Membrane domains

Organization of higher-order oligomers of the serotonin₁(A) receptor explored utilizing homo-FRET in live cells

The serotonin₁(A) receptor is a representative member of the GPCR superfamily and serves as an important drug target. The possible role of GPCR oligomerization in receptor function is an active area of research. We monitored the oligomerization state of serotonin₁(A) receptors using homo-FRET and fluorescence lifetime measurements. Homo-FRET is estimated by a reduction in fluorescence anisotropy and provides a superior approach for exploring oligomerization. In addition, homo-FRET offers the possibility of detecting higher-order oligomers. On the basis of an observed increase in fluorescence anisotropy upon progressive photobleaching and analysis of the difference between the extrapolated anisotropy and the predicted anisotropy of an immobile monomer, we propose the presence of constitutive oligomers of the serotonin₁(A) receptor. To the best of our knowledge, these results constitute the first report of higher-order oligomers for the serotonin₁(A) receptor. We further show that cholesterol depletion and antagonist treatment result in a reduced population of higher-order oligomers. In contrast, agonist stimulation and destabilization of the actin cytoskeleton lead to an increased contribution from higher oligomers. These results provide novel insight into the oligomerization status of the serotonin₁(A) receptor that could enhance the ability to design better therapeutic strategies to combat diseases related to malfunctioning of GPCRs.

FCS diffusion laws in two-phase lipid membranes: determination of domain mean size by experiments and Monte Carlo simulations

Many efforts have been undertaken over the last few decades to characterize the diffusion process in model and cellular lipid membranes. One of the techniques developed for this purpose, fluorescence correlation spectroscopy (FCS), has proved to be a very efficient approach, especially if the analysis is extended to measurements on different spatial scales (referred to as FCS diffusion laws). In this work, we examine the relevance of FCS diffusion laws for probing the behavior of a pure lipid and a lipid mixture at temperatures below, within and above the phase transitions, both experimentally and numerically. The accuracy of the microscopic description of the lipid mixtures found here extends previous work to a more complex model in which the geometry is unknown and the molecular motion is driven only by the thermodynamic parameters of the system itself. For multilamellar vesicles of both pure lipid and lipid mixtures, the FCS diffusion laws recorded at different temperatures exhibit large deviations from pure Brownian motion and reveal the existence of nanodomains. The variation of the mean size of these domains with temperature is in perfect correlation with the enthalpy fluctuation. This study highlights the advantages of using FCS diffusion laws in complex lipid systems to describe their temporal and spatial structure.

Imaging the early stages of phospholipase C/sphingomyelinase activity on vesicles containing coexisting ordered-disordered and gel-fluid domains

The binding and early stages of activity of a phospholipase C/sphingomyelinase from Pseudomonas aeruginosa on giant unilamellar vesicles (GUV) have been monitored using fluorescence confocal microscopy. Both the lipids and the enzyme were labeled with specific fluorescent markers. GUV consisted of a mixture of phosphatidylcholine, sphingomyelin, phosphatidylethanolamine, and cholesterol in equimolar ratios, to which 5-10 mol% of the enzyme end-product ceramide and/or diacylglycerol were occasionally added. Morphological examination of the GUV in the presence of enzyme reveals that, although the enzyme diffuses rapidly throughout the observation chamber, detectable enzyme binding appears to be a slow, random process, with new bound-enzyme-containing vesicles appearing for several minutes. Enzyme binding to the vesicles appears to be a cooperative process. After the initial cluster of bound enzyme is detected, further binding and catalytic activity follow rapidly. After the activity has started, the enzyme is not released by repeated washing, suggesting a "scooting" mechanism for the hydrolytic activity. The enzyme preferentially binds the more disordered domains, and, in most cases, the catalytic activity causes the disordering of the other domains. Simultaneously, peanut- or figure-eight-shaped vesicles containing two separate lipid domains become spherical. At a further stage of lipid hydrolysis, lipid aggregates are formed and vesicles disintegrate.

Membrane organization and dynamics of the serotonin1A receptor in live cells

The G-protein coupled receptor (GPCR) superfamily is one of the largest classes of molecules involved in signal transduction across the plasma membrane. The serotonin(1A) receptor is a representative member of the GPCR superfamily and serves as an important target in the development of therapeutic agents for neuropsychiatric disorders such as anxiety and depression. In the context of the pharmacological relevance of the serotonin(1A) receptor, the membrane organization and dynamics of this receptor in the cellular environment assume relevance. We have highlighted results, obtained from fluorescence microscopy-based approaches, related to domain organization and dynamics of the serotonin(1A) receptor. A fraction of serotonin(1A) receptors displays detergent insolubility, monitored using green fluorescent protein, that increases upon depletion of membrane cholesterol. Fluorescence recovery after photobleaching measurements with varying bleach spot sizes show that lateral diffusion parameters of serotonin(1A) receptors in normal cells are consistent with models describing diffusion of molecules in a homogenous membrane. Interestingly, these characteristics are altered in cholesterol-depleted cells. Taken together, we conclude that the serotonin(1A) receptor exhibits dynamic confinement in the cellular plasma membranes. Progress in understanding GPCR organization and dynamics would result in better insight into our overall understanding of GPCR function in health and disease.

Cytochrome c-lipid interactions: new insights from resonance energy transfer

Resonance energy transfer (RET) from anthrylvinyl-labeled phosphatidylcholine (AV-PC) or cardiolipin (AV-CL) to cytochrome c (cyt c) heme moiety was employed to assess the molecular-level details of protein interactions with lipid bilayers composed of PC with 2.5 (CL2.5), 5 (CL5), 10 (CL10), or 20 (CL20) mol % CL under conditions of varying ionic strength and lipid/protein molar ratio. Monte Carlo analysis of multiple data sets revealed a subtle interplay between 1), exchange of the neutral and acidic lipid in the protein-lipid interaction zone; 2), CL transition into the extended conformation; and 3), formation of the hexagonal phase. The switch between these states was found to be controlled by CL content and salt concentration. At ionic strengths ≥ 40 mM, lipid bilayers with CL fraction not exceeding 5 mol % exhibited the tendency to transform from lamellar to hexagonal phase upon cyt c adsorption, whereas at higher contents of CL, transition into the extended conformation seems to become thermodynamically favorable. At lower ionic strengths, deviations from homogeneous lipid distributions were observed only for model membranes containing 2.5 mol % CL, suggesting the existence of a certain surface potential critical for assembly of lipid lateral domains in protein-lipid systems that may subsequently undergo morphological transformations depending on ambient conditions. These characteristics of cyt c-CL interaction are of great interest, not only from the viewpoint of regulating cyt c electron transfer and apoptotic propensities, but also to elucidate the general mechanisms by which membrane functional activities can be modulated by protein-lipid interactions.

Modeling of intracellular transport and compartmentation

The complexity and internal organization of mammalian cells as well as the regulation of intracellular transport processes has increasingly moved into the focus of investigation during the past two decades. Advanced staining and microscopy techniques help to shed light onto spatial cellular compartmentation and regulation, increasing the demand for improved modeling techniques. In this chapter, we summarize recent developments in the field of quantitative simulation approaches and frameworks for the description of intracellular transport processes. Special focus is therefore laid on compartmented and spatiotemporally resolved simulation approaches. The processes considered include free and facilitated diffusion of molecules, active transport via the microtubule and actin filament network, vesicle distribution, membrane transport, cell cycle-dependent cell growth and morphology variation, and protein production. Commercially and freely available simulation packages are summarized as well as model data exchange and harmonization issues.

The human cytomegalovirus protein UL37 exon 1 associates with internal lipid rafts

The human cytomegalovirus (HCMV) protein UL37 exon 1 (pUL37x1), also known as viral mitochondrion-localized inhibitor of apoptosis (vMIA), sequentially traffics from the endoplasmic reticulum (ER) through mitochondrion-associated membranes (MAMs) to the outer mitochondrial membrane (OMM), where it robustly inhibits apoptosis. Here, we report the association of pUL37x1/vMIA with internal lipid rafts (LRs) in the ER/MAM. The MAM, which serves as a site for lipid transfer and calcium signaling to mitochondria, is enriched in detergent-resistant membrane (DRM)-forming lipids, including cholesterol and ceramide, which are found in lower concentrations in the bulk ER. Sigma 1 receptor (Sig-1R), a MAM chaperone affecting calcium signaling to mitochondria, is anchored in the MAM by its LR association. Because of its trafficking through the MAM and partial colocalization with Sig-1R, we tested whether pUL37x1/vMIA associates with MAM LRs. Extraction with methyl-β-cyclodextrin (MβCD) removed pUL37x1/vMIA from lysed but not intact cells, indicating its association with internal LRs. Furthermore, the isolation of DRMs from purified intracellular organelles independently verified the localization of pUL37x1/vMIA within ER/MAM LRs. However, pUL37x1/vMIA was not detected in DRMs from mitochondria. pUL37x1/vMIA associated with LRs during all temporal phases of HCMV infection, indicating the likely importance of this location for HCMV growth. Although detected during its sequential trafficking to the OMM, the pUL37x1/vMIA LR association was independent of its mitochondrial targeting signals. Rather, it was dependent upon cholesterol binding. These studies suggest a conserved ability of UL37 proteins to interact with cholesterol and LRs, which is functionally distinguishable from their sequential trafficking to mitochondria.

Identification of cholesterol recognition amino acid consensus (CRAC) motif in G-protein coupled receptors

G-protein coupled receptors (GPCRs) are the largest class of molecules involved in signal transduction across membranes, and represent major targets in the development of novel drug candidates in all clinical areas. Membrane cholesterol has been reported to have an important role in the function of a number of GPCRs. Several structural features of proteins, believed to result in preferential association with cholesterol, have been recognized. Cholesterol recognition/interaction amino acid consensus (CRAC) sequence represents such a motif. Many proteins that interact with cholesterol have been shown to contain the CRAC motif in their sequence. We report here the presence of CRAC motifs in three representative GPCRs, namely, rhodopsin, the β(2)-adrenergic receptor, and the serotonin(1A) receptor. Interestingly, the function of these GPCRs has been previously shown to be dependent on membrane cholesterol. The presence of CRAC motifs in GPCRs indicates that interaction of cholesterol with GPCRs could be specific in nature. Further analysis shows that CRAC motifs are inherent characteristic features of the serotonin(1A) receptor and are conserved over natural evolution. These results constitute the first report of the presence of CRAC motifs in GPCRs and provide novel insight in the molecular nature of GPCR-cholesterol interaction.

Surface characterization of bovine milk phospholipid monolayers by Langmuir isotherms and microscopic techniques

Monolayers were prepared from phospholipids extracted from bovine milk and used as a model system to mimic the native milk fat globule membrane (MFGM) surface structure in various microscopic experiments. The natural complex mixtures of phospholipids were isolated from bovine raw milk, raw cream, processed whole milk, and buttermilk powder by total lipid extraction and solid-phase extraction. A Langmuir film balance mounted on an epifluorescence microscope was used to analyze the physical behavior of the monolayer films and the phase coexistence resulting from the formation of phospholipid microdomains within these films. Atomic force microscopy was used for nanometer-scale topographic resolution of the microdomains. This study allowed comparison of the behavior of phospholipid monolayers from dairy products at different stages of processing, analysis of the formation of microdomains, and the study of the effect of milk processing on lipid-lipid interactions and phase coexistence. It was observed that milk processing changes the physical behavior of phospholipid monolayers by altering the phospholipid profile and the fatty acid distribution.

Liposome and lipid bilayer arrays towards biosensing applications

Sensitive and selective biosensors for high-throughput screening are having an increasing impact in modern medical care. The establishment of robust protein biosensing platforms however remains challenging, especially when membrane proteins are involved. Although this type of proteins is of enormous relevance since they are considered in >60% of the pharmaceutical drug targets, their fragile nature (i.e., the requirement to preserve their natural lipid environment to avoid denaturation and loss of function) puts strong additional prerequisites onto a successful biochip. In this review, the leading approaches to create lipid membrane-based arrays towards the creation of membrane protein biosensing platforms are described. Liposomes assembled in micro- and nanoarrays and the successful set-ups containing functional membrane proteins, as well as the use of liposomes in networks, are discussed in the first part. Then, the complementary approaches to create cell-mimicking supported membrane patches on a substrate in an array format will be addressed. Finally, the progress in assembling free-standing (functional) lipid bilayers over nanopore arrays for ion channel sensing will be reported. This review illustrates the rapid pace by which advances are being made towards the creation of a heterogeneous biochip for the high-throughput screening of membrane proteins for diagnostics, drug screening, or drug discovery purposes.

Plasma membrane sterol complexation, generated by filipin, triggers signaling responses in tobacco cells

The effects of changes in plasma membrane (PM) sterol lateral organization and availability on the control of signaling pathways have been reported in various animal systems, but rarely assessed in plant cells. In the present study, the pentaene macrolide antibiotic filipin III, commonly used in animal systems as a sterol sequestrating agent, was applied to tobacco cells. We show that filipin can be used at a non-lethal concentration that still allows an homogeneous labeling of the plasma membrane and the formation of filipin-sterol complexes at the ultrastructural level. This filipin concentration triggers a rapid and transient NADPH oxidase-dependent production of reactive oxygen species, together with an increase in both medium alkalinization and conductivity. Pharmacological inhibition studies suggest that these signaling events may be regulated by phosphorylations and free calcium. By conducting FRAP experiments using the di-4-ANEPPDHQ probe and spectrofluorimetry using the Laurdan probe, we provide evidence for a filipin-induced increase in PM viscosity that is also regulated by phosphorylations. We conclude that filipin triggers ligand-independent signaling responses in plant cells. The present findings strongly suggest that changes in PM sterol availability could act as a sensor of the modifications of cell environment in plants leading to adaptive cell responses through regulated signaling processes.

Transbilayer organization of membrane cholesterol at low concentrations: Implications in health and disease

Cholesterol is an essential and representative lipid in higher eukaryotic cellular membranes and is often found distributed nonrandomly in domains in biological membranes. A large body of literature exists on the organization of cholesterol in plasma membranes or membranes with high cholesterol content. However, very little is known about organization of cholesterol in membranes containing low amounts of cholesterol such as the endoplasmic reticulum or inner mitochondrial membranes. In this review, we have traced the discovery and subsequent development of the concept of transbilayer cholesterol dimers (domains) in membranes at low concentrations. We have further discussed the role of membrane curvature and thickness on the transbilayer organization of cholesterol. Interestingly, this type of cholesterol organization could be relevant in cellular sorting and trafficking, and in pathological conditions.

Morphological changes of supported lipid bilayers induced by lysozyme: planar domain formation vs. multilayer stacking

Total internal reflection fluorescence microscopy (TIRFM) has been utilized to explore the effect of cationic protein lysozyme (Lz) on the morphology of solid-supported lipid bilayers (SLBs) comprised of zwitterionic lipid phosphatidylcholine (PC) and its mixture with anionic lipid cardiolipin (CL). Kinetic TIRFM imaging of different systems revealed subtle interplay between lipid lateral segregation accompanied by exchange of neutral and acidic lipids in the protein-lipid interaction zone, and the formation of lipid multilayer stacks. The switch between these states was shown to be controlled by CL content. In weakly charged SLBs containing 5 mol% CL, assembling of CL molecules into planar domains upon Lz adsorption has been observed while at higher content of anionic lipid (25 mol%) in-plane domains tend to transform into multilayer stacks, thereby ensuring the most thermodynamically-favorable membrane conformation.

Composition and fatty acid distribution of bovine milk phospholipids from processed milk products

The aim of this work was to assess the accuracy of different extraction methods of phospholipids and to measure the effect that processing has on phospholipid composition. Four methods of extracting phospholipids from buttermilk powder were compared to optimize recovery of sphingomyelin. Using the optimal method, the phospholipid profile of four dairy products (raw milk, raw cream, homogenized and pasteurized milk, and buttermilk powder) was determined. A total lipid extraction by the Folch method followed by a solid-phase extraction using the Bitman method was the most efficient technique to recover milk sphingomyelin. Milk processing (churning, centrifuging, homogenization, spray-drying) affected the profile of milk phospholipids, leading to a loss of sphingomyelin and phosphatidylcholine after centrifugation for cream separation. A corresponding decrease in the saturation content of the raw cream phospholipids and a loss of phosphatidylethanolamine after spray-drying to produce buttermilk powder were also observed.

Cholesterol depletion enhances adrenergic signaling in cardiac myocytes

Cardiac myocytes endogenously express α and β adrenergic receptors, prototypes of the G-protein coupled receptor superfamily. Depending upon the dose of norepinephrine (agonist) exposure, hypertrophy and apoptosis are initiated by differential induction of two discrete constituents of the transcription factor AP-1, i.e., FosB and Fra-1. We explored differential adrenergic signaling as a paradigm for understanding how cholesterol dictates cells to choose hypertrophy or apoptosis. For this, we used fosB and fra-1 promoter-reporter constructs for monitoring adrenergic signaling. We show that cholesterol depletion enhances norepinephrine-mediated signaling in cardiac myocytes. Importantly, this increased signaling is reduced to original level upon cholesterol replenishment. We used specific ligands for α and β adrenergic receptors and show that the enhanced signaling upon cholesterol depletion is a combined effect of both α and β adrenergic receptors. These results constitute the first report demonstrating the effect of cholesterol on adrenergic signaling using a direct end-point gene expression.

Cholesterol depletion mimics the effect of cytoskeletal destabilization on membrane dynamics of the serotonin1A receptor: A zFCS study

Single-point fluorescence correlation spectroscopy (FCS) of membrane-bound molecules suffers from a number of limitations leading to inaccurate estimation of diffusion parameters. To overcome such problems and with the overall goal of addressing membrane heterogeneities, we performed z-scan FCS (zFCS) of the serotonin(1A) receptor. We analyzed the results according to FCS diffusion laws that provide information on the organization of the diffusing species. Analysis of our results shows that the diffusion coefficients of the receptor and a fluorescently labeled phospholipid are similar when probed at length scales approximately 210 nm. We discuss the significance of the spatiotemporal evolution of dynamics of membrane-bound molecules in the overall context of membrane domains and heterogeneity. Importantly, our results show that the serotonin(1A) receptor exhibits confinement in cell membranes, possibly due to interaction with the actin cytoskeleton. Surprisingly, depletion of membrane cholesterol appears to reduce receptor confinement in a manner similar to that observed in the case of cytoskeletal destabilization, implying possible changes in the actin cytoskeleton induced upon cholesterol depletion. These results constitute the first report on G-protein-coupled receptor dynamics utilizing a combination of zFCS and the FCS diffusion laws, and present a convenient approach to explore cell membrane heterogeneity at the submicron level.

Synaptic membrane proteins form stable microdomains in early endosomes

In the plasma membrane, membrane proteins are frequently organized in microdomains that are stabilized both by protein-protein and protein-lipid interactions, with the membrane lipid cholesterol being instrumental for microdomain stability. However, it is unclear whether such microdomains persist during endocytotic membrane trafficking. We used stimulated emission-depletion microscopy to investigate the domain structure of the endosomes. We developed a semiautomatic method for counting the individual domains, an approach that we have validated by immunoelectron microscopy. We found that in endosomes derived from neuroendocrine PC12 cells synaptophysin and several SNARE proteins are organized in microdomains. Cholesterol depletion by methyl-beta-cyclodextrin disintegrates most of the domains. Interestingly, no change in the frequency of microdomains was observed when endosomes were fused with protein-free liposomes of similar size (in what constitutes a novel approach in modifying acutely the lipid composition of organelles), regardless of whether the membrane lipid composition of the liposomes was similar or very different from that of the endosomes. Similarly, Rab depletion from the endosome membranes left the domain structure unaffected. Furthermore, labeled exogenous protein, introduced into endosomes by liposome fusion, equilibrated with the corresponding microdomains. We conclude that synaptic membrane proteins are organized in stable but dynamic clusters within endosomes, which are likely to persist during membrane recycling.

Behavior of plant plasma membranes under hydrostatic pressure as monitored by fluorescent environment-sensitive probes

We monitored the behavior of plasma membrane (PM) isolated from tobacco cells (BY-2) under hydrostatic pressures up to 3.5kbar at 30 degrees C, by steady-state fluorescence spectroscopy using the newly introduced environment-sensitive probe F2N12S and also Laurdan and di-4-ANEPPDHQ. The consequences of sterol depletion by methyl-beta-cyclodextrin were also studied. We found that application of hydrostatic pressure led to a marked decrease of hydration as probed by F2N12S and to an increase of the generalized polarization excitation (GPex) of Laurdan. We observed that the hydration effect of sterol depletion was maximal between 1 and 1.5 kbar but was much less important at higher pressures (above 2 kbar) where both parameters reached a plateau value. The presence of a highly dehydrated gel state, insensitive to the sterol content, was thus proposed above 2.5 kbar. However, the F2N12S polarity parameter and the di-4-ANEPPDHQ intensity ratio showed strong effect on sterol depletion, even at very high pressures (2.5-3.5 kbar), and supported the ability of sterols to modify the electrostatic properties of membrane, notably its dipole potential, in a highly dehydrated gel phase. We thus suggested that BY-2 PM undergoes a complex phase behavior in response to the hydrostatic pressure and we also emphasized the role of phytosterols to regulate the effects of high hydrostatic pressure on plant PM.

Intracellular phospholipase A1 and acyltransferase, which are involved in Caenorhabditis elegans stem cell divisions, determine the sn-1 fatty acyl chain of phosphatidylinositol

Phosphatidylinositol (PI), an important constituent of membranes, contains stearic acid as the major fatty acid at the sn-1 position. This fatty acid is thought to be incorporated into PI through fatty acid remodeling by sequential deacylation and reacylation. However, the genes responsible for the reaction are unknown, and consequently, the physiological significance of the sn-1 fatty acid remains to be elucidated. Here, we identified acl-8, -9, and -10, which are closely related to each other, and ipla-1 as strong candidates for genes involved in fatty acid remodeling at the sn-1 position of PI. In both ipla-1 mutants and acl-8 acl-9 acl-10 triple mutants of Caenorhabditis elegans, the stearic acid content of PI is reduced, and asymmetric division of stem cell-like epithelial cells is defective. The defects in asymmetric division of these mutants are suppressed by a mutation of the same genes involved in intracellular retrograde transport, suggesting that ipla-1 and acl genes act in the same pathway. IPLA-1 and ACL-10 have phospholipase A(1) and acyltransferase activity, respectively, both of which recognize the sn-1 position of PI as their substrate. We propose that the sn-1 fatty acid of PI is determined by ipla-1 and acl-8, -9, -10 and crucial for asymmetric divisions.

Rafting trips into the cell

Lipid rafts are small, heterogeneous and short-lived assemblies of cholesterol, sphingolipids and few proteins in biological membranes. They can be converted to larger and more permanent membrane domains by coalescence. Cells appear to be able to modulate the size and the longevity of lipid rafts and thus exploit the local enrichment of membrane components for processes ranging from signaling to intracellular sorting and transport. In a recent paper, we provided evidence for the internalization of MHC I and MHC II along two distinct endocytosis pathways in mouse B-lymphocytes. Both pathways were much more dependent on membrane cholesterol than the clathrin-mediated uptake of transferrin receptor, which implicated lipid rafts in the internalization of MHC molecules. Indeed, MHC I and MHC II prefer distinct raft-like membrane environments as revealed by a co-clustering analysis with the sphingolipids G(M)1 and G(M)2. Moreover, MHC I and MHC II distributed to different types of detergent resistant membranes (DRMs) prepared by a novel detergent extraction procedure. In this article addendum we discuss the relationship between DRMs, small lipid rafts and stabilized rafts/membrane domains and propose a role for membrane domains in the endocytosis of MHC proteins.

Designed low amphipathic peptides with alpha-helical propensity exhibiting antimicrobial activity via a lipid domain formation mechanism

Although several low amphipathic peptides have been known to exhibit antimicrobial activity, their mode of action has not been completely elucidated. In this study, using designed low amphipathic peptides that retain different alpha-helical content and hydrophobicity, we attempted to investigate the mechanism of these properties. Calorimetric and thermodynamic analyses demonstrated that the peptides induce formation of two lipid domains in an anionic liposome at a high peptide-to-lipid ratio. On the other hand, even at a low peptide-to-lipid ratio, they caused minimal membrane damage, such as flip-flop of membrane lipids or leakage of calcein molecules from liposomes, and never translocated across membranes. Interaction energies between the peptides and anionic liposomes showed good correlation with antimicrobial activity for both Escherichia coli and Bacillus subtilis. We thus propose that the domain formation mechanism in which antimicrobial peptides exhibit activity solely by forming lipid domains without membrane damage is a major determinant of the antimicrobial activity of low amphipathic peptides. These peptides appear to stiffen the membrane such that it is deprived of the fluidity necessary for biological functions. We also showed that to construct the lipid domains, peptides need not form stable and cooperative structures. Rather, it is essential for peptides to only interact tightly with the membrane interface via strong electrostatic interactions, and slight differences in binding strength are invoked by differences in hydrophobicity. The peptides thus designed might pave the way for "clean" antimicrobial reagents that never cause release of membrane elements and efflux of their inner components.

Involvement of cholesterol in synaptic vesicle swelling

Studies demonstrate that cholesterol plays a critical role in the regulation of neurotransmitter release and that secretory vesicle swelling is a requirement for the regulated expulsion of intravesicular contents during cell secretion. In view of this, the involvement of cholesterol in synaptic vesicle swelling was hypothesized and tested in the present study, using isolated synaptic vesicles from rat brain and the determination of their swelling competency in the presence and absence of cholesterol. The involvement of the water channel aquaporin-6 (AQP-6) and proton pump vH(+)-ATPase in GTP-G(alpha o)-mediated synaptic vesicle swelling has been reported previously. Mastoparan, the amphiphilic tetradecapeptide from wasp venom, known to activate the GTPase activity of G(alpha o/i) proteins, stimulates synaptic vesicle swelling in the presence of GTP. In the current study, using nanometer-scale precision measurements of isolated synaptic vesicles, we report for the first time that depletion of cholesterol from synaptic vesicle membrane results in a significant loss of GTP-mastoparan-stimulable synaptic vesicle swelling. In contrast, incorporation of cholesterol into the synaptic vesicle membrane potentiates GTP-mastoparan-stimulable vesicle swelling. Our study further demonstrates that this effect of cholesterol is due, in part, to its involvement in the interactions between AQP-6, vH(+)-ATPase and the GTP-binding G(alpha o) protein at the synaptic vesicle membrane.

Lipidomics reveals membrane lipid remodelling and release of potential lipid mediators during early stress responses in a murine melanoma cell line

Membranes are known to respond rapidly to various environmental perturbations by changing their composition and microdomain organization. In previous work we showed that a membrane fluidizer benzyl alcohol (BA) could mimic the effects of heat stress and enhance heat shock protein synthesis in different mammalian cells. Here we explore heat- and BA-induced stress further by characterizing stress-induced membrane lipid changes in mouse melanoma B16 cells. Lipidomic fingerprints revealed that membrane stress achieved either by heat or BA resulted in pronounced and highly specific alterations in lipid metabolism. The loss in polyenes with the concomitant increase in saturated lipid species was shown to be a consequence of the activation of phopholipases (mainly phopholipase A(2) and C). A phospholipase C-diacylglycerol lipase-monoacylglycerol lipase pathway was identified in B16 cells and contributed significantly to the production of several lipid mediators upon stress including the potent heat shock modulator, arachidonic acid. The accumulation of cholesterol, ceramide and saturated phosphoglyceride species with raft-forming properties observed upon both heat and BA treatments of B16 cells may explain the condensation of ordered plasma membrane domains previously detected by fluorescence microscopy and may serve as a signalling platform in stress responses or as a primary defence mechanism against the noxious effects of stresses.

Using confocal laser scanning microscopy to probe the milk fat globule membrane and associated proteins

The bovine milk fat globule membrane (MFGM) is an important, biologically relevant membrane due to its functional and health properties. Its composition has been thoroughly studied, but its structure, especially the lateral organization of its components, still remains unclear. We have used confocal laser scanning microscopy (CLSM) to investigate the surface structure of the MFGM in globules with different degrees of processing using two types of fluorescently labeled phospholipid probes and a protein dye. Using this technique, we have observed heterogeneities in the distribution of MFGM lipids and proteins relating to the processing and size of the globules. The effect of pretreating the milk (centrifugation, pasteurization-homogenization and churning) was studied by double-staining the surface of the milk fat globules, followed by observation using CLSM, and by determining the phospholipid profile of raw milk, raw cream, processed milk and buttermilk powder. Our findings agree with other techniques by showing that the composition of the MFGM changes with processing through the loss of phospholipids and the adsorption of caseins and whey proteins onto the surface.

Plasma membrane proteomics and its application in clinical cancer biomarker discovery

Plasma membrane proteins that are exposed on the cell surface have important biological functions, such as signaling into and out of the cells, ion transport, and cell-cell and cell-matrix interactions. The expression level of many of the plasma membrane proteins involved in these key functions is altered on cancer cells, and these proteins may also be subject to post-translational modification, such as altered phosphorylation and glycosylation. Additional protein alterations on cancer cells confer metastatic capacities, and some of these cell surface proteins have already been successfully targeted by protein drugs, such as human antibodies, that have enhanced survival of several groups of cancer patients. The combination of novel analytical approaches and subcellular fractionation procedures has made it possible to study the plasma membrane proteome in more detail, which will elucidate cancer biology, particularly metastasis, and guide future development of novel drug targets. The technical advances in plasma membrane proteomics and the consequent biological revelations will be discussed herein. Many of the advances have been made using cancer cell lines, but because the main goal of this research is to improve individualized treatment and increase cancer patient survival, further development is crucial to direct analysis of clinically relevant patient samples. These efforts include optimized specimen handling and preparation as well as improved proteomics platforms. Identification of potentially useful proteomics-based biomarkers must be validated in larger, well defined retrospective and prospective clinical studies, and these combined efforts should result in identification of biomarkers that will greatly improve early detection, prognosis, and prediction of treatment response.

Sterols and sphingolipids: dynamic duo or partners in crime?

One manner in which eukaryotic cells respond to their environments is by optimizing the composition and proportions of sterols and sphingolipids in membranes. The physical association of the planar ring of sterols with the acyl chains of phospholipids, particularly sphingolipids, produces membrane micro-heterogeneity that is exploited to coordinate several crucial pathways. We hypothesize that these lipid molecules play an integrated role in human disease; when one of the partners is mis-regulated, pathology frequently ensues. Sterols and sphingolipid levels are not coordinated by the action of a single master regulator, however the cross-talk between their metabolic pathways is considerable. We describe our perspectives on the key components of synthesis, catabolism and transport of these lipid partners with an emphasis on evolutionarily conserved reactions that produce disease states when defective.

Virus entry by endocytosis

Although viruses are simple in structure and composition, their interactions with host cells are complex. Merely to gain entry, animal viruses make use of a repertoire of cellular processes that involve hundreds of cellular proteins. Although some viruses have the capacity to penetrate into the cytosol directly through the plasma membrane, most depend on endocytic uptake, vesicular transport through the cytoplasm, and delivery to endosomes and other intracellular organelles. The internalization may involve clathrin-mediated endocytosis (CME), macropinocytosis, caveolar/lipid raft-mediated endocytosis, or a variety of other still poorly characterized mechanisms. This review focuses on the cell biology of virus entry and the different strategies and endocytic mechanisms used by animal viruses.

Diffusion, transport, and cell membrane organization investigated by imaging fluorescence cross-correlation spectroscopy

Cell membrane organization is dynamic and is assumed to have different characteristic length scales. These length scales, which are influenced by lipid and protein composition as well as by the cytoskeleton, can range from below the optical resolution limit (as with rafts or microdomains) to far above the resolution limit (as with capping phenomena or the formation of lipid "platforms"). The measurement of these membrane features poses a significant problem because membrane dynamics are on the millisecond timescale and are thus beyond the time resolution of conventional imaging approaches. Fluorescence correlation spectroscopy (FCS), a widely used spectroscopic technique to measure membrane dynamics, has the required time resolution but lacks imaging capabilities. A promising solution is the recently introduced method known as imaging total internal reflection (ITIR)-FCS, which can probe diffusion phenomena in lipid membranes with good temporal and spatial resolution. In this work, we extend ITIR-FCS to perform ITIR fluorescence cross-correlation spectroscopy (ITIR-FCCS) between pixel areas of arbitrary shape and derive a generalized expression that is applicable to active transport and diffusion. ITIR-FCCS is applied to model systems exhibiting diffusion, active transport, or a combination of the two. To demonstrate its applicability to live cells, we observe the diffusion of a marker, the sphingolipid-binding domain (SBD) derived from the amyloid peptide Abeta, on live neuroblastoma cells. We investigate the organization and dynamics of SBD-bound lipid microdomains under the conditions of cholesterol removal and cytoskeleton disruption.

FLIM-FRET and FRAP reveal association of influenza virus haemagglutinin with membrane rafts

It has been supposed that the HA (haemagglutinin) of influenza virus must be recruited to membrane rafts to perform its function in membrane fusion and virus budding. In the present study, we aimed at substantiating this association in living cells by biophysical methods. To this end, we fused the cyan fluorescent protein Cer (Cerulean) to the cytoplasmic tail of HA. Upon expression in CHO (Chinese-hamster ovary) cells HA–Cer was glycosylated and transported to the plasma membrane in a similar manner to authentic HA. We measured FLIM-FRET (Förster resonance energy transfer by fluorescence lifetime imaging microscopy) and showed strong association of HA–Cer with Myr-Pal–YFP (myristoylated and palmitoylated peptide fused to yellow fluorescent protein), an established marker for rafts of the inner leaflet of the plasma membrane. Clustering was significantly reduced when rafts were disintegrated by cholesterol extraction and when the known raft-targeting signals of HA, the palmitoylation sites and amino acids in its transmembrane region, were removed. FRAP (fluorescence recovery after photobleaching) showed that removal of raft-targeting signals moderately increased the mobility of HA in the plasma membrane, indicating that the signals influence access of HA to slowly diffusing rafts. However, Myr-Pal–YFP exhibited a much faster mobility compared with HA–Cer, demonstrating that HA and the raft marker do not diffuse together in a stable raft complex for long periods of time.

Membrane cholesterol in the function and organization of G-protein coupled receptors

Cholesterol is an essential component of higher eukaryotic membranes and plays a crucial role in membrane organization, dynamics and function. The G-protein coupled receptors (GPCRs) are the largest class of molecules involved in signal transduction across membranes, and represent major targets in the development of novel drug candidates in all clinical areas. Membrane cholesterol has been reported to have a modulatory role in the function of a number of GPCRs. Two possible mechanisms have been previously suggested by which membrane cholesterol could influence the structure and function of GPCRs (i) through a direct/specific interaction with GPCRs, or (ii) through an indirect way by altering membrane physical properties in which the receptor is embedded, or due to a combination of both. Recently reported crystal structures of GPCRs have shown structural evidence of cholesterol binding sites. Against this backdrop, we recently proposed a novel mechanism by which membrane cholesterol could affect structure and function of GPCRs. According to our hypothesis, cholesterol binding sites in GPCRs could represent 'nonannular' binding sites. Interestingly, previous work from our laboratory has demonstrated that membrane cholesterol is required for the function of the serotonin(1A) receptor (a representative GPCR), which could be due to specific interaction of the receptor with cholesterol. Based on these results, we envisage that there could be specific/nonannular cholesterol binding site(s) in the serotonin(1A) receptor. We have analyzed putative cholesterol binding sites from protein databases in the serotonin(1A) receptor. Our analysis shows that cholesterol binding sites are inherent characteristic features of serotonin(1A) receptors and are conserved through natural evolution. Progress in deciphering molecular details of the GPCR-cholesterol interaction in the membrane would lead to better insight into our overall understanding of GPCR function in health and disease, thereby enhancing our ability to design better therapeutic strategies to combat diseases related to malfunctioning of GPCRs.

Membrane cholesterol depletion enhances ligand binding function of human serotonin1A receptors in neuronal cells

Membrane lipid composition of cells in the nervous system is unique and displays remarkable diversity. Cholesterol metabolism and homeostasis in the central nervous system and their role in neuronal function represent important determinants in neuropathogenesis. The serotonin(1A) receptor is an important member of the G-protein coupled receptor superfamily, and is involved in a variety of cognitive, behavioral, and developmental functions. We report here, for the first time, that the ligand binding function of human serotonin(1A) receptors exhibits an increase in membranes isolated from cholesterol-depleted neuronal cells. Our results gain pharmacological significance in view of the recently described structural evidence of specific cholesterol binding site(s) in GPCRs, and could be useful in designing better therapeutic strategies for neurodegenerative diseases associated with GPCRs.

IgE receptor-mediated alteration of membrane-cytoskeleton interactions revealed by mass spectrometric analysis of detergent-resistant membranes

We use electrospray ionization mass spectrometry to quantify >100 phospholipid (PL) components in detergent-resistant membrane (DRM) domains that are related to ordered membrane compartments commonly known as lipid rafts. We previously compared PL compositions of DRMs with plasma membrane vesicles and whole cell lipid extracts from RBL mast cells, and we made the initial observation that antigen stimulation of IgE receptors (FcepsilonRI) causes a significant change in the PL composition of DRMs [Fridriksson, E. K., et al. (1999) Biochemistry 38, 8056-8063]. We now characterize the signaling requirements and time course for this change, which is manifested as an increase in the recovery of polyunsaturated PL in DRM, particularly in phosphatidylinositol species. We find that this change is largely independent of tyrosine phosphorylation, stimulated by engagement of FcepsilonRI, and can be activated by Ca(2+) ionophore in a manner independent of antigen stimulation. Unexpectedly, we found that inhibitors of actin polymerization (cytochalasin D and latrunculin A) cause a similar, but more rapid, change in the PL composition of DRMs in the absence of FcepsilonRI activation, indicating that perturbations in the actin cytoskeleton affect the organization of plasma membrane domains. Consistent with this interpretation, a membrane-permeable stabilizer of F-actin, jasplakinolide, prevents antigen-stimulated changes in DRM PL composition. These results are confirmed by a detailed analysis of multiple experiments, showing that receptor and cytochalasin D-stimulated changes in DRM lipid composition follow first-order kinetics. Analysis in terms of the number of double bonds in the fatty acid chains is valid for total PL of the major headgroups and for headgroups individually. In this manner, we show that, on average, concentrations of saturated or monounsaturated PL decrease in the DRM, whereas concentrations of PL with two or more double bonds (polyunsaturated PL) increase due to cytoskeletal perturbation. We find that these changes are independent of fatty acid chain length. Our mass spectrometric analyses provide a detailed accounting of receptor-activated alterations in the plasma membrane that are regulated by the actin cytoskeleton.

Differential effects of cholesterol and desmosterol on the ligand binding function of the hippocampal serotonin(1A) receptor: implications in desmosterolosis

Cholesterol is a unique molecule in terms of high level of in-built stringency, fine tuned by natural evolution for its ability to optimize physical properties of higher eukaryotic cell membranes in relation to biological functions. We previously demonstrated the requirement of membrane cholesterol in maintaining the ligand binding activity of the hippocampal serotonin(1A) receptor. In order to test the molecular stringency of the requirement of cholesterol, we depleted cholesterol from native hippocampal membranes followed by replenishment with desmosterol. Desmosterol is an immediate biosynthetic precursor of cholesterol in the Bloch pathway differing only in a double bond at the 24th position in the alkyl side chain. Our results show that replenishment with desmosterol does not restore ligand binding activity of the serotonin(1A) receptor although replenishment with cholesterol led to significant recovery of ligand binding. This is in spite of similar membrane organization (order) in these membranes, as monitored by fluorescence anisotropy measurements. The requirement for restoration of ligand binding activity therefore appears to be more stringent than the requirement for the recovery of overall membrane order. These novel results have potential implications in understanding the interaction of membrane lipids with this important neuronal receptor in diseases such as desmosterolosis.

Illuminating the life of GPCRs

The investigation of biological systems highly depends on the possibilities that allow scientists to visualize and quantify biomolecules and their related activities in real-time and non-invasively. G-protein coupled receptors represent a family of very dynamic and highly regulated transmembrane proteins that are involved in various important physiological processes. Since their localization is not confined to the cell surface they have been a very attractive "moving target" and the understanding of their intracellular pathways as well as the identified protein-protein-interactions has had implications for therapeutic interventions. Recent and ongoing advances in both the establishment of a variety of labeling methods and the improvement of measuring and analyzing instrumentation, have made fluorescence techniques to an indispensable tool for GPCR imaging. The illumination of their complex life cycle, which includes receptor biosynthesis, membrane targeting, ligand binding, signaling, internalization, recycling and degradation, will provide new insights into the relationship between spatial receptor distribution and function. This review covers the existing technologies to track GPCRs in living cells. Fluorescent ligands, antibodies, auto-fluorescent proteins as well as the evolving technologies for chemical labeling with peptide- and protein-tags are described and their major applications concerning the GPCR life cycle are presented.

Membrane organization of the human serotonin1Areceptor monitored by detergent insolubility using GFP fluorescence

Insolubility in non-ionic detergents such as Triton X-100 at low temperature is a widely used biochemical criterion for characterization of membrane domains. In view of the emerging role of membrane organization in the function of G-protein coupled receptors, we have examined detergent insolubility of the 5-HT(1A) receptor in CHO cells using a novel GFP fluorescence approach developed by us. Using this approach, we have explored the membrane organization of the serotonin(1A) receptor tagged to enhanced yellow fluorescent protein (5-HT(1A)R-EYFP) stably expressed in CHO-K1 cells under conditions of varying detergent concentration, reduced membrane cholesterol and agonist stimulation. Our results show that a small yet significant fraction of the 5-HT(1A) receptor exhibits detergent insolubility, which increases upon depletion of membrane cholesterol. Stimulation of 5-HT(1A)R-EYFP by its endogenous ligand, serotonin, did not cause a significant change in the detergent insolubility of the receptor. Taken together, our results on detergent insolubility of 5-HT(1A)R-EYFP provide new insights into the membrane organization of the 5-HT(1A) receptor and could be relevant in the analysis of membrane organization of other G-protein coupled receptors.

Cholesterol-regulated stress fiber formation

Dynamic interactions between cellular membranes and the cytoskeleton are known to play major roles in many cellular responses to environmental cues. External signals resulting in proliferation, differentiation, polarization, and motility must be translated from chemical signals into changes of state, often involving the cytoskeleton-dependent altering of cell shape and redistribution of molecules. Cholesterol, a critical component of eukaryotic cell membranes, performs vital roles in regulating membrane dynamics and function. Here we demonstrate, using mesenchymal and epithelial cell lines, that depletion of membrane cholesterol results in Src kinase-mediated Rho activation and caveolin phosphorylation, which together collaborate to form stress fibers. These results demonstrate that cholesterol is a critical regulator of membrane-cytoskeletal dynamics and suggest that altered cholesterol concentrations may result in dramatic changes in cellular responses mediated by the cytoskeleton.

Stability and liquid-liquid phase separation in mixed saturated lipid bilayers

The phase stability of a fluid lipid bilayer composed of a mixture of DC(18)PC, (DSPC), and a shorter DCn(s) PC, with n(s) from 8 to 17, has been studied using a self-consistent field theory that explicitly includes molecular details and configurational properties of the lipid molecules. Phase separation between two liquid phases was found when there was a sufficient mismatch between the hydrophobic thicknesses of the two bilayers composed entirely of one component or the other. This occurs when n(s) 12, we observe that the effect of the shorter lipid is to increase the susceptibility of the system to fluctuations in the concentration. This is of interest, given that a common motif for the anchoring of proteins to the plasma membrane is via a myristoyl chain, that is, one with 14 carbons.

Phase-field modeling of the dynamics of multicomponent vesicles: Spinodal decomposition, coarsening, budding, and fission

We develop a thermodynamically consistent phase-field model to simulate the dynamics of multicomponent vesicles. The model accounts for bending stiffness, spontaneous curvature, excess (surface) energy, and a line tension between the coexisting surface phases. Our approach is similar to that recently used by Wang and Du [J. Math. Biol. 56, 347 (2008)] with a key difference. Here, we concentrate on the dynamic evolution and solve the surface mass conservation equation explicitly; this equation was not considered by Wang and Du. The resulting fourth-order strongly coupled system of nonlinear nonlocal equations are solved numerically using an adaptive finite element numerical method. Although the system is valid for three dimensions, we limit our studies here to two dimensions where the vesicle is a curve. Differences between the spontaneous curvatures and the bending rigidities of the surface phases are found numerically to lead to the formation of buds, asymmetric vesicle shapes and vesicle fission even in two dimensions. In addition, simulations of configurations far from equilibrium indicate that phase separation via spinodal decomposition and coarsening not only affect the vesicle shape but also that the vesicle shape affects the phase separation dynamics, especially the coarsening and may lead to lower energy states than might be achieved by evolving initially phase-separated configurations.