Lipid microdomains in cell surface membranes

Role of membrane organization and membrane domains in endocytic lipid trafficking

Lipid compositions vary greatly among organelles, and specific sorting mechanisms are required to establish and maintain these distinct compositions. In this review, we discuss how the biophysical properties of the membrane bilayer and the chemistry of individual lipid molecules play a role in the intracellular trafficking of the lipids themselves, as well as influencing the trafficking of transmembrane proteins. The large diversity of lipid head groups and acyl chains lead to a variety of weak interactions, such as ionic and hydrogen bonding at the lipid/water interfacial region, hydrophobic interactions, and van-der-Waals interactions based on packing density. In simple model bilayers, these weak interactions can lead to large-scale phase separations, but in more complex mixtures, which mimic cell membranes, such phase separations are not observed. Nevertheless, there is growing evidence that domains (i.e., localized regions with non-random lipid compositions) exist in biological membranes, and it is likely that the formation of these domains are based on interactions similar to those that lead to phase separations in model systems. Sorting of lipids appears to be based in part on the inclusion or exclusion of certain types of lipids in vesicles or tubules as they bud from membrane organelles.

Detection of peptide-lipid interactions in mixed monolayers, using isotherms, atomic force microscopy, and fourier transform infrared analyses

To improve the understanding of the membrane uptake of an amphipathic and positively charged vector peptide, we studied the interactions of this peptide with different phospholipids, the nature of whose polar headgroups and physical states were varied. Three lipids were considered: dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylglycerol (DPPG), and dioleoylphosphatidylglycerol (DOPG). The approach was carried out by three complementary methods: compression isotherms of monolayers and atomic force microscopy observations associated with Fourier transform infrared investigations. From analysis of the compression isotherms, it was concluded that the peptide interacts with all lipids and with an expansion of the mean molecular area, implying that both components form nonideal mixtures. The expansion was larger in the case of DOPG than for DPPC and DPPG because of an alpha to beta conformational transition with an increase in the peptide molar fraction. Atomic force microscopy observations showed that the presence of small amounts of peptide led to the appearance of bowl-like particles and that an increase in the peptide amounts generated the formation of filaments. In the case of DOPG, filaments were found at higher peptide molar fractions than already observed for DOPC because of the presence of negatively charged lipid headgroups.

Kinetics of amphiphile association with two-phase lipid bilayer vesicles

We examined the consequences of membrane heterogeneity for the association of a simple amphiphilic molecule with phospholipid vesicles with solid-liquid and liquid-liquid phase coexistence. To address this problem we studied the association of a single-chain, fluorescent amphiphile with dimyristoylphosphatidylcholine (DMPC) vesicles containing varying amounts of cholesterol. DMPC bilayers containing 15 mol% cholesterol show a region of solid-liquid-ordered (s-l(o)) coexistence below the T(m) of pure DMPC (23.9 degrees C) and a region of liquid-disordered-liquid-ordered coexistence (l(d)-l(o)) above the T(m). We first examined equilibrium binding and kinetics of amphiphile insertion into single-phase vesicles (s, l(d), and l(o) phase). The data obtained were then used to predict the behavior of the equivalent process in a two-phase system, taking into account the fractions of phases present. Next, the predicted kinetics were compared to experimental kinetics obtained from a two-phase system. We found that association of the amphiphile with lipid vesicles is not influenced by the existence of l(d)-l(o) phase boundaries but occurs much more slowly in the s-l(o) phase coexistence region than expected on the basis of phase composition.

A model for membrane patchiness: lateral diffusion in the presence of barriers and vesicle traffic

Patches (lateral heterogeneities) of cell surface membrane proteins and lipids have been imaged by a number of different microscopy techniques. This patchiness has been taken as evidence for the organization of membranes into domains whose composition differs from the average for the entire membrane. However, the mechanism and specificity of patch formation are not understood. Here we show how vesicle traffic to and from a cell surface membrane can create patches of molecules of the size observed experimentally. Our computer model takes into account lateral diffusion, barriers to lateral diffusion, and vesicle traffic to and from the plasma membrane. Neither barriers nor vesicle traffic alone create and maintain patches. Only the combination of these produces a dynamic but persistent patchiness of membrane proteins and lipids.

Plasma membrane microdomains act as concentration platforms to facilitate intoxication by aerolysin

It has been proposed that the plasma membrane of many cell types contains cholesterol-sphingolipid-rich microdomains. Here, we analyze the role of these microdomains in promoting oligomerization of the bacterial pore-forming toxin aerolysin. Aerolysin binds to cells, via glycosyl phosphatidylinositol-anchored receptors, as a hydrophilic soluble protein that must polymerize into an amphipathic ring-like complex to form a pore. We first show that oligomerization can occur at >10(5)-fold lower toxin concentration at the surface of living cells than in solution. Our observations indicate that it is not merely the number of receptors on the target cell that is important for toxin sensitivity, but their ability to associate transiently with detergent resistant microdomains. Oligomerization appears to be promoted by the fact that the toxin bound to its glycosyl phosphatidylinositol-anchored receptors, can be recruited into these microdomains, which act as concentration devices.

The structure, biosynthesis and functions of glycosylphosphatidylinositol anchors, and the contributions of trypanosome research

The discovery of glycosylphosphatidylinositol (GPI) membrane anchors has had a significant impact on several areas of eukaryote cell biology. Studies of the African trypanosome, which expresses a dense surface coat of GPI-anchored variant surface glycoprotein, have played important roles in establishing the general structure of GPI membrane anchors and in delineating the pathway of GPI biosynthesis. The major cell-surface molecules of related parasites are also rich in GPI-anchored glycoproteins and/or GPI-related glycophospholipids, and differences in substrate specificity between enzymes of trypanosomal and mammalian GPI biosynthesis may have potential for the development of anti-parasite therapies. Apart from providing stable membrane anchorage, GPI anchors have been implicated in the sequestration of GPI-anchored proteins into specialised membrane microdomains, known as lipid rafts, and in signal transduction events.

The electrostatics of lipid surfaces

Charged lipids constitute a substantial fraction of all membrane lipids. Their charges vary in quantity and distribution within their headgroup regions. In long range interactions, their charges' value and electrostatic potential in the vicinity of the membrane surface can be approximated by the Guy-Chapman theory. This theory treats the interface as a charged structureless plain surrounded by uniform environments. However, if one considers intermolecular interactions, such assumptions need to be revised. The interface is in reality a thick region containing the residual charges of lipid headgroups. Their arrangement depends on the type of lipid present in the membrane. The variety of lipids and their biological functions suggests that charge distribution determines the extent and type of interaction with surface associated molecules. Numerous examples show that protein behavior at the lipid bilayer surface is determined by the type of lipid present, indicating protein specificity towards certain surface locations and local properties (determined by lipid composition) of a particular type. Such specificity is achieved by a combination of electrostatic, hydrophobic and enthropic effects. Comparing lipid biological activity, it can be stated that residual charge distribution is one of the factors of intermolecular recognition leading to the specific interaction of lipid molecules and selected proteins in various processes, particularly those involved with signal transduction pathways. Such specificity enables a variety of processes occurring simultaneously on the same membrane surface to function without cross-reaction interference.

Characterization of lipid bilayer phases by confocal microscopy and fluorescence correlation spectroscopy

We report the application of confocal imaging and fluorescence correlation spectroscopy (FCS) to characterize chemically well-defined lipid bilayer models for biomembranes. Giant unilamellar vesicles of dilauroyl phosphatidylcholine/dipalmitoyl phosphatidylcholine (DLPC/DPPC)/cholesterol were imaged by confocal fluorescence microscopy with two fluorescent probes, 1, 1'-dieicosanyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI-C(20)) and 2-(4,4-difluoro-5,7-dimethyl-4-bora-3a, 4a-diaza-s-indacene-3-pentanoyl)-1-hexadecanoyl-sn-glycero-3 -phosphoc holine (Bodipy-PC). Phase separation was visualized by differential probe partition into the coexisting phases. Three-dimensional image reconstructions of confocal z-scans through giant unilamellar vesicles reveal the anisotropic morphology of coexisting phase domains on the surface of these vesicles with full two-dimensional resolution. This method demonstrates by direct visualization the exact superposition of like phase domains in apposing monolayers, thus answering a long-standing open question. Cholesterol was found to induce a marked change in the phase boundary shapes of the coexisting phase domains. To further characterize the phases, the translational diffusion coefficient, D(T), of the DiI-C(20) was measured by FCS. D(T) values at approximately 25 degrees C ranged from approximately 3 x 10(-8) cm(2)/s in the fluid phase, to approximately 2 x 10(-9) cm(2)/s in high-cholesterol-content phases, to approximately 2 x 10(-10) cm(2)/s in the spatially ordered phases that coexist with fluid phases. In favorable cases, FCS could distinguish two different values of D(T) in a region of two-phase coexistence on a single vesicle.

Fluorescence correlation spectroscopy with single-molecule sensitivity on cell and model membranes

We report on the successful application of fluorescence correlation spectroscopy (FCS) to the analysis of single fluorescently labeled lipid analogue molecules diffusing laterally in lipid bilayers, as exemplified by time traces of fluorescence bursts of individual molecules entering and leaving the excitation area. FCS measurements performed on lipid probes in rat basophilic leukemia cell membranes showed deviations from two-dimensional Brownian motion with a single uniform diffusion constant. Giant unilamellar vesicles were employed as model systems to characterize diffusion of fluorescent lipid analogues in both homogeneous and mixed lipid phases with diffusion heterogeneity. Comparing the results of cell membrane diffusion with the findings on the model systems suggests possible explanations for the observations: (a) anomalous subdiffusion in which evanescent attractive interactions with disparate mobile molecules modifies the diffusion statistics; (b) alternatively, probe molecules are localized in microdomains of submicroscopic size, possibly in heterogeneous membrane phases.

Anomalous diffusion of major histocompatibility complex class I molecules on HeLa cells determined by single particle tracking

Single-particle tracking (SPT) was used to determine the mobility characteristics of MHC (major histocompatibility complex) class I molecules at the surface of HeLa cells at 22 degrees C and on different time scales. MHC class I was labeled using the Fab fragment of a monoclonal antibody (W6/32), covalently bound to either R-phycoerythrin or fluorescent microspheres, and the particles were tracked using high-sensitivity fluorescence imaging. Analysis of the data for a fixed time interval suggests a reasonable fit to a random diffusion model. The best fit values of the diffusion coefficient D decreased markedly, however, with increasing time interval, demonstrating the existence of anomalous diffusion. Further analysis of the data shows that the diffusion is anomalous over the complete time range investigated, 4-300 s. Fitting the results obtained with the R-phycoerythrin probe to D = D0talpha-1, where Do is a constant and t is the time, gave D0 = (6.7 +/- 4.5) x 10(-11) cm2 s-1 and alpha = 0.49 +/- 0.16. Experiments with fluorescent microspheres were less reproducible and gave slower anomalous diffusion. The R-phycoerythrin probe is considered more reliable for fluorescent SPT because it is small (11 x 8 nm) and monovalent. The type of motion exhibited by the class I molecules will greatly affect their ability to migrate in the plane of the membrane. Anomalous diffusion, in particular, greatly reduces the distance a class I molecule can travel on the time scale of minutes. The present data are discussed in relation to the possible role of diffusion and clustering in T-cell activation.

Activation-dependent clustering of the erbB2 receptor tyrosine kinase detected by scanning near-field optical microscopy

ErbB2 (HER2, Neu), a member of the epidermal growth factor (EGF) receptor tyrosine kinase family, is often overexpressed in breast cancer and other malignancies. ErbB2 homodimerizes but also presents as a common auxiliary subunit of the EGF and heregulin receptors (erbB1 or EGFR; and erbB3-4, respectively), with which it heteroassociates. ErbB2 is generally regarded as an orphan (ligand-less) receptor with a very potent kinase domain activated either via its associated partners or constitutively as a consequence of discrete mutations. It follows that the extent and regulation of its cell surface interactions are of central importance. We have studied the large-scale association pattern of erbB2 in quiescent and activated cells labeled with fluorescent anti-erbB2 monoclonal antibodies using scanning near-field optical microscopy (SNOM). ErbB2 was found to be concentrated in irregular membrane patches with a mean diameter of approx. 0.5 microm in nonactivated SKBR3 and MDA453 human breast tumor cells. The average number of erbB2 proteins in a single cluster on nonactivated SKBR3 cells was about 10(3). Activation of SKBR3 cells with EGF, heregulin as well as a partially agonistic anti-erbB2 monoclonal antibody led to an increase in the mean cluster diameter to 0.6-0.9 microm, irrespective of the ligand. The EGF-induced increase in the erbB2 cluster size was inhibited by the EGFR-specific tyrosine kinase inhibitor PD153035. The average size of erbB2 clusters on the erbB2-transfected line of CHO cells (CB2) was similar to that of activated SKBR3 cells, a finding correlated with the increased base-line tyrosine phosphorylation of erbB2 in cells expressing only erbB2. We conclude that an increase in cluster size may constitute a general phenomenon in the activation of erbB2.

Critical role for cholesterol in Lyn-mediated tyrosine phosphorylation of FcepsilonRI and their association with detergent-resistant membranes

Tyrosine phosphorylation of the high affinity immunoglobulin (Ig)E receptor (FcepsilonRI) by the Src family kinase Lyn is the first known biochemical step that occurs during activation of mast cells and basophils after cross-linking of FcepsilonRI by antigen. The hypothesis that specialized regions in the plasma membrane, enriched in sphingolipids and cholesterol, facilitate the coupling of Lyn and FcepsilonRI was tested by investigating functional and structural effects of cholesterol depletion on Lyn/FcepsilonRI interactions. We find that cholesterol depletion with methyl-beta-cyclodextrin substantially reduces stimulated tyrosine phosphorylation of FcepsilonRI and other proteins while enhancing more downstream events that lead to stimulated exocytosis. In parallel to its inhibition of tyrosine phosphorylation, cholesterol depletion disrupts the interactions of aggregated FcepsilonRI and Lyn on intact cells and also disrupts those interactions with detergent-resistant membranes that are isolated by sucrose gradient ultracentrifugation of lysed cells. Importantly, cholesterol repletion restores receptor phosphorylation together with the structural interactions. These results provide strong evidence that membrane structure, maintained by cholesterol, plays a critical role in the initiation of FcepsilonRI signaling.

Lipid composition and the lateral pressure profile in bilayers

The mechanisms by which variations in the lipid composition of cell membranes influence the function of membrane proteins are not yet well understood. In recent work, a nonlocal thermodynamic mechanism was suggested in which changes in lipid composition cause a redistribution of lateral pressures that in turn modulates protein conformational (or aggregation) equilibria. In the present study, results of statistical thermodynamic calculations of the equilibrium pressure profile and bilayer thickness are reported for a range of lipids and lipid mixtures. Large redistributions of lateral pressure are predicted to accompany variation in chain length, degree and position of chain unsaturation, head group repulsion, and incorporation of cholesterol and interfacially active solutes. Combinations of compositional changes are found that compensate with respect to bilayer thickness, thus eliminating effects of hydrophobic mismatch, while still effecting significant shifts of the pressure profile. It is also predicted that the effect on the pressure profile of addition of short alkanols can be reproduced with certain unnatural lipids. These results suggest possible roles of cholesterol, highly unsaturated fatty acids and small solutes in modulating membrane protein function and suggest unambiguous experimental tests of the pressure profile hypothesis. As a test of the methodology, calculated molecular areas and area elastic moduli are compared with experimental and simulation results.

Docosahexaenoic acid (DHA) alters the phospholipid molecular species composition of membranous vesicles exfoliated from the surface of a murine leukemia cell line

Previously, we presented evidence that the vesicles routinely exfoliated from the surface of T27A tumor cells arise from vesicle-forming regions of the plasma membrane and possess a set of lateral microdomains distinct from those of the plasma membrane as a whole. We also showed that docosahexaenoic acid (DHA, or 22:6n-3), a fatty acyl chain known to alter microdomain structure in model membranes, also alters the structure and composition of exfoliated vesicles, implying a DHA-induced change in microdomain structure on the cell surface. In this report we show that enrichment of the cells with DHA reverses some of the characteristic differences in composition between the parent plasma membrane and shed microdomain vesicles, but does not alter their phospholipid class composition. In untreated cells, DHA-containing species were found to be a much greater proportion of the total phosphatidylethanolamine (PE) pool than the total phosphatidylcholine (PC) pool in both the plasma membrane and the shed vesicles. After DHA treatment, the proportion of DHA-containing species in the PE and PC pools of the plasma membrane were elevated, and unlike in untreated cells, their proportions were equal in the two pools. In the vesicles shed from DHA-loaded cells, the proportion of DHA-containing species of PE was the same as in the plasma membrane. However, the proportion of DHA-containing species of PC in the vesicles (0.089) was much lower than that found in the plasma membrane (0.194), and was relatively devoid of species with 16-carbon acyl components. These data suggested that DHA-containing species of PC, particularly those having a 16-carbon chain in the sn-1 position, were preferentially retained in the plasma membrane. The data can be interpreted as indicating that DHA induces a restructuring of lateral microdomains on the surface of living cells similar to that predicted by its behavior in model membranes.

Looking at lipid rafts?

The notion that microdomains enriched in certain specialized lipids exist in membranes has been both attractive and controversial since it was first proposed that such domains, termed rafts, might act as apical sorting devices in epithelial cells. The observation that certain lipids are not extractable in cold nonionic detergent supports the raft concept, but the nature of the in vivo correlate of such detergent-resistant membranes remains enigmatic. In principle, microscopy should be able to determine whether the postulated rafts exist. This article focuses on recent microscopy experiments addressing this question. Several, but not all, results support the raft concept, but further definition of the structure, dynamics and function of lipid domains in various biological contexts is urgently required.

Membrane organization in immunoglobulin E receptor signaling

The structure and dynamics of the plasma membrane are proposed to be critical for the initial steps of signal transduction by the high-affinity immunoglobulin E receptor. Recent experimental advances indicate that interactions between the high-affinity immunoglobulin E receptor and the tyrosine kinase Lyn with cholesterol- and sphingolipid-rich regions within the plasma membrane are important for receptor function. This accumulating evidence points to spatio-temporal control of immunoglobulin E receptor signaling by the organization of the plasma membrane; an attractive hypothesis is that ligand-dependent receptor aggregation causes the segregation of Lyn-containing ordered regions of the plasma membrane from disordered regions.

Lipid diffusion in the plasma membrane of ram and boar spermatozoa during maturation in the epididymis measured by fluorescence recovery after photobleaching

Maturation of spermatozoa in the epididymis involves remodelling of many protein and lipid components of the plasma membrane. In this investigation we have examined whether (a) diffusion of lipid molecules in the surface membrane changes during epididymal maturation; (b) diffusion is spatially restricted; and (c) differences in lipid diffusion can be related to known changes in membrane composition. For this purpose we have used the technique of fluorescence recovery after photobleaching (FRAP) to measure diffusion of the lipid reporter probe ODAF (5-(octa-decanoyl)aminofluorescein) in spermatozoa from two species: ram, where substantial changes in membrane lipids occur during passage through the epididymis, and boar, where there are relatively few changes. Results on ram spermatozoa show that between the testis and cauda epididymidis, diffusion coefficients values (D) for ODAF increase significantly in all the surface domains. Percentage recovery values (%R) remain constant irrespective of maturational status. In boar spermatozoa, however, D and %R values do not change significantly between epididymal regions. Cholesterol, which has widespread effects on the behaviour of lipid molecules in cell membranes, was visualized by binding of filipin. In both species filipin was concentrated over the acrosomal domain and cytoplasmic droplet of testicular spermatozoa, but in the epididymis it had a heterogenous distribution over the whole head and tail. These results are discussed in relation to the establishment and maintenance of lipid domains in spermatozoa and their influence on development of fertilizing capacity.

Kinetic analysis of secretory protein traffic and characterization of golgi to plasma membrane transport intermediates in living cells

Quantitative time-lapse imaging data of single cells expressing the transmembrane protein, vesicular stomatitis virus ts045 G protein fused to green fluorescent protein (VSVG-GFP), were used for kinetic modeling of protein traffic through the various compartments of the secretory pathway. A series of first order rate laws was sufficient to accurately describe VSVG-GFP transport, and provided compartment residence times and rate constants for transport into and out of the Golgi complex and delivery to the plasma membrane. For ER to Golgi transport the mean rate constant (i.e., the fraction of VSVG-GFP moved per unit of time) was 2.8% per min, for Golgi to plasma membrane transport it was 3.0% per min, and for transport from the plasma membrane to a degradative site it was 0.25% per min. Because these rate constants did not change as the concentration of VSVG-GFP in different compartments went from high (early in the experiment) to low (late in the experiment), secretory transport machinery was never saturated during the experiments. The processes of budding, translocation, and fusion of post-Golgi transport intermediates carrying VSVG- GFP to the plasma membrane were also analyzed using quantitative imaging techniques. Large pleiomorphic tubular structures, rather than small vesicles, were found to be the primary vehicles for Golgi to plasma membrane transport of VSVG-GFP. These structures budded as entire domains from the Golgi complex and underwent dynamic shape changes as they moved along microtubule tracks to the cell periphery. They carried up to 10,000 VSVG-GFP molecules and had a mean life time in COS cells of 3.8 min. In addition, they fused with the plasma membrane without intersecting other membrane transport pathways in the cell. These properties suggest that the post-Golgi intermediates represent a unique transport organelle for conveying large quantities of protein cargo from the Golgi complex directly to the plasma membrane.

Regionalized lipid diffusion in the plasma membrane of mammalian spermatozoa

The plasma membrane of mammalian spermatozoa shows pronounced lateral asymmetry with many glycoproteins restricted to specific domains. Some of these antigens are freely diffusing throughout the membrane whereas others appear static in position. It is not clear whether these concepts also apply to membrane lipids. In this investigation we have used fluorescence recovery after photobleaching (FRAP) techniques to spatially resolve lipid dynamics in various surface domains of 5 species of mammalian spermatozoa (bull, boar, ram, mouse, and guinea pig). Sperm plasma membranes were loaded with 5-(N-octadecanoyl)aminofluorescein (ODAF) reporter probe, and its diffusion was measured in various domains by FRAP analysis. Results showed that in live bull, boar, ram, and mouse spermatozoa, diffusion coefficients (D) were significantly higher over the acrosome and postacrosome than on the midpiece and principal piece of the tail. In dead or permeabilized cells, on the other hand, large immobile phases developed, particularly on the sperm tail, that severely reduced D values. ODAF diffusion was also sensitive to temperature and cross-linking of protein components within the membrane with paraformaldehyde. Guinea pig spermatozoa were different in almost all respects from those of the other species tested. It is concluded that lipid diffusion in the plasma membrane of live spermatozoa varies significantly between surface domains, because of either compositional heterogeneity, or differences in bilayer disposition, or the presence of intramembranous barriers that impede free exchange between domains. This study emphasizes the important role of membrane lipids in regulating polarized migration of sperm surface antigens during developmental processes such as maturation and capacitation.

Minimal radius of curvature of lipid bilayers in the gel phase state corresponds to the dimension of biomembrane structures "caveolae"

Caveolae are membrane invaginations with a radius of curvature in the range of 40 nm for the bulb; 10-15 nm is the minimal radius for lipid bilayers in the liquid-crystalline Lalpha (liquid-disordered: ld) phase state. A minimal radius of 20-30 nm could be detected for the gel phase state by analysis of convex-concave bilayer deformations. Circular protrusions with a diameter in the range of only about 40 nm are closed by a flat lid, and those with diameters of 60 nm or more are closed by hemispherical caps. These structures are found primarily in phosphatidylcholine/sterol mixtures, where the gel phase state "liquid ordered" (lo) has been introduced. As a further example the mixture of dimyristoylphosphatidylcholine (DMPC) with an unusual sterol (diflucortolon-21-valerat) is presented. In the usual hydration at temperatures above the phase transition the deformation requires an incubation at 4 degrees C for several weeks or months to form. Using a low temperature hydration procedure (at 4 degrees C), surprisingly bilayers of pure DMPC and DPPC (dipalmitoylphosphatidylcholine) are found to deform in the same convex-concave manner, and this takes place within hours and days. The dependence on hydration protocol is also observed for formation of a sponge-like bilayer network with 30-35 nm radius of curvature in brain sphingomyelin and its mixtures with cholesterol. Caveolae are microdomains enriched in cholesterol and sphingomyelin and are simultaneously discussed to be in the lo state. Direct evidence by investigation of bilayers formed by the lipids isolated from caveolae is still lacking, but structures similar to caveolae which are in the gel phase state (very probably the lo state) are also formed by lipids extracted from bacterial membranes. A further analogy exists because both natural lipid mixtures (brain sphingomyelin and bacterial lipids) transform during heating from the curved bilayer structures into microvesicles above the phase transition. Internalization of caveolae is a process of vesicle formation.

Probing the dynamics of planar supported membranes by Nile red fluorescence lifetime distribution

The structure and dynamics of planar supported membranes were studied by using the fluorescence probe Nile red. The width of fluorescence lifetime distribution of Nile red was used to infer the heterogeneity of membranes. The width of fluorescence lifetime was larger and the lifetime was shorter in supported membranes when compared to vesicle membranes. This was interpreted as due to the presence of water-filled membrane discontinuity leading to a heterogeneous surface in supported membranes. Microdomain causing agents such as cholesterol, sphingomyelin, etc. caused a larger level of heterogeneity in supported membranes when compared to vesicle membranes.

The differential miscibility of lipids as the basis for the formation of functional membrane rafts

The formation of sphingolipid-cholesterol microdomains in cellular membranes has been proposed to function in sorting and transport of lipids and proteins as well as in signal transduction. An increasing number of cell biological and biochemical studies now supports this concept. Here we discuss the structural properties of lipids in a cell biological context. The sphingolipid-cholesterol microdomains or rafts are described as dispersed liquid ordered phase domains. These domains are dynamic assemblies to which specific proteins are selectively sequestered while others are excluded. The proteins associated to rafts can act as organizers and can modulate raft size and function.

Surface pressure-dependent cross-modulation of sphingomyelinase and phospholipase A2 in monolayers

We investigated the ways in which phospholipase A2 and sphingomyelinase are mutually modulated at lipid interfaces. The activity of one enzyme is affected by its own reaction products and by substrates and products of the other enzyme; all this depends differently on the lateral surface pressure. Ceramide inhibits both the sphingomyelinase activity rate and the extent of degradation, and decreases the lag time at all surface pressures. Dilauroyl- and dipalmitoylphosphatidylcholine, the substrates of phospholipase A2 (PLA2), do not affect sphingomyelinase activity. The products of PLA2, palmitic acid and lysopalmitoylphosphatidylcholine, strongly enhance and shift to high surface pressures the activity optimum and the cutoff point of sphingomyelinase. Palmitic acid also shifts to high surface pressures the cut-off point of PLA2 activity. Sphingomyelin strongly inhibits PLA2 at surface pressures above 5 mN/m, while ceramide shifts the cut-off point and the activity optimum to high surface pressures. The sphingolipids increase the lag time of PLA2 at low surface pressures. Both phosphohydrolytic pathways involve different levels of control on precatalytic steps and on the rate of activity that appear independent on specific alterations of molecular packing and surface potential. The mutual lipid-mediated interfacial modulation between both phosphohydrolytic pathways indicates that phospholipid degradation may be self-amplified or dampened depending on subtle changes of surface pressure and composition.

Effects of cholesterol depletion by cyclodextrin on the sphingolipid microdomains of the plasma membrane

Sphingolipid microdomains are thought to result from the organization of plasma membrane sphingolipids and cholesterol into a liquid ordered phase, wherein the glycosylphosphatidylinositol (GPI)-anchored proteins are enriched. These domains, resistant to extraction by cold Triton X-100, can be isolated as buoyant membrane complexes (detergent-resistant membranes) in isopycnic density gradients. Here the effects of methyl-beta-cyclodextrin (MBCD), a specific cholesterol-binding agent that neither binds nor inserts into the plasma membrane, were investigated on the sphingolipid microdomains of lymphocytes. MBCD released substantial quantities of GPI-anchored Thy-1 and glycosphingolipid GM1, and also other surface proteins including CD45, and intracellular Lck and Fyn kinases. From endothelial cells, MBCD released GPI-anchored CD59, and CD44, but only a negligible amount of caveolin. Most MBCD-released Thy-1 and CD59 were not sedimentable and thus differed from Thy-1 released by membrane-active cholesterol-binding agents such as saponin and streptolysin O, or Triton X-100. Unlike that released by Triton X-100, only part of the Thy-1 molecules released by MBCD was buoyant in density gradients and co-isolated with GM1. Finally, treatment of Triton X-100-isolated detergent-resistant membranes with MBCD extracted most of the cholesterol without affecting the buoyant properties of Thy-1 or GM1. We suggest that (1) MBCD preferentially extracts cholesterol from outside, rather than within the sphingolipid microdomains and (2) this partly solubilizes GPI-anchored and transmembrane proteins from the glycerophospholipid-rich membrane and releases sphingolipid microdomains in both vesicular and non-vesicular form.

A new look at lipid-membrane structure in relation to drug research

Lipid-bilayer membranes are key objects in drug research in relation to (i) interaction of drugs with membrane-bound receptors, (ii) drug targeting, penetration, and permeation of cell membranes, and (iii) use of liposomes in micro-encapsulation technologies for drug delivery. Rational design of new drugs and drug-delivery systems therefore requires insight into the physical properties of lipid-bilayer membranes. This mini-review provides a perspective on the current view of lipid-bilayer structure and dynamics based on information obtained from a variety of recent experimental and theoretical studies. Special attention is paid to trans-bilayer structure, lateral molecular organization of the lipid bilayer, lipid-mediated protein assembly, and lipid-bilayer permeability. It is argued that lipids play a major role in lipid membrane-organization and functionality.

GPI-anchored proteins are organized in submicron domains at the cell surface

Lateral heterogeneities in the classical fluid-mosaic model of cell membranes are now envisaged as domains or 'rafts' that are enriched in (glyco)sphingolipids, cholesterol, specific membrane proteins and glycosylphosphatidylinositol (GPI)-anchored proteins. These rafts dictate the sorting of associated proteins and/or provide sites for assembling cytoplasmic signalling molecules. However, there is no direct evidence that rafts exist in living cells. We have now measured the extent of energy transfer between isoforms of the folate receptor bound to a fluorescent analogue of folic acid, in terms of the dependence of fluorescence polarization on fluorophore densities in membranes. We find that the extent of energy transfer for the GPI-anchored folate-receptor isoform is density-independent, which is characteristic of organization in sub-pixel-sized domains at the surface of living cells; however, the extent of energy transfer for the transmembrane-anchored folate-receptor isoform was density-dependent, which is consistent with a random distribution. These domains are likely to be less than 70 nm in diameter and are disrupted by removal of cellular cholesterol. These results indicate that lipid-linked proteins are organized in cholesterol-dependent submicron-sized domains. Our methodology offers a new way of monitoring nanometre-scale association between molecules in living cells.

Distribution of a glycosylphosphatidylinositol-anchored protein at the apical surface of MDCK cells examined at a resolution of <100 A using imaging fluorescence resonance energy transfer

Membrane microdomains ("lipid rafts") enriched in glycosylphosphatidylinositol (GPI)-anchored proteins, glycosphingolipids, and cholesterol have been implicated in events ranging from membrane trafficking to signal transduction. Although there is biochemical evidence for such membrane microdomains, they have not been visualized by light or electron microscopy. To probe for microdomains enriched in GPI- anchored proteins in intact cell membranes, we used a novel form of digital microscopy, imaging fluorescence resonance energy transfer (FRET), which extends the resolution of fluorescence microscopy to the molecular level (<100 A). We detected significant energy transfer between donor- and acceptor-labeled antibodies against the GPI-anchored protein 5' nucleotidase (5' NT) at the apical membrane of MDCK cells. The efficiency of energy transfer correlated strongly with the surface density of the acceptor-labeled antibody. The FRET data conformed to theoretical predictions for two-dimensional FRET between randomly distributed molecules and were inconsistent with a model in which 5' NT is constitutively clustered. Though we cannot completely exclude the possibility that some 5' NT is in clusters, the data imply that most 5' NT molecules are randomly distributed across the apical surface of MDCK cells. These findings constrain current models for lipid rafts and the membrane organization of GPI-anchored proteins.

Docosahexaenoic acid (DHA) alters the structure and composition of membranous vesicles exfoliated from the surface of a murine leukemia cell line

Membrane lipid microdomains are regions of the membrane thought to be functionally important, but which have remained poorly characterized because they have proven to be difficult to isolate. The exfoliation of small membranous vesicles from the cell surface is a continuous and normal activity in many cells. If microdomains are relatively large or stable, they may influence the structure and composition of exfoliated vesicles, which are easy to isolate. We tested the ability of docosahexaenoic acid (DHA), a fatty acid proposed to alter the structure of microdomains, to change the structure and composition of vesicles exfoliated from a murine leukemia cell line. Cells were cultured in normal and DHA-enriched media for 72 h, then washed and given a 15-h exfoliation period. Afterwards, the pooled vesicles and their parent plasma membrane were collected and analyzed. Vesicles and plasma membrane from cells grown in normal culture medium had similar fatty acid compositions, including equal, and low, proportions of DHA, but the vesicles had much more cholesterol and displayed higher anisotropy than the plasma membrane. When cells were grown in DHA-enriched medium, both the plasma membrane and exfoliated vesicles had 10-fold elevated levels of DHA in their phospholipids, with the DHA displacing other polyunsaturates. These cells released vesicles having significantly reduced levels of cholesterol and monoenoic fatty acids than those in normal culture. The anisotropy of these vesicles was also dramatically reduced. These data are consistent with DHA altering the structure and composition of membrane microdomains on the cell surface, and suggest that exfoliated vesicles may prove useful in the further study of membrane microdomains.

Domains in cell plasma membranes investigated by near-field scanning optical microscopy

Near-field scanning optical microscopy (NSOM) uses the near-field interaction of light from a sharp fiber-optic probe with a sample of interest to image surfaces at a resolution beyond the diffraction limit of conventional optics. We used NSOM to image fluorescently labeled plasma membranes of fixed human skin fibroblasts, either dried or in buffer. A patchy distribution of a fluorescent lipid analog suggestive of lipid domains was observed in the fixed, dried cells. The sizes of these patches were consistent with the sizes of domains implied by fluorescence photobleaching recovery measurements. Patches of fluorescent lipid analog were not spatially correlated with patches of transmembrane proteins, HLA class I molecules labeled with fluorescent antibody; the patchiness of the HLA class I molecules was on a smaller scale and was not localized to the same regions of membrane as the lipid analog. Sizes of HLA patches were deduced from a two-dimensional spatial autocorrelation analysis of NSOM images that resolved patches with radii of approximately 70 and approximately 600 nm on fixed, dried cells labeled with IgG and 300-600 nm on cells labeled with Fab and imaged in buffer. The large-size patches were also resolved by far-field microscopy. Both the spatial autocorrelation analysis and estimates from fluorescence intensity indicate that the small patches seen on fixed, dried cells contain approximately 25-125 HLA-I molecules each.

Structure of detergent-resistant membrane domains: does phase separation occur in biological membranes?

Detergent-resistant membrane domains (DRMs) can be isolated from a variety of eukaryotic cells. DRMs are of interest because of their potential importance in processes such as intracellular membrane sorting, and signal transduction at the cell surface. One type of DRM is also present in caveolae, non clathrin-coated plasma membrane pits with proposed roles in endocytosis, lipid transport, and signal transduction. Here we review recent advances in understanding the structure of these domains, and explore the possibility that DRMs are present in a phase separate from the surrounding bilayer. DRMs are rich in sphingolipids and cholesterol. The long saturated acyl chains and high acyl chain melting temperature of sphingolipids mediate their association in detergent resistant domains. These sphingolipid and cholesterol-rich domains have the properties of the liquid-ordered phase previously described in model membranes. Several lines of investigation support the idea that DRMs are not detergent-induced artifacts, but exist as domains in cell membranes. A striking feature of the proteins in DRMs is that many of them are linked to lipids. These include both GPI anchored proteins, and acylated proteins such as Src-family kinases. The linkage of these proteins to saturated acyl chains may help in targeting them to ordered membrane domains. Caveolin, the major structural protein of caveolae, is multiply palmitoylated. The presence of a high concentration of palmitate chains in DRMs in caveolae may help stabilize ordered domains.