, and applications of the various cholesterol probes for these studies.

Biophysical characterization of a new phospholipid analogue with a spin-labeled unsaturated fatty acyl chain

Spin-labeled analogs of phospholipids have been used widely to characterize the biophysical properties of membranes. We describe synthesis and application of a new spin-labeled phospholipid analog, SL-POPC. The advantage of this molecule is that the EPR active doxyl group is linked to an unsaturated fatty acyl chain different to saturated phospholipid analogs used so far. The need for those analogs arises from the fact that biological membranes contain unsaturated phospholipids to a large extent. The biophysical properties of SL-POPC in membranes were characterized using EPR and NMR spectroscopy and compared with those of the saturated spin-labeled phospholipid, SL-PSPC. To this end, POPC membranes were labeled with either analog to assess whether the spin-labeled counterpart SL-POPC mimics the membrane properties better than the often used SL-PSPC. The results show that SL-POPC and SL-PSPC explore different molecular environments of the bilayer, and that the type and degree of perturbation of bilayer caused by the label moiety also differs between both analogs. We found that SL-POPC is more appropriate to assess the versatile dynamics of POPC membranes than SL-PSPC.

Detection of lipid domains in model and cell membranes by fluorescence lifetime imaging microscopy of fluorescent lipid analogues

The presence of lipid domains in cellular membranes and their characteristic features are still an issue of dividing discussion. Several recent studies implicate lipid domains in plasma membranes of mammalian cells as short lived and in the submicron range. Measuring the fluorescence lifetime of appropriate lipid analogues is a proper approach to detect domains with such properties. Here, the sensitivity of the fluorescence lifetime of1-palmitoyl-2-[6-[(7-nitro-2-1,3-benzoxadiazol-4-yl)amino]-hexanoyl]-sn-glycero-3-phospholipid (C6-NBD-phospholipid) analogues has been employed to characterize lipid domains in giant unilamellar vesicles (GUVs) and the plasma membrane of mammalian cells by fluorescence lifetime imaging (FLIM). Fluorescence decay of C6-NBD-phosphatidylcholine is characterized by a short and long lifetime. For GUVs forming microscopically visible lipid domains the longer lifetime in the liquid disordered (ld) and the liquid ordered (lo) phase was clearly distinct, being approximately 7 ns and 11 ns, respectively. Lifetimes were not sensitive to variation of cholesterol concentration of domain-forming GUVs indicating that the lipid composition and physical properties of those lipid domains are well defined entities. Even the existence of submicroscopic domains can be detected by FLIM as demonstrated for GUVs of palmitoyloleoyl phosphatidylcholine/N-palmitoyl-d-sphingomyelin/cholesterol mixtures. A broad distribution of the long lifetime was found for C6-NBD-phosphatidylcholine inserted in the plasma membrane of HepG2 and HeLa cells centered around 11 ns. FLIM studies on lipid domains forming giant vesicles derived from the plasma membrane of HeLa cells may suggest that a variety of submicroscopic lipid domains exists in the plasma membrane of intact cells.