Role of lipid modifications in targeting proteins to detergent-resistant membrane rafts. Many raft proteins are acylated, while few are prenylated

ROLES OF BILAYER MATERIAL PROPERTIES IN FUNCTION AND DISTRIBUTION OF MEMBRANE PROTEINS

Structural, compositional, and material (elastic) properties of lipid bilayers exert strong influences on the interactions of water-soluble proteins and peptides with membranes, the distribution of transmembrane proteins in the plane of the membrane, and the function of specific membrane channels. Theoretical and experimental studies show that the binding of either cytoplasmic proteins or extracellular peptides to membranes is regulated by the presence of charged lipids and that the sorting of transmembrane proteins into or out of membrane microdomains (rafts) depends on several factors, including bilayer material properties governed by the presence of cholesterol. Recent studies have also shown that bilayer material properties modify the permeability of membrane pores, formed either by protein channels or by cell-lytic peptides.

Acute ethanol treatment modulates Toll-like receptor-4 association with lipid rafts

BACKGROUND

Alcohol, a substance that is most frequently abused, suppresses innate immune responses to microbial pathogens. The host senses pathogens via Toll-like receptors (TLRs). Recent studies indicate that alcohol affects TLR signaling.

METHODS

Here, we hypothesized that acute alcohol treatment may interfere with early steps of membrane-associated TLR2 and TLR4 signaling at the level of lipid rafts. Human monocytes and Chinese hamster ovary (CHO) cells, transfected with human TLR2, TLR4, or CD14, were stimulated with peptidoglycan (PGN, TLR2 ligand) or lipopolysaccharide (LPS, TLR4 ligand) with or without alcohol (50 mM) and analyzed for cytokine production (enzyme-linked immunosorbent assay), nuclear factor-kappaB (NF-kappaB) activation (electrophoretic mobility shift assay), membrane fluidity (fluorescent pyrene eximer formation), and partition of cellular membrane into cholesterol-rich, detergent-resistant domains (DRMs; Western blot).

RESULTS

We determined that both TLR2 and TLR4 were located outside the rafts; flotillin, a DRM marker, was resident in the rafts, while CD14 was equally distributed in and outside the rafts in a steady-state condition. PGN forced TLR2 to migrate into DRMs. Engagement of TLR4 and CD14 with LPS induced their migration into the rafts. Alcohol prevented TLR4 partitioning; however, it did not affect TLR2 migration into the rafts. Furthermore, alcohol downregulated TLR4-induced, but not TLR2-induced, NF-kappaB activation and cytokine production in monocytes. We found that alcohol increased membrane fluidity and depleted cellular cholesterol in CHO cells without affecting cell viability.

CONCLUSIONS

These data demonstrate for the first time that alcohol disturbs TLR4 and CD14 association with lipid rafts. We propose that alcohol-induced effects on lipid rafts may contribute to modulation of TLR4-CD14-triggered early cellular responses.

Quantitative proteomic analysis of B cell lipid rafts reveals that ezrin regulates antigen receptor-mediated lipid raft dynamics

Ligation of the B cell antigen receptor (BCR) with antigen induces lipid raft coalescence, a process that occurs after crosslinking of a variety of signaling receptors and is thought to potentiate cellular activation. To investigate lipid raft dynamics during BCR signaling, we quantitatively analyzed the B cell lipid raft proteome. BCR engagement induced dissociation of the adaptor protein ezrin from lipid rafts as well as threonine dephosphorylation of ezrin and its concomitant detachment from actin, indicating a transient uncoupling of lipid rafts from the actin cytoskeleton. Expression of constitutively active ezrin chimeras inhibited the BCR-induced coalescence of lipid rafts. Our data demonstrate that the release of ezrin from lipid rafts acts as a critical trigger that regulates lipid raft dynamics during BCR signaling.

Lipid rafts in Cryptococcus neoformans concentrate the virulence determinants phospholipase B1 and Cu/Zn superoxide dismutase

Lipid rafts have been identified in the membranes of mammalian cells, the yeast Saccharomyces cerevisiae, and the pathogenic fungus Candida albicans. Formed by a lateral association of sphingolipids and sterols, rafts concentrate proteins carrying a glycosylphosphatidylinositol (GPI) anchor. We report the isolation of membranes with the characteristics of rafts from the fungal pathogen Cryptococcus neoformans. These characteristics include insolubility in Triton X-100 (TX100) at 4 degrees C, more-buoyant density within a sucrose gradient than the remaining membranes, and threefold enrichment with sterols. The virulence determinant phospholipase B1 (PLB1), a GPI-anchored protein, was highly concentrated in raft membranes and could be displaced from them by treatment with the sterol-sequestering agent methyl-beta-cyclodextrin (MbetaCD). Phospholipase B enzyme activity was inhibited in the raft environment and increased 15-fold following disruption of rafts with TX100 at 37 degrees C. Treatment of viable cryptococcal cells in suspension with MbetaCD also released PLB1 protein and enzyme activity, consistent with localization of PLB1 in plasma membrane rafts prior to secretion. The antioxidant virulence factor Cu/Zn superoxide dismutase (SOD1) was concentrated six- to ninefold in raft membrane fractions compared with nonraft membranes, whereas the cell wall-associated virulence factor laccase was not detected in membranes. We hypothesize that raft membranes function to cluster certain virulence factors at the cell surface to allow efficient access to enzyme substrate and/or to provide rapid release to the external environment.

Membrane accumulation of influenza A virus hemagglutinin triggers nuclear export of the viral genome via protein kinase Calpha-mediated activation of ERK signaling

Replication and transcription of the influenza virus genome takes place exclusively within the nucleus of the infected cells. The viral RNA genome, polymerase subunits, and nucleoprotein form ribonucleoprotein (RNP) complexes. Late in the infectious cycle RNPs have to be exported from the nucleus to be enwrapped into budding progeny virions at the cell membrane. This process requires viral activation of the cellular Raf/MEK/ERK (mitogen-activated protein kinase (MAPK)) signaling cascade that is activated late in the infection cycle. Accordingly, block of the cascade results in retardation of RNP export and reduced titers of progeny virus. In the present study we have analyzed the importance of cell-membrane association of the viral hemagglutinin glycoprotein for viral MAPK activation. We show that hemagglutinin membrane accumulation and its tight association with lipid-raft domains trigger activation of the MAPK cascade via protein kinase Calpha activation and induces RNP export. This may represent an auto-regulative mechanism that coordinates timing of RNP export to a point when all viral components are ready for virus budding.

Essential role for cholesterol in the delivery of exogenous antigens to the MHC class I-presentation pathway

Cross-presentation, which is crucial for the generation of immunity against virus-infected and tumor cells, requires exogenous antigens to be internalized into antigen-presenting cells (APCs) followed by translocation to the cytosol by unknown mechanisms. One important entry route for such antigens is macropinocytosis. We here describe that cholesterol is essential for cross-presentation of antigens loaded via macropinocytosis into APCs. Modification of antigens by palmitoylation to target antigens to cholesterol-enriched plasma membrane domains resulted in a dramatically increased T cell activation. These results define cholesterol as an essential factor for cross-presentation and suggest that specific modification of antigens to increase their affinity for cholesterol may be utilized to enhance immunity.

Cholesterol and the interaction of proteins with membrane domains

Cholesterol is not uniformly distributed in biological membranes. One of the factors influencing the formation of cholesterol-rich domains in membranes is the unequal lateral distribution of proteins in membranes. Certain proteins are found in cholesterol-rich domains. In some of these cases, it is as a consequence of the proteins interacting directly with cholesterol. There are several structural features of a protein that result in the protein preferentially associating with cholesterol-rich domains. One of the best documented of these is certain types of lipidations. In addition, however, there are segments of a protein that can preferentially sequester cholesterol. We discuss two examples of these cholesterol-recognition elements: the cholesterol recognition/interaction amino acid consensus (CRAC) domain and the sterol-sensing domain (SSD). The requirements for a CRAC motif are quite flexible and predict that a large number of sequences could recognize cholesterol. There are, however, certain proteins that are known to interact with cholesterol-rich domains of cell membranes that have CRAC motifs, and synthetic peptides corresponding to these segments also promote the formation of cholesterol-rich domains. Modeling studies have provided a rationale for certain requirements of the CRAC motif. The SSD is a larger protein segment comprising five transmembrane domains. The amino acid sequence YIYF is found in several SSD and in certain other proteins for which there is evidence that they interact with cholesterol-rich domains. The CRAC sequences as well as YIYF are generally found adjacent to a transmembrane helical segment. These regions appear to have a strong influence of the localization of certain proteins into domains in biological membranes. In addition to the SSD, there is also a domain found in soluble proteins, the START domain, that binds lipids. Certain proteins with START domains specifically bind cholesterol and are believed to function in intracellular cholesterol transport. One of these proteins is StAR-D1, that also has a mitochondrial targeting sequence and plays an important role in delivering cholesterol to the mitochondria of steroidogenic cells.

Fas ligand is localized to membrane rafts, where it displays increased cell death–inducing activity

Fas ligand (FasL), a member of the TNF protein family, potently induces cell death by activating its matching receptor Fas. Fas-mediated killing plays a critical role in naturally and pathologically occurring cell death, including development and homeostasis of the immune system. In addition to its receptor-interacting and cell death–inducing extracellular domain, FasL has a well-conserved intracellular portion with a proline-rich SH3 domain–binding site probably involved in non-apoptotic functions. We report here that, as with the Fas receptor, a fraction of FasL is constitutively localized in rafts. These dynamic membrane microdomains, enriched in sphingolipids and cholesterol, are important for cell signaling and trafficking processes. We show that FasL is partially localized in rafts and that increased amounts of FasL are found in rafts after efficient FasL/Fas receptor interactions. Raft disorganization after cholesterol oxidase treatment and deletions within the intracellular FasL domain diminish raft partitioning and, most important, lead to decreased FasL killing. We conclude that FasL is recruited into lipid rafts for maximum Fas receptor contact and cell death–inducing potency. These findings raise the possibility that certain pathologic conditions may be treated by altering the cell death–inducing capability of FasL with drugs affecting its raft localization.

Location of major histocompatibility complex class II molecules in rafts on dendritic cells enhances the efficiency of T-cell activation and proliferation

The existence of major histocompatibility complex (MHC) class II molecules in lipid rafts has been described in dendritic cells (DC); however, the importance of rafts in T-cell activation has not been clarified. In this study, the distribution of the lipid raft components (CD59 and GM1 ganglioside) in human monocyte-derived DC was investigated. DC had an even distribution of these components at the cell surface. In addition, raft-associated GM1 ganglioside colocalized with cross-linked MHC class II. This implies coaggregation of raft components with these MHC molecules, which may be important in the interaction between T cells and antigen-presenting cells. In studies carried out to investigate the effect of the DC : T-cell interaction on raft distribution, we found a clustering of the lipid raft component CD59 on DC at the synaptic interface, with associated activation of the interacting T cell. In an antigen-specific response between DC and CD4+ T-cell clones, disruption of lipid rafts resulted in inhibition of both CD59 clustering and T-cell activation. This was most pronounced when limiting amounts of cognate peptide were used. Together, these data demonstrate the association of MHC class II with lipid rafts during DC : T-cell interaction and suggest an important role for DC lipid rafts in T-cell activation.

Single-cell analysis of lipid rafts in lymphocytes and in T cell-containing immunoconjugates

Within the plasma membranes of many different cell types, certain membrane lipids, including cholesterol and sphingolipids, form lateral assemblies surrounded by unsaturated glycerophospholipids. The concentration of such membrane lipids and associated proteins results in the formation of microdomains termed lipid rafts" (or glycolipid-enriched membranes or detergent-insoluble glycosphingolipid-enriched domains). Proteins that possess saturated acyl chains are generally associated with lipid rafts. Lipid rafts are believed to be involved in a number of cellular processes including cell activation. When material is limiting, raft-associated proteins may be identified on single cells using microscopy. This unit describes the application of this technique in an immunological example, examining the location and movement of signal transduction complexes in single T lymphocytes and in interactive conjugates between T cells and antigen-presenting cells (APCs).

Biochemical and functional characterization of the membrane association and membrane permeabilizing activity of the severe acute respiratory syndrome coronavirus envelope protein

A diverse group of cytolytic animal viruses encodes small, hydrophobic proteins to modify host cell membrane permeability to ions and small molecules during their infection cycles. In this study, we show that expression of the SARS-CoV E protein in mammalian cells alters the membrane permeability of these cells. Immunofluorescent staining and cell fractionation studies demonstrate that this protein is an integral membrane protein. It is mainly localized to the ER and the Golgi apparatus. The protein can be translocated to the cell surface and is partially associated with lipid rafts. Further biochemical characterization of the protein reveals that it is posttranslationally modified by palmitoylation on all three cysteine residues. Systematic mutagenesis studies confirm that the membrane permeabilizing activity of the SARS-CoV E protein is associated with its transmembrane domain.

Recycling of MUC1 is dependent on its palmitoylation

MUC1 is a mucin-like transmembrane protein expressed on the apical surface of epithelia, where it protects the cell surface. The cytoplasmic domain has numerous sites for phosphorylation and docking of proteins involved in signal transduction. In a previous study, we showed that the cytoplasmic YXXphi motif Y20HPM and the tyrosine-phosphorylated Y60TNP motif are required for MUC1 clathrin-mediated endocytosis through binding AP-2 and Grb2, respectively (Kinlough, C. L., Poland, P. A., Bruns, J. B., Harkleroad, K. L., and Hughey, R. P. (2004) J. Biol. Chem. 279, 53071-53077). Palmitoylation of transmembrane proteins can affect their membrane trafficking, and the MUC1 sequence CQC3RRK at the boundary of the transmembrane and cytoplasmic domains mimics reported site(s) of S-palmitoylation. [3H]Palmitate labeling of Chinese hamster ovary cells expressing MUC1 with mutations in CQC3RRK revealed that MUC1 is dually palmitoylated at the CQC motif independent of RRK. Lack of palmitoylation did not affect the cold detergent solubility profile of a chimera (Tac ectodomain and MUC1 transmembrane and cytoplasmic domains), the rate of chimera delivery to the cell surface, or its half-life. Calculation of rate constants for membrane trafficking of wild-type and mutant Tac-MUC1 indicated that the lack of palmitoylation blocked recycling, but not endocytosis, and caused the chimera to accumulate in a EGFP-Rab11-positive endosomal compartment. Mutations CQC/AQA and Y20N inhibited Tac-MUC1 co-immunoprecipitation with AP-1, although mutant Y20N had reduced rates of both endocytosis and recycling, but a normal subcellular distribution. The double mutant chimera AQA+Y20N had reduced endocytosis and recycling rates and accumulated in EGFP-Rab11-positive endosomes, indicating that palmitoylation is the dominant feature modulating MUC1 recycling from endosomes back to the plasma membrane.

Palmitoylations on murine coronavirus spike proteins are essential for virion assembly and infectivity

Coronavirus spike (S) proteins are palmitoylated at several cysteine residues clustered near their transmembrane-spanning domains. This is achieved by cellular palmitoyl acyltransferases (PATs), which can modify newly synthesized S proteins before they are assembled into virion envelopes at the intermediate compartment of the exocytic pathway. To address the importance of these fatty acylations to coronavirus infection, we exposed infected cells to 2-bromopalmitate (2-BP), a specific PAT inhibitor. 2-BP profoundly reduced the specific infectivities of murine coronaviruses at very low, nontoxic doses that were inert to alphavirus and rhabdovirus infections. 2-BP effected only two- to fivefold reductions in S palmitoylation, yet this correlated with reduced S complexing with virion membrane (M) proteins and consequent exclusion of S from virions. At defined 2-BP doses, underpalmitoylated S proteins instead trafficked to infected cell surfaces and elicited cell-cell membrane fusions, suggesting that the acyl chain adducts are more critical to virion assembly than to S-induced syncytial developments. These studies involving pharmacologic inhibition of S protein palmitoylation were complemented with molecular genetic analyses in which cysteine acylation substrates were mutated. Notably, some mutations (C1347F and C1348S) did not interfere with S incorporation into virions, indicating that only a subset of the cysteine-rich region provides the essential S-assembly functions. However, the C1347F/C1348S mutant viruses exhibited relatively low specific infectivities, similar to virions secreted from 2-BP-treated cultures. Our collective results indicate that the palmitate adducts on coronavirus S proteins are necessary in assembly and also in positioning the assembled envelope proteins for maximal infectivity.

Visualizing association of N-ras in lipid microdomains: influence of domain structure and interfacial adsorption

In this study, two-photon fluorescence microscopy on giant unilamellar vesicles and tapping-mode atomic force microscopy (AFM) are applied to follow the insertion of a fluorescently (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene, BODIPY) labeled and completely lipidated (hexadecylated and farnesylated) N-Ras protein into heterogeneous lipid bilayer systems. The bilayers consist of the canonical raft mixture 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), sphingomyelin, and cholesterol, which-depending on the concentration of the constituents-separates into liquid-disordered (l(d)), liquid-ordered (l(o)), and solid-ordered (s(o)) phases. The results provide direct evidence that partitioning of N-Ras occurs preferentially into liquid-disordered lipid domains, which is also reflected in a faster kinetics of incorporation into the fluid lipid bilayers. The phase sequence of preferential binding of N-Ras to mixed-domain lipid vesicles is l(d) > l(o) >> s(o). Intriguingly, we detect, using the better spatial resolution of AFM, also a large proportion of the lipidated protein located at the l(d)/l(o) phase boundary, thus leading to a favorable decrease in line tension that is associated with the rim of the demixed phases. Such an interfacial adsorption effect may serve as an alternative vehicle for association processes of signaling proteins in membranes.

The polybasic juxtamembrane region of Sso1p is required for SNARE function in vivo

Exocytosis in Saccharomyces cerevisiae requires the specific interaction between the plasma membrane t-SNARE complex (Sso1/2p;Sec9p)and a vesicular v-SNARE (Snc1/2p). While SNARE proteins drive membrane fusion, many aspects of SNARE assembly and regulation are ill defined. Plasma membrane syntaxin homologs (including Sso1p) contain a highly charged juxtamembrane region between the transmembrane helix and the "SNARE domain" or core complex domain. We examined this region in vitro and in vivo by targeted sequence modification, including insertions and replacements. These modified Sso1 proteins were expressed as the sole copy of Sso in S. cerevisiae and examined for viability. We found that mutant Sso1 proteins with insertions or duplications show limited function, whereas replacement of as few as three amino acids preceding the transmembrane domain resulted in a nonfunctional SNARE in vivo. Viability is also maintained when two proline residues are inserted in the juxtamembrane of Sso1p, suggesting that helical continuity between the transmembrane domain and the core coiled-coil domain is not absolutely required. Analysis of these mutations in vitro utilizing a reconstituted fusion assay illustrates that the mutant Sso1 proteins are only moderately impaired in fusion. These results suggest that the sequence of the juxtamembrane region of Sso1p is vital for function in vivo, independent of the ability of these proteins to direct membrane fusion.

Docosahexaenoic acid selectively inhibits plasma membrane targeting of lipidated proteins

Membrane localization of lipidated cytosolic signaling proteins is mediated by interactions between specific lipid anchors and membranes, but little is known about the regulatory role of membrane composition in lipidated protein membrane targeting. Here, using green fluorescent protein (GFP) chimeras and quantitative fluorescence microscopy in living mouse colonocytes, we show that docosahexaenoic acid (DHA), a dietary polyunsaturated fatty acid (PUFA) with membrane lipid-modifying properties, selectively inhibits plasma membrane (PM) targeting and increases the endomembrane localization of lipidated proteins that are cytoplasmic cargo in the exocytic pathway, without affecting the exocytic pathway itself. DHA selectivity seems to be dictated by the protein trafficking route, independent of the functional state of proteins and the location and composition of membrane anchors. DHA enrichment in cell membranes was required to elicit the inhibitory effect. These data reveal that membrane lipid composition influences cell signaling by modulating intracellular trafficking and localization of membrane proteins, providing a potential molecular mechanism for the documented health benefits of DHA.

Role of lipids in the MAPK signaling pathway

The mitogen-activated protein kinase (MAPK) signaling pathway is activated in response to a variety of extracellular stimuli such as growth factor stimulation. The best-characterized MAPK pathway involves the sequential activation of Raf, MEK and ERK proteins, capable of regulating the gene expression required for cell proliferation. Binding to specific lipids can regulate both the subcellular localization of these MAPK signaling proteins as well as their kinase activities. More recently it has become increasingly clear that the majority of MAPK signaling takes place intracellularly on endosomes and that the perturbation of endocytic pathways has dramatic effects on the MAPK pathway. This review highlights the direct effects of lipids on the localization and regulation of MAPK pathway proteins. In addition, the indirect effects lipids have on MAPK signaling via their regulation of endocytosis and the biophysical properties of different membrane lipids as a result of growth factor stimulation are discussed. The ability of a protein to bind to both lipids and proteins at the same time may act like a "ZIP code" to target that protein to a highly specific microlocation and could also allow a protein to be "handed off" to maintain tight control over its binding partners and location.

Role of chirality in peptide-induced formation of cholesterol-rich domains

The chiral specificity of the interactions of peptides that induce the formation of cholesterol-rich domains has not been extensively investigated. Both the peptide and most lipids are chiral, so there is a possibility that interactions between peptide and lipid could require chiral recognition. On the other hand, in our models with small peptides, the extent of folding of the peptide to form a specific binding pocket is limited. We have determined that replacing cholesterol with its enantiomer, ent-cholesterol, alters the modulation of lipid organization by peptides. The phase-transition properties of SOPC (1-stearoyl-2-oleoylphosphatidylcholine):cholesterol [in a 6:4 ratio with 0.2 mol% PtdIns(4,5)P2] are not significantly altered when ent-cholesterol replaces cholesterol. However, in the presence of 10 mol% of a 19-amino-acid, N-terminally myristoylated fragment (myristoyl-GGKLSKKKKGYNVNDEKAK-amide) of the protein NAP-22 (neuronal axonal membrane protein), the lipid mixture containing cholesterol undergoes separation into cholesterol-rich and cholesterol-depleted domains. This does not occur when ent-cholesterol replaces cholesterol. In another example, when N-acetyl-Leu-Trp-Tyr-Ile-Lys-amide (N-acetyl-LWYIK-amide) is added to SOPC:cholesterol (7:3 ratio), there is a marked increase in the transition enthalpy of the phospholipid, indicating separation of a cholesterol-depleted domain of SOPC. This phenomenon completely disappears when ent-cholesterol replaces cholesterol. The all-D-isomer of N-acetyl-LWYIK-amide also induces the formation of cholesterol-rich domains with natural cholesterol, but does so to a lesser extent with ent-cholesterol. Thus specific peptide chirality is not required for interaction with cholesterol-containing membranes. However, a specific chirality of membrane lipids is required for peptide-induced formation of cholesterol-rich domains.

A live cell, image-based approach to understanding the enzymology and pharmacology of 2-bromopalmitate and palmitoylation

The addition of a lipid moiety to a protein increases its hydrophobicity and subsequently its attraction to lipophilic environments like membranes. Indeed most lipid-modified proteins are localized to membranes where they associate with multiprotein signaling complexes. Acylation and prenylation are the two common categories of lipidation. The enzymology and pharmacology of prenylation are well understood but relatively very little is known about palmitoylation, the most common form of acylation. One distinguishing characteristic of palmitoylation is that it is a dynamic modification. To understand more about how palmitoylation is regulated, we fused palmitoylation substrates to fluorescent proteins and reported their subcellular distribution and trafficking. We used automated high-throughput fluorescence microscopy and a specialized computer algorithm to image and measure the fraction of palmitoylation reporter on the plasma membrane versus the cytoplasm. Using this system we determined the residence half-life of palmitate on the dipalmitoyl substrate peptide from GAP43 as well as the EC(50) for 2-bromopalmitate, a common inhibitor of palmitoylation.

Role of cholesterol and lipid organization in disease

Membrane lipids are essential for biological functions ranging from membrane trafficking to signal transduction. The composition of lipid membranes influences their organization and properties, so it is not surprising that disorders in lipid metabolism and transport have a role in human disease. Significant recent progress has enhanced our understanding of the molecular and cellular basis of lipid-associated disorders such as Tangier disease, Niemann-Pick disease type C and atherosclerosis. These insights have also led to improved understanding of normal physiology.

Thermodynamics of membrane domains

The concept of lipid rafts and the intense work toward their characterization in biological membranes has spurred a renewed interest in the understanding of domain formation, particularly in the case of cholesterol-containing membranes. The thermodynamic principles underlying formation of domains, rafts, or cholesterol/phospholipid complexes are reviewed here, along with recent work in model and biological membranes. A major motivation for this review was to present those concepts in a way appropriate for the broad readership that has been drawn to the field. Evidence from a number of different techniques points to the conclusion that lipid-lipid interactions are generally weak; therefore, in most cases, massive phase separations are not to be expected in membranes. On the contrary, small, dynamic lipid domains, possibly stabilized by proteins are the most likely outcome. The results on mixed lipid bilayers are used to discuss recent experiments in biological membranes. The clear indication is that proteins partition preferentially into fluid, disordered lipid domains, which is contrary to their localization in ordered, cholesterol/sphingomyelin rafts inferred from detergent extraction experiments on cell membranes. Globally, the evidence appears most consistent with a membrane model in which the majority of the lipid is in a liquid-ordered phase, with dispersed, small, liquid-disordered domains, where most proteins reside. Co-clustering of proteins and their concentration in some membrane areas may occur because of similar preferences for a particular domain but also because of simultaneous exclusion from other lipid phases. Specialized structures, such as caveolae, which contain high concentrations of cholesterol and caveolin are not necessarily similar to bulk liquid-ordered phase.

How principles of domain formation in model membranes may explain ambiguities concerning lipid raft formation in cells

Sphingolipid and cholesterol-rich liquid ordered lipid domains (lipid rafts) have been studied in both eukaryotic cells and model membranes. However, while the coexistence of ordered and disordered liquid phases can now be easily demonstrated in model membranes, the situation in cell membranes remains ambiguous. Unlike the usual situation in model membranes, under most conditions, cell membranes rich in sphingolipid and cholesterol may have a "granular" organization in which the size of ordered and/or disordered domains is extremely small and domains may be of borderline stability. This review attempts to explain the origin of the divergence between of our understanding of rafts in model membranes and in cells, and how the physical properties of model membranes can help explain many of the ambiguities concerning raft formation and properties in cells. How physical principles of ordered domain formation relate to limitations of detergent insolubility and cholesterol depletion methods used to infer the presence of rafts in cells is also discussed. Possible modifications of these techniques that may increase their reliability are considered. It will be necessary to study model membrane systems more closely approximating cell membranes in order gain a complete understanding of raft properties in cells. Very high concentrations of membrane cholesterol and proteins may explain key physical characteristics of domains in cellular membranes, and are the two of the most obvious factors requiring additional study.

Transmembrane sequences are determinants of immunoreceptor signaling

To investigate structural features critical for signal initiation by Ag-stimulated immunoreceptors, we constructed a series of single-chain chimeric receptors that incorporate extracellular human Fc epsilonRIalpha for IgE binding, a variable transmembrane (TM) segment, and the ITAM-containing cytoplasmic tail of the TCR zeta-chain. We find that functional responses mediated by these receptors are strongly dependent on their TM sequences, and these responses are highly correlated to cross-link-dependent association with detergent-resistant lipid rafts. For one chimera designated alpha Fzeta, mutation of a TM cysteine abolishes robust signaling and lipid raft association. In addition, TM disulfide-mediated oligomerization of another chimeric receptor, alpha zetazeta, enhances signaling. These results demonstrate an important role for TM segments in immunoreceptor signaling and a strong correspondence between strength of signaling and cross-link-dependent partitioning into ordered membrane domains.

Influenza virus hemagglutinin (H3 subtype) requires palmitoylation of its cytoplasmic tail for assembly: M1 proteins of two subtypes differ in their ability to support assembly

The influenza A virus hemagglutinin (HA) transmembrane domain boundary region and the cytoplasmic tail contain three cysteines (residues 555, 562, and 565 for the H3 HA subtype) that are highly conserved among the 16 HA subtypes and which are each modified by the covalent addition of palmitic acid. Previous analysis of the role of these conserved cysteine residues led to differing data, suggesting either no role for HA palmitoylation or an important role for HA palmitoylation. To reexamine the role of these residues in the influenza virus life cycle, a series of cysteine-to-serine mutations were introduced into the HA gene of influenza virus A/Udorn/72 (Ud) (H3N2) by using a highly efficient reverse genetics system. Mutant viruses containing HA-C562S and HA-C565S mutations had reduced growth and failed to form plaques in MDCK cells but formed wild-type-like plaques in an MDCK cell line expressing wild-type HA. In cell-cell fusion assays, nonpalmitoylated H3 HA, in both cDNA-transfected and virus-infected cells, was fully competent for HA-mediated membrane fusion. When the HA cytoplasmic tail cysteine mutants were examined for lipid raft association, using as the criterion Triton X-100 insolubility, loss of raft association did not show a direct correlation with a reduction in virus replication. However, mutant virus assembly was reduced in parallel with reduced virus replication. Additionally, a reassortant of strain A/WSN/33 (WSN), containing the Ud HA gene with mutations C555S, C562S, and C565S, produced virus that could form plaques on regular MDCK cells and had only moderately decreased replication, suggesting differences in the interactions between Ud and WSN HA and internal viral proteins. Analysis of M1 mutants containing substitutions in the six residues that differ between the Ud and WSN M1 proteins indicated that a constellation of residues are responsible for the difference between the M1 proteins in their ability to support virus assembly with nonpalmitoylated H3 HA.

Acute in vivo elevation of intravascular triacylglycerol lipolysis impairs peripheral T cell activation in humans

BACKGROUND

Previous studies have shown suppressive effects of polyunsaturated fatty acids (PUFAs) on T cell proliferation, but the precise mechanism for this effect has not been fully investigated in vivo in humans.

OBJECTIVE

The objective was to determine whether this effect is the result of altered T cell membrane properties and impaired CD3- and CD28-mediated signaling in vivo in humans.

DESIGN

Peripheral T cells were isolated from healthy subjects before and 2 h after an intravenous infusion of heparin plus a PUFA-rich lipid emulsion during a euglycemic hyperinsulinemic clamp to induce a 2.5-fold elevation in plasma linoleic acid concentration without significant change in plasma total free fatty acid concentrations.

RESULTS

Intravenous infusion of heparin plus the lipid emulsion reduced peripheral T cell membrane fluidity and altered lipid raft organization, both of which were associated with reduced T cell proliferation after stimulation with CD3 plus CD28. Tyrosine phosphorylation of linker of activated T cells and activation of protein kinase B in T cells were also impaired without a reduction in T cell receptor expression. In addition, acute PUFA elevation was associated with a reduction in T cell membrane cholesterol exchange with the cellular milieu ex vivo.

CONCLUSIONS

A selective increase in plasma linoleic acid concentration and in intravascular lipolysis has a suppressive effect on peripheral T cell CD28-dependent activation, and this effect is associated with changes in plasma membrane properties. Our results have important implications for nutritional therapy in patients at high risk of septic complications and may also be of relevance to postprandial lipid metabolism disorders such as insulin resistance and type 2 diabetes.

Influenza virus assembly and budding in raft-derived microdomains: a quantitative analysis of the surface distribution of HA, NA and M2 proteins

Influenza virus hemagglutinin (HA) and neuraminidase (NA) are known to associate with lipid rafts, membrane microdomains comprised of densely packed cholesterol and sphingolipids. These specialized membrane regions are believed to be involved in the budding of many enveloped viruses including influenza virus. Quantitative analysis of HA distribution on the surface of virus-infected cells by immunogold staining shows an organization into clusters that grow in size as the expression level of HA increases with time post-infection (p.i.) ( approximately 325-500 nm at 4 h p.i. and approximately 425-600 nm at 6 h p.i.). These HA-containing clusters are likely derived from lipid rafts as they contain a high density of the raft marker ganglioside GM1 and are dependent upon the presence of cholesterol. The clustering of HA is an intrinsic property of the HA protein and occurs in the absence of expression of other viral proteins. NA is also found sequestered within the same microdomains as HA, whereas the M2 ion channel protein does not concentrate within the raft-like microdomains. Quantification of the distribution of surface expressed HA by examining serial sections of virus-infected cells suggests that the HA-containing microdomains give rise to regions of influenza assembly and budding.

Effects of farnesol on the physical properties of DMPC membranes

Farnesol interacts with membranes in a wide variety of biological contexts, yet our understanding of how it affects lipid bilayers is not yet complete. This study investigates how the 15-carbon isoprenoid, farnesol, influences the phase behaviour, lateral organization, and mechanical stability of dimyristol phosphatidylcholine (DMPC) model membranes. Differential scanning calorimetry (DSC) of multilamellar DMPC-farnesol mixtures (up to 26 mol% farnesol) demonstrates how this isoprenoid lowers and broadens the gel-fluid phase transition. A gel-fluid coexistence region becomes progressively more dominant with increasing farnesol concentration and at concentrations of and greater than 10.8 mol%, an upper transition emerges at about 35 degrees C. Atomic force microscopy images of supported farnesol-DMPC bilayers containing 10 and 20 mol% farnesol provide structural evidence of gel-fluid coexistence around the main transition. Above this coexistence region, membranes exhibit homogeneous lateral organization but at temperatures below the main gel-fluid coexistence region, another form of phase coexistence is observed. The solid nature of the gel phase is confirmed using micropipette aspiration. The combined thermodynamic, structural, and mechanical data allow us to construct a phase diagram. Our results show that farnesol preferentially partitions into the fluid phase and induces phase coexistence in membranes below the main transition of the pure lipid.

Chimeric GnRH-LH receptors and LH receptors lacking C-terminus palmitoylation sites do not localize to plasma membrane rafts

Luteinizing hormone and gonadotropin releasing hormone receptors (LHR and GnRHR, respectively) are G protein-coupled receptors with important functions in reproduction. We have developed chimeric GnRHR-LHR that contain the full GnRHR coupled to various forms of the LH receptor C-terminus to explore the role of the LH receptor C-terminus in raft localization of the receptor and signaling. Addition of the full-length LHR C-terminus to GnRHR resulted in localization of the resting chimeric receptor in the bulk membrane rather than plasma membrane rafts as has been reported for the wild-type GnRHR [A. Navratil, S. Bliss, K. Berghorn, J. Haughian, T. Farmerie, J. Graham, C. Clay, M. Roberson, Constitutive localization of the gonadotropin-releasing hormone (GnRH) receptor to low density membrane microdomains is necessary for GnRH signaling to ERK, J. Biol. Chem. 278 (2003) 31593-31602]. With truncation of the LHR C-terminus, approximately 3% of chimeric receptors appeared in low density membrane fractions. Palmitoylation of sites on the LHR C-terminus appears important for raft localization. Mutations to C-terminus palmitoylation sites eliminated translocation of LH receptors from the bulk membrane to rafts upon binding of hCG although these mutant receptors retained the ability to signal via cAMP.

Two types of detergent-insoluble, glycosphingolipid/cholesterol-rich membrane domains from isolated myelin

Two different types of low-density detergent-insoluble glycosphingolipid-enriched membrane domain (DIG) fractions were isolated from myelin by extraction with Triton X-100 (TX-100) in 50 mM sodium phosphate buffer at room temperature (20 degrees C) (procedure 1), in contrast to a single low-density fraction obtained by extraction with TX-100 in Tris buffer containing 150 mM NaCl and 5 mM EDTA at 4 degrees C (procedure 2). Procedure 1 has been used in the past by others for myelin extraction to preserve the cytoskeleton and/or radial component of oligodendrocytes and myelin, whereas procedure 2 is now more commonly used to isolate myelin DIG fractions. The two DIG fractions obtained by procedure 1 gave opaque bands, B1 and B2, at somewhat lower and higher sucrose density respectively than myelin itself. The single DIG fraction obtained by procedure 2 gave a single opaque band at a similar sucrose density to B1. Both B1 and B2 had characteristics of lipid rafts, i.e. high galactosylceramide and cholesterol content and enrichment in GPI-linked 120-kDa neural cell adhesion molecule (NCAM)120, as found by others for the single low-density DIG fraction obtained by procedure 2. However, B2 had most of the myelin GM1 and more of the sulfatide than B1, and they differed significantly in their protein composition. B2 contained 41% of the actin, 100% of the tubulin, and most of the flotillin-1 and caveolin in myelin, whereas B1 contained more NCAM120 and other proteins than B2. The single low-density DIG fraction obtained by procedure 2 contained only low amounts of actin and tubulin. B1 and B2 also had size-isoform selectivity for some proteins, suggesting specific interactions and different functions of the two membrane domains. We propose that B1 may come from non-caveolar raft domains whereas B2 may derive from caveolin-containing raft domains associated with cytoskeletal proteins. Some kinases present were active on myelin basic protein suggesting that the DIGs may come from signaling domains.

Cholesterol content regulates acrosomal exocytosis by enhancing Rab3A plasma membrane association

The acrosome is an exocytic granule that overlies the spermatozoan nucleus. In response to different stimuli, it undergoes calcium-regulated exocytosis. Freshly ejaculated mammalian sperm are not immediately capable of undergoing acrosome reaction. The acquisition of this ability is called capacitation and involves a series of still not well-characterized changes in the sperm physiology. Plasma membrane cholesterol removal is one of the sperm modifications that are associated with capacitation. However, how sterols affect acrosomal exocytosis is unknown. Here, we show that short incubations with cyclodextrin, a cholesterol removal agent, just before stimulation promote acrosomal exocytosis. Moreover, the effect was also observed in permeabilized cells stimulated with calcium, indicating that cholesterol plays a direct role in the calcium-dependent exocytosis associated with acrosome reaction. Using a photo-inhibitable calcium chelator, we show that cholesterol affects an early event of the exocytic cascade rather than the lipid bilayers mixing. Functional data indicate that one target for the cholesterol effect is Rab3A. The sterol content does not affect the Rab3A activation-deactivation cycle but regulates its membrane anchoring. Western blot analysis and immunoelectron microscopy confirmed that cholesterol efflux facilitates Rab3A association to sperm plasma membrane. Our data indicate that the cholesterol efflux occurring during capacitation optimizes the conditions for the productive assembly of the fusion machinery required for acrosome reaction.

Mastoparan changes the cellular localization of Galphaq/11 and Gbeta through its binding to ganglioside in lipid rafts

Although it is known that mastoparan, a wasp venom toxin, directly activates Gi/o, mastoparan-induced biological responses are not always explained by this mechanism. For instance, we have demonstrated previously that mastoparan suppressed phosphoinositide hydrolysis induced by carbachol in human astrocytoma cells (FEBS Lett 206:91-94, 1990). In the present study, we examined whether mastoparan affected phosphoinositide hydrolysis by interacting with lipid rafts in PC-12 cells. Mastoparan inhibited UTP-induced increase in [Ca2+]i and phosphoinositide hydrolysis in a concentration-dependent manner. UTP-induced phosphoinositide hydrolysis occurred in lipid rafts, because methyl-beta-cyclodextrin, a disrupting regent of lipid rafts, inhibited the hydrolysis. Mastoparan changed the localization of Galphaq/11 and Gbeta together with cholesterol from lipid rafts to nonraft fractions or cytosol. These changes were inhibited by ganglioside mixtures, suggesting that mastoparan interacts with gangliosides in lipid rafts. In fact, ganglioside mixtures and neuraminidase, but not sialic acid, attenuated the inhibitory effect of mastoparan on phosphoinositide hydrolysis. Furthermore, fluorescence intensity of tyrosine residue of [Tyr3]mastoparan was potentiated by ganglioside mixtures, suggesting the direct binding of mastoparan to gangliosides. Mastoparan caused cytotoxicity of PC-12 cells in a concentration-dependent manner, determined by LDH release. The mastoparan-induced cytotoxicity was significantly inhibited by neuraminidase or gangliosides. The order of inhibitory potency of gangliosides was GT1b approximately GD1b > GD1a > GM1 >> GQ1b, but asialo-GM1 and sialic acid were inactive. These results suggest that mastoparan initially binds to gangliosides in lipid rafts and then it inhibits phosphoinositide hydrolysis by changing the localization of Galphaq/11 and Gbeta in lipid rafts.

Modified cell ELISA to determine the solubilization of cell surface proteins: Applications in GPI-anchored protein purification

A major step in purifying membrane bound proteins involves the solubilization of the protein of interest from the cell membranes. Glycosylphosphatidyl inositol (GPI)-anchored proteins pose a singular problem in this solubilization step since they are found in detergent-resistant membrane complexes and accordingly are insoluble in cold Triton X-100. In this study we have developed a modified cell ELISA that determines the solubility of these cell surface proteins under various solubilization conditions. Using this non-radioactive method we show that the combination of saponin/Triton X-100 at 4 degrees C solubilized GPI-anchored proteins more efficiently than Triton X-100 at 4 degrees C. The combination of saponin/Triton X-100 at 4 degrees C avoids the potential of activating proteases that occurs when using Triton X-100 at 37 degrees C. Furthermore, our method also shows the saponin/Triton X-100 solubilized GPI-anchored proteins equivalent to the more expensive octyl beta-glucoside. This is a particularly important consideration in large-scale protein purification. This method obviates the need to use radioactivity, gel electrophoresis and immunoblotting procedures. The solubilization conditions determined by this modified ELISA are readily translated to the practical application of large-scale protein purification as demonstrated in the purification of two different recombinant GPI-anchored proteins, GPI-hB7-1 (CD80) and GPI-mICAM-1 (CD54).

Receptors, rafts, and microvesicles in thrombosis and inflammation

Hemostasis at sites of blood vessel injury and its pathologic counterpart, thrombosis, involve a complex interplay between several blood elements: soluble proteins of the blood coagulation system, blood cells (most prominently platelets) and cell fragments, and elements of the vessel wall (endothelial cells and, at sites of injury, the exposed matrix and deeper cellular components). In this review, we focus on ways in which specialized membrane microdomains known as lipid rafts are involved in various phases of hemostasis and thrombosis.

Platelet GPIb-IX-V-dependent signaling

Although the signaling pathways related to GPIb-IX-V have not been fully elucidated, an accumulating body of evidence suggests that phospholipase C (PLC)gamma2 activation, subsequent Ca++ release and oscillations constitute an essential signal transduction pathway related to GPIb-IX-V. Src family kinases are required for PLCgamma2 activation, while FcR gamma-chain/Fc gammaRIIA may be dispensable for PLCgamma2 activation. Although PI-3K serves to potentiate various signaling events culminating in alpha(IIb)beta3 activation, PI-3K activity may be dispensable for Src-PLCgamma2 activation in GPIb-IX-V-mediated signaling. Glycosphingolipid-enriched microdomains (GEMs) appear to provide platforms for the signal transduction pathway related to GIb-IX-V, as the interaction between GPIb-IX-V and Src or PLCgamma2 tyrosine phosphorylation occurs exclusively in GEMs.

Epidermal growth factor receptors are localized to lipid rafts that contain a balance of inner and outer leaflet lipids: a shotgun lipidomics study

The epidermal growth factor (EGF) receptor partitions into lipid rafts made using a detergent-free method, but is extracted from low density fractions by Triton X-100. By screening several detergents, we identified Brij 98 as a detergent in which the EGF receptor is retained in detergent-resistant membrane fractions. To identify the difference in lipid composition between those rafts that harbored the EGF receptor (detergent-free and Brij 98-resistant) and those that did not (Triton X-100-resistant), we used multidimensional electrospray ionization mass spectrometry to perform a lipidomics study on these three raft preparations. Although all three raft preparations were similarly enriched in cholesterol, the EGF receptor-containing rafts contained more ethanolamine glycerophospholipids and less sphingomyelin than did the non-EGF receptor-containing Triton X-100 rafts. As a result, the detergent-free and Brij 98-resistant rafts exhibited a balance of inner and outer leaflet lipids, whereas the Triton X-100 rafts contained a preponderance of outer leaflet lipids. Furthermore, in all raft preparations, the outer leaflet phospholipid species were significantly different from those in the bulk membrane, whereas the inner leaflet lipids were quite similar to those found in the bulk membrane. These findings indicate that the EGF receptor is retained only in rafts that exhibit a lipid distribution compatible with a bilayer structure and that the selection of phospholipids for inclusion into rafts occurs mainly on the outer leaflet lipids.

Acylation-mediated membrane anchoring of avian influenza virus hemagglutinin is essential for fusion pore formation and virus infectivity

Attachment of palmitic acid to cysteine residues is a common modification of viral glycoproteins. The influenza virus hemagglutinin (HA) has three conserved cysteine residues at its C terminus serving as acylation sites. To analyze the structural and functional roles of acylation, we have generated by reverse genetics a series of mutants (Ac1, Ac2, and Ac3) of fowl plague virus (FPV) containing HA in which the acylation sites at positions 551, 559, and 562, respectively, have been abolished. When virus growth in CV1 and MDCK cells was analyzed, similar amounts of virus particles were observed with the mutants and the wild type. Protein patterns and lipid compositions, characterized by high cholesterol and glycolipid contents, were also indistinguishable. However, compared to wild-type virus, Ac2 and Ac3 virions were 10 and almost 1,000 times less infectious, respectively. Fluorescence transfer experiments revealed that loss of acyl chains impeded formation of fusion pores, whereas hemifusion was not affected. When the affinity to detergent-insoluble glycolipid (DIG) domains was analyzed by Triton X-100 treatment of infected cells and virions, solubilization of Ac2 and Ac3 HAs was markedly facilitated. These observations show that acylation of the cytoplasmic tail, while not necessary for targeting to DIG domains, promotes the firm anchoring and retention of FPV HA in these domains. They also indicate that tight DIG association of FPV HA is essential for formation of fusion pores and thus probably for infectivity.

Increased phosphorylation and redistribution of NMDA receptors between synaptic lipid rafts and post-synaptic densities following transient global ischemia in the rat brain

Ischemia results in increased phosphorylation of NMDA receptors. To investigate the possible role of lipid rafts in this increase, lipid rafts and post-synaptic densities (PSDs) were isolated by the extraction of rat brain synaptosomes with Triton X-100 followed by sucrose density gradient centrifugation. Lipid rafts accounted for the majority of PSD-95, whereas SAP102 was predominantly located in PSDs. Between 50 and 60% of NMDA receptors were associated with lipid rafts. Greater than 85-90% of Src and Fyn were present in lipid rafts, whereas Pyk2 was mainly associated with PSDs. Lipid rafts and PSDs were isolated from animals subjected to 15 min of global ischemia followed by 6 h of recovery. Ischemia did not affect the yield, density, flotillin-1 or cholesterol content of lipid rafts. Following ischemia, the phosphorylation of NR1 by protein kinase C and tyrosine phosphorylation of NR2A and NR2B was increased in both lipid rafts and PSDs, with a greater increase in tyrosine phosphorylation occurring in the raft fraction. Following ischemia, NR1, NR2A and NR2B levels were elevated in PSDs and reduced in lipid rafts. The findings are consistent with a model involving close interaction between lipid rafts and PSDs and a role for lipid rafts in ischemia-induced signaling pathways.

Localization of Src homology 2 domain-containing phosphatase 1 (SHP-1) to lipid rafts in T lymphocytes: functional implications and a role for the SHP-1 carboxyl terminus

The protein tyrosine phosphatase Src homology 2 domain-containing phosphatase 1 (SHP-1) has previously been shown to be a negative regulator of signaling mediated via the TCR. A growing body of evidence indicates that the regulated localization of proteins within certain membrane subdomains, referred to as lipid rafts, is important for the successful transduction of signaling events downstream of the TCR. However, considerably less is known about the localization of negative regulators during these lipid raft-dependent signaling events. In this study we have investigated the subcellular localization of SHP-1 and its role in regulation of TCR-mediated signaling. Our studies demonstrate that in a murine T cell hybridoma as well as in primary murine thymocytes, a fraction of SHP-1 localizes to the lipid rafts, both basally and after TCR stimulation. Interestingly, although SHP-1 localized in the nonraft fractions is tyrosine phosphorylated, the SHP-1 isolated from the lipid rafts lacks the TCR-induced tyrosine phosphorylation, suggesting physical and/or functional differences between these two subpopulations. We identify a requirement for the C-terminal residues of SHP-1 in optimal localization to the lipid rafts. Although expression of SHP-1 that localizes to lipid rafts potently inhibits TCR-mediated early signaling events and IL-2 production, the expression of lipid raft-excluded SHP-1 mutants fails to elicit any of the inhibitory effects. Taken together these studies reveal a key role for lipid raft localization of SHP-1 in mediating the inhibitory effects on T cell signaling events.

A rapid translocation of CD45RO but not CD45RA to lipid rafts in IL-6-induced proliferation in myeloma

CD45, a receptor-type tyrosine phosphatase, is required for interleukin-6 (IL-6)-induced proliferation in human myeloma cells, which express the shortest isoform, CD45RO, but not the longest isoform, CD45RA. Here, we showed that IL-6 induced the translocation of CD45 to lipid rafts in an isoform-dependent manner. In myeloma cells, CD45RO was translocated to lipid rafts more rapidly than CD45RB, but exogenously expressed CD45RA was not translocated. When an IL-6Rα-transfected B-cell line was stimulated with IL-6, CD45RA was not translocated, although CD45RB was. We further confirmed that the translocated CD45 bound to IL-6Rα, Lyn, and flotillin-2, and this was followed by the dephosphorylation of the negative regulatory Tyr507 of Lyn. CD45 also bound to phosphoprotein associated with glycosphingolipid-enriched microdomains (PAGs), which were subsequently dephosphorylated, resulting in the release of C-terminal src kinase (Csk) from lipid rafts. Therefore, these results indicate that a rapid translocation of CD45RO to lipid rafts may be responsible for IL-6-induced proliferation, and that the change from CD45RA to CD45RO confers the ability to respond to IL-6 in human myeloma cells. (Blood. 2005;105:3295-3302)

Loss of stearoyl-CoA desaturase expression is a frequent event in prostate carcinoma

Prostate carcinogenesis is influenced by genetic alterations resulting in a biochemical condition that favors cell proliferation and survival. Studies of prostate carcinoma using comparative genomic hybridization and cDNA microarray analysis indicate that numerous biochemical processes may be affected during cellular transformation and progression to an invasive phenotype. Among the consistently observed tumor-associated changes are alterations in fatty acid metabolism that influence diverse cellular activities such as signaling, energy utilization, and membrane fluidity. Increases in fatty acid synthase (FAS) levels have been shown to be one of the earliest and most frequent molecular alterations in prostate carcinogenesis. We sought to identify tumor-associated changes in the expression of genes with functional roles associated with lipid metabolism. Defined populations of normal and neoplastic prostate epithelium were acquired by laser capture microdissection and transcript levels were measured by cDNA microarray hybridization. We determined that stearoyl-CoA desaturase (SCD) transcripts were downregulated in cancer relative to normal epithelium. These results were confirmed by quantitative PCR. Further analysis by immunohistochemical evaluation of radical prostatectomy samples employed a quantitative scoring system with a range of 0-300. The median SCD expression levels were 150, 45 and 10 for normal, PIN and carcinoma samples, respectively. Statistically significant differential SCD expression between normal and cancerous epithelium was determined at the p=0.001 level, and between PIN and prostate carcinoma at the p=0.03 level. Of these cases, 92% overexpressed fatty acid synthase (FAS) in cancerous cells and 84.7% exhibited the signature of FAS overexpression and SCD loss in prostate carcinoma as compared to normal prostate epithelium. These results indicate that loss of SCD expression is a frequent event in prostate adenocarcinoma, and further supports a role for altered lipid metabolism as a factor in the process of carcinogenesis.

Induction of raft-like domains by a myristoylated NAP-22 peptide and its Tyr mutant

The N-terminally myristoylated, 19-amino acid peptide, corresponding to the amino terminus of the neuronal protein NAP-22 (NAP-22 peptide) is a naturally occurring peptide that had been shown by fluorescence to cause the sequestering of a Bodipy-labeled PtdIns(4,5)P2 in a cholesterol-dependent manner. The present work, using differential scanning calorimetry (DSC), extends the observation that formation of a PtdIns(4,5)P2-rich domain is cholesterol dependent and shows that it also leads to the formation of a cholesterol-depleted domain. The PtdIns(4,5)P2 used in the present work is extracted from natural sources and does not contain any label and has the native acyl chain composition. Peptide-induced formation of a cholesterol-depleted domain is abolished when the sole aromatic amino acid, Tyr11 is replaced with a Leu. Despite this, the modified peptide can still sequester PtdIns(4,5)P2 into domains, probably because of the presence of a cluster of cationic residues in the peptide. Cholesterol and PtdIns(4,5)P2 also modulate the insertion of the peptide into the bilayer as revealed by 1H NOESY MAS/NMR. The intensity of cross peaks between the aromatic protons of the Tyr residue and the protons of the lipid indicate that in the presence of cholesterol there is a change in the nature of the interaction of the peptide with the membrane. These results have important implications for the mechanism by which NAP-22 affects actin reorganization in neurons.

Palmitoylation of claudins is required for efficient tight-junction localization

Palmitoylation of integral membrane proteins can affect intracellular trafficking, protein-protein interactions and protein stability. The goal of the present study was to determine whether claudins, transmembrane-barrier-forming proteins of the tight junction, are palmitoylated and whether this modification has functional implications for the tight-junction barrier. Claudin-14, like other members of the claudin family, contains membrane-proximal cysteines following both the second and the fourth transmembrane domains, which we speculated could be modified by S-acylation with palmitic acid. We observed that [(3)H]-palmitic acid was incorporated into claudin-14 expressed by transfection in both cultured epithelial cells and fibroblasts. Mutation of cysteines to serines following either the second or the fourth transmembrane segments decreased the incorporation of [(3)H]-palmitic acid, and mutation of all four cysteines eliminated palmitoylation. We previously reported that expression of claudin-14 in epithelial monolayers results in a fivefold increase in electrical resistance. By contrast, expression of the mutant claudin-14 resulted in smaller increases in resistance. The mutants localized less well to tight junctions and were also found in lysosomes, suggesting an alteration in trafficking or stability. However, we observed no change in protein half-life and only a small shift in fractionation out of caveolin-enriched detergent-resistant membranes. Although less well localized to the tight junction, palmitoylation-deficient claudin-14 was still concentrated at sites of cell-cell contact and was competent to assemble into freeze-fracture strands when expressed in fibroblasts. These results demonstrate that palmitoylation of claudin-14 is required for efficient localization into tight junctions but not stability or strand assembly. Decreased ability of the mutants to alter resistance is probably the result of their less efficient localization into the barrier.

Palmitoylation and intracellular domain interactions both contribute to raft targeting of linker for activation of T cells

Some transmembrane proteins must associate with lipid rafts to function. However, even if acylated, transmembrane proteins should not pack well with ordered raft lipids, and raft targeting is puzzling. Acylation is necessary for raft targeting of linker for activation of T cells (LAT). To determine whether an acylated transmembrane domain is sufficient, we examined raft association of palmitoylated and nonpalmitoylated LAT transmembrane peptides in lipid vesicles by a fluorescence quenching assay, by microscopic examination, and by association with detergent-resistant membranes (DRMs). All three assays detected very low raft association of the nonacylated LAT peptide. DRM association was the same as a control random transmembrane peptide. Acylation did not measurably enhance raft association by the first two assays but slightly enhanced DRM association. The palmitoylated LAT peptide and a FLAG-tagged LAT transmembrane domain construct expressed in cells showed similar DRM association when both were reconstituted into mixed vesicles (containing cell-derived proteins and lipids and excess artificial raft-forming lipids) before detergent extraction. We conclude that the acylated LAT transmembrane domain has low inherent raft affinity. Full-length LAT in mixed vesicles associated better with DRMs than the peptide. However, cells appeared to contain two pools of LAT, with very different raft affinities. Since some LAT (but not the transmembrane domain construct) was isolated in a protein complex, and the Myc- and FLAG-tagged forms of LAT could be mutually co-immunoprecipitated, oligomerization or interactions with other proteins may enhance raft affinity of one pool of LAT. We conclude that both acylation and other factors, possibly protein-protein interactions, target LAT to rafts.

A simplified method for the preparation of detergent-free lipid rafts

Lipid rafts are small plasma membrane domains that contain high levels of cholesterol and sphingolipids. Traditional methods for the biochemical isolation of lipid rafts involve the extraction of cells with nonionic detergents followed by the separation of a low-density, detergent-resistant membrane fraction on density gradients. Because of concerns regarding the possible introduction of artifacts through the use of detergents, it is important to develop procedures for the isolation of lipid rafts that do not involve detergent extraction. We report here a simplified method for the purification of detergent-free lipid rafts that requires only one short density gradient centrifugation, but yields a membrane fraction that is highly enriched in cholesterol and protein markers of lipid rafts, with no contamination from nonraft plasma membrane or intracellular membranes.

Membrane domains

Considerable evidence shows that lateral inhomogeneities in lipid composition and physical properties exist in biological membranes. These membrane lipid domains are proposed to play important roles in processes such as signal transduction and membrane traffic. However, there is not at present an adequate description of the nature of these lipid domains in terms of their size, abundance, composition, or dynamics. We discuss the current analyses of the properties and function of membrane domains in cells and compare their properties with chemically simpler model membrane systems that can be understood in greater detail.

The SNARE proteins SNAP-25 and SNAP-23 display different affinities for lipid rafts in PC12 cells. Regulation by distinct cysteine-rich domains

SNAP-25 and its ubiquitously expressed homologue, SNAP-23, are SNARE proteins that are essential for regulated exocytosis in diverse cell types. Recent work has shown that SNAP-25 and SNAP-23 are partly localized in sphingolipid/cholesterol-rich lipid raft domains of the plasma membrane and that the integrity of these domains is important for exocytosis. Here, we show that raft localization is mediated by a 36-amino-acid region of SNAP-25 that is also the minimal sequence required for membrane targeting; this domain contains 4 closely spaced cysteine residues that are sites for palmitoylation. Analysis of endogenous levels of SNAP-25 and SNAP-23 present in lipid rafts in PC12 cells revealed that SNAP-23 (54% raft-associated) was almost 3-fold more enriched in rafts when compared with SNAP-25 (20% raft-associated). We report that the increased raft association of SNAP-23 occurs due to the substitution of a highly conserved phenylalanine residue present in SNAP-25 with a cysteine residue. Intriguingly, although the extra cysteine in SNAP-23 enhances its raft association, the phenylalanine at the same position in SNAP-25 acts to repress the raft association of this protein. These different raft-targeting signals within SNAP-25 and SNAP-23 are likely important for fine-tuning the exocytic pathways in which these proteins operate.

Do proteins facilitate the formation of cholesterol-rich domains?

Both biological and model membranes can exhibit the formation of domains. A brief review of some of the diverse methodologies used to identify the presence of domains in membranes is given. Some of these domains are enriched in cholesterol. The segregation of lipids into cholesterol-rich domains can occur in both pure lipid systems as well as membranes containing peptides and proteins. Peptides and proteins can promote the formation of cholesterol-rich domains not only by preferentially interacting with cholesterol and being sequestered into these regions of the membrane, but also indirectly as a consequence of being excluded from cholesterol-rich domains. The redistribution of components is dictated by the thermodynamics of the system. The formation of domains in a biological membrane is a consequence of all of the intermolecular interactions including those among lipid molecules as well as between lipids and proteins.

Signaling cascades activated upon antibody cross-linking of myelin oligodendrocyte glycoprotein: potential implications for multiple sclerosis

Antibody-induced demyelination is an important component of pathology in multiple sclerosis. In particular, antibodies to myelin oligodendrocyte glycoprotein (MOG) are elevated in multiple sclerosis patients, and they have been implicated as mediators of demyelination. We have shown previously that antibody cross-linking of MOG in oligodendrocytes results in the repartitioning of MOG into glycosphingolipid-cholesterol membrane microdomains ("lipid rafts"), followed by changes in the phosphorylation of specific proteins, including dephosphorylation of beta-tubulin and the beta subunit of the trimeric G protein and culminating in rapid and dramatic morphological alterations. In order to further elucidate the mechanism of anti-MOG-mediated demyelination, we have carried out a proteomic analysis to identify the set of proteins for which the phosphorylation states or expression levels are altered upon anti-MOG treatment. We demonstrate that treatment of oligodendrocytes with anti-MOG alone leads to an increase in calcium influx and activation of the MAPK/Akt pathways that is independent of MOG repartitioning. However, further cross-linking of anti-MOG.MOG complexes with a secondary anti-IgG results in the lipid raft-dependent phosphorylation of specific proteins related to cellular stress response and cytoskeletal stability. Oligodendrocyte survival is not compromised by these treatments. We discuss the possible significance of the anti-MOG-induced signaling cascade in relation to the initial steps of MOG-mediated demyelination.