A lipid raft environment enhances Lyn kinase activity by protecting the active site tyrosine from dephosphorylation

Chemoattractant-induced signaling via the Ras-ERK and PI3K-Akt networks, along with leukotriene C4 release, is dependent on the tyrosine kinase Lyn in IL-5- and IL-3-primed human blood eosinophils

Human blood eosinophils exhibit a hyperactive phenotype in response to chemotactic factors after cell "priming" with IL-5 family cytokines. Earlier work has identified ERK1/2 as molecular markers for IL-5 priming, and in this article, we show that IL-3, a member of the IL-5 family, also augments fMLP-stimulated ERK1/2 phosphorylation in primary eosinophils. Besides ERK1/2, we also observed an enhancement of chemotactic factor-induced Akt phosphorylation after IL-5 priming of human blood eosinophils. Administration of a peptide antagonist that targets the Src family member Lyn before cytokine (IL-5/IL-3) priming of blood eosinophils inhibited the synergistic increase of fMLP-induced activation of Ras, ERK1/2 and Akt, as well as the release of the proinflammatory factor leukotriene C(4). In this study, we also examined a human eosinophil-like cell line HL-60 clone-15 and observed that these cells exhibited significant surface expression of IL-3Rs and GM-CSFRs, as well as ERK1/2 phosphorylation in response to the addition of IL-5 family cytokines or the chemotactic factors fMLP, CCL5, and CCL11. Consistent with the surface profile of IL-5 family receptors, HL-60 clone-15 recapitulated the enhanced fMLP-induced ERK1/2 phosphorylation observed in primary blood eosinophils after priming with IL-3/GM-CSF, and small interfering RNA-mediated knockdown of Lyn expression completely abolished the synergistic effects of IL-3 priming on fMLP-induced ERK1/2 phosphorylation. Altogether, our data demonstrate a central role for Lyn in the mechanisms of IL-5 family priming and suggest that Lyn contributes to the upregulation of the Ras-ERK1/2 and PI3K-Akt cascades, as well as the increased leukotriene C(4) release observed in response to fMLP in "primed" eosinophils.

Ubiquitination and endocytosis of the high affinity receptor for IgE

The high affinity receptor for IgE (FcvarepsilonRI) is constitutivelly expressed on the surface of mast cells and basophils as a multimeric complex. Upon antigen ligation to FcvarepsilonRI-bound IgE molecules, the receptor complex transduces intracellular signals leading to the release of preformed and newly synthesised pro-inflammatory mediators. FcvarepsilonRI engagement also generates negative intracellular signals involving the coordinated action of adapters, phosphatases and ubiquitin ligases that limits the intensity and duration of positive signals. Relevant to this, antigen-induced FcvarepsilonRI ubiquitination has become recognized as an important signal for the internalization and delivery of engaged receptor complexes to lysosomes for degradation. In this article, we review recent advances in our understanding of molecular mechanisms that guarantee the clearance of antigen-stimulated FcvarepsilonRI complexes from the cell surface. A particular emphasis will be given on how lipid rafts and the ubiquitin pathway cooperate to ensure receptor internalization and sorting along the endocytic compartments. A brief discussion regarding how ubiquitination regulates the endocytosis of Fc receptors other than FcvarepsilonRI will be included.

Adapters in the organization of mast cell signaling

Mast cells are pivotal in innate immunity and play an important role in amplifying adaptive immunity. Nonetheless, they have long been known to be central to the initiation of allergic disorders. This results from the dysregulation of the immune response whereby normally innocuous substances are recognized as non-self, resulting in the production of IgE antibodies to these 'allergens'. Preformed and newly synthesized inflammatory (allergic) mediators are released from the mast cell following allergen-mediated aggregation of allergen-specific IgE bound to the high-affinity receptors for IgE (FcepsilonRI). Thus, the process by which the mast cell is able to interpret the engagement of FcepsilonRI into the molecular events necessary for release of their allergic mediators is of considerable therapeutic interest. Unraveling these molecular events has led to the discovery of a functional class of proteins that are essential in organizing activated signaling molecules and in coordinating and compartmentalizing their activity. These so-called 'adapters' bind multiple signaling proteins and localize them to specific cellular compartments, such as the plasma membrane. This organization is essential for normal mast cell responses. Here, we summarize the role of adapter proteins in mast cells focusing on the most recent advances toward understanding how these molecules work upon FcepsilonRI engagement.

Hierarchical organization of the plasma membrane: investigations by single-molecule tracking vs. fluorescence correlation spectroscopy

Single-molecule tracking and fluorescence correlation spectroscopy (FCS) applied to the plasma membrane in living cells have allowed a number of unprecedented observations, thus fostering a new basic understanding of molecular diffusion, interaction, and signal transduction in the plasma membrane. It is becoming clear that the plasma membrane is a heterogeneous entity, containing diverse structures on nano-meso-scales (2-200 nm) with a variety of lifetimes, where certain membrane molecules stay together for limited durations. Molecular interactions occur in the time-dependent inhomogeneous two-dimensional liquid of the plasma membrane, which might be a key for plasma membrane functions.

Mechanisms of mast cell signaling in anaphylaxis

The recent development of a consensus definition and proposed diagnostic criteria for anaphylaxis offers promise for research efforts and a better understanding of the epidemiology and pathogenesis of this enigmatic and life-threatening disease. This review examines basic principles and recent research advances in the mechanisms of mast cell signaling believed to underlie anaphylaxis. The unfolding complexity of mast cell signaling suggests that the system is sensitive to regulation by any of several individual signaling pathways and intermediates and that complementary pathways regulate mast cell activation by amplified signals. The signaling events underlying anaphylactic reactions have largely been identified through experiments in genetically modified mice and supported by biochemical studies of mast cells derived from these mice. These studies have revealed that signaling pathways exist to both upregulate and downregulate mast cell responses. In this review we will thus describe the key molecular players in these pathways in the context of anaphylaxis.

Are all n-3 polyunsaturated fatty acids created equal?

N-3 Polyunsaturated fatty acids have been shown to have potential beneficial effects for chronic diseases including cancer, insulin resistance and cardiovascular disease. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in particular have been studied extensively, whereas substantive evidence for a biological role for the precursor, alpha-linolenic acid (ALA), is lacking. It is not enough to assume that ALA exerts effects through conversion to EPA and DHA, as the process is highly inefficient in humans. Thus, clarification of ALA's involvement in health and disease is essential, as it is the principle n-3 polyunsaturated fatty acid consumed in the North American diet and intakes of EPA and DHA are typically very low. There is evidence suggesting that ALA, EPA and DHA have specific and potentially independent effects on chronic disease. Therefore, this review will assess our current understanding of the differential effects of ALA, EPA and DHA on cancer, insulin resistance, and cardiovascular disease. Potential mechanisms of action will also be reviewed. Overall, a better understanding of the individual role for ALA, EPA and DHA is needed in order to make appropriate dietary recommendations regarding n-3 polyunsaturated fatty acid consumption.

Lipidomic profiling in mouse brain reveals differences between ages and genders, with smaller changes associated with alpha-synuclein genotype

Advances in lipidomics technology have facilitated the precise detection, identification and profiling of lipid species within tissues. Mass spectrometry allows for identification of lipids as a function of the total number of carbons and double bonds in their acyl chains. Such detailed descriptions of lipid composition can provide a basis for further investigation of cell signaling and metabolic pathways, both physiological and pathological. Here, we applied phospholipid profiling to mouse models relevant to Parkinson's disease, using mice that were transgenic for human alpha-synuclein (alphaSyn) or deleted of endogenous alphaSyn. Proposed functions of alphaSyn include phospholipid binding, regulation of membrane composition, and regulation of vesicular pools. We investigated whether alphaSyn gene dosage interacts with differences in phospholipid composition across brain regions or with age-related changes in brain phospholipid composition. The most dramatic phospholipid changes were observed in alphaSyn wild-type animals as a function of age and gender. alphaSyn genotype-specific changes were also observed in aged, but not young, mice. Our results provide a detailed and systematic characterization of brain phospholipid composition in mice and identify age-related changes relevant both to Parkinson's disease and to normal aging.

Developmental acquisition of the Lyn-CD22-SHP-1 inhibitory pathway promotes B cell tolerance

To better understand whether autoimmunity in Lyn-deficient mice arises from compromised central or peripheral B cell tolerance, we examined BCR signaling properties of wild-type and Lyn-deficient B cells at different stages of development. Wild-type mature follicular B cells were less sensitive to BCR stimulation than were immature transitional stage 1 B cells with regard to BCR-induced calcium elevation and ERK MAPK activation. In the absence of Lyn, mature B cell signaling was greatly enhanced, whereas immature B cell signaling was minimally affected. Correspondingly, Lyn deficiency substantially enhanced the sensitivity of mature B cells to activation via the BCR, but minimally affected events associated with tolerance induction at the immature stage. The effects of CD22 deficiency on BCR signaling were very similar in B cells at different stages of maturation. These results indicate that the Lyn-CD22-Src homology region 2 domain-containing phosphatase-1 inhibitory pathway largely becomes operational as B cell mature, and sets a threshold for activation that appears to be critical for the maintenance of tolerance in the B cell compartment.

Association of FcgammaRIIa (CD32a) with lipid rafts regulates ligand binding activity

Binding of Igs to myeloid cells via FcR is a key event in the control of innate and acquired immunity. FcgammaRIIa (CD32a) is a receptor for multivalent IgG expressed predominantly by myeloid cells, and its association with microdomains rich in cholesterol and sphingolipids, termed as lipid rafts, has been reported to be essential for efficient signaling. However, for many myeloid cell types, ligand binding to CD32a is suppressed by as yet undefined mechanisms. In this study, we have examined the role of CD32a-lipid raft interactions in the regulation of IgG binding to CD32a. Disruption of lipid raft structure following depletion or sequestration of membrane cholesterol greatly inhibited CD32a-mediated IgG binding. Furthermore, specific CD32a mutants, which show reduced association with lipid rafts (A224S and C241A), displayed decreased levels of IgG binding compared with wild-type CD32a. In contrast, constitutively lipid raft-associated CD32a (GPI-anchored CD32a) exhibited increased capacity for IgG binding compared with the full-length transmembrane CD32a. Our findings clearly suggest a major role for lipid rafts in the regulation of IgG binding and, more specifically, that suppression of CD32a-mediated IgG binding in myeloid cells is achieved by receptor exclusion from lipid raft membrane microdomains.

Lipid raft-dependent FcepsilonRI ubiquitination regulates receptor endocytosis through the action of ubiquitin binding adaptors

The best characterized role for ubiquitination of membrane receptors is to negatively regulate signaling by targeting receptors for lysosomal degradation. The high affinity receptor for IgE (FcεRI) expressed on mast cells and basophils is rapidly ubiquitinated upon antigen stimulation. However, the nature and the role of this covalent modification are still largelly unknown. Here, we show that FcεRI subunits are preferentially ubiquitinated at multiple sites upon stimulation, and provide evidence for a role of ubiquitin as an internalization signal: under conditions of impaired receptor ubiquitination a decrease of receptor entry is observed by FACS analysis and fluorescence microscopy. We also used biochemical approaches combined with fluorescence microscopy, to demonstrate that receptor endocytosis requires the integrity of specific membrane domains, namely lipid rafts. Additionally, by RNA interference we demonstrate the involvement of ubiquitin-binding endocytic adaptors in FcεRI internalization and sorting. Notably, the triple depletion of Eps15, Eps15R and Epsin1 negatively affects the early steps of Ag-induced receptor endocytosis, whereas Hrs depletion retains ubiquitinated receptors into early endosomes and partially prevents their sorting into lysosomes for degradation. Our results are compatible with a scenario in which the accumulation of engaged receptor subunits into lipid rafts is required for receptor ubiquitination, a prerequisite for efficient receptor internalization, sorting and delivery to a lysosomal compartment.

The tyrosine kinase network regulating mast cell activation

Mast cell mediator release represents a pivotal event in the initiation of inflammatory reactions associated with allergic disorders. These responses follow antigen-mediated aggregation of immunoglobulin E (IgE)-occupied high-affinity receptors for IgE (Fc epsilon RI) on the mast cell surface, a response which can be further enhanced following stem cell factor-induced ligation of the mast cell growth factor receptor KIT (CD117). Activation of tyrosine kinases is central to the ability of both Fc epsilon RI and KIT to transmit downstream signaling events required for the regulation of mast cell activation. Whereas KIT possesses inherent tyrosine kinase activity, Fc epsilon RI requires the recruitment of Src family tyrosine kinases and Syk to control the early receptor-proximal signaling events. The signaling pathways propagated by these tyrosine kinases can be further upregulated by the Tec kinase Bruton's tyrosine kinase and downregulated by the actions of the tyrosine Src homology 2 domain-containing phosphatase 1 (SHP-1) and SHP-2. In this review, we discuss the regulation and role of specific members of this tyrosine kinase network in KIT and Fc epsilon RI-mediated mast cell activation.

Signal transduction by CD58: the transmembrane isoform transmits signals outside lipid rafts independently of the GPI-anchored isoform

The adhesion molecule CD58 is natively expressed in both a glycosylphosphatidylinositol (GPI)-anchored form and a transmembrane form. We previously demonstrated that the two isoforms of CD58 are differentially distributed in the cell membrane. The GPI-linked form resides in lipid rafts while the transmembrane form resides outside lipid rafts. Following cross-linking a fraction of transmembrane CD58 redistributes to lipid rafts. It has also been demonstrated that ligand binding to CD58 induces biological functions such as cytokine production and immunoglobulin isotype switching, indicating that cell-cell interactions result in CD58-mediated signal transduction. However, the signaling pathways involved in these activation processes are poorly defined. Here we show for the first time that cross-linking of CD58 induces protein tyrosine phosphorylation of BLNK, Syk and PLCgamma, and activation of ERK and Akt/PKB. In addition, we studied how these signaling events relate to the distinct membrane localization of the two isoforms of CD58. We demonstrate that cross-linking of CD58 triggers signaling that is predominantly associated with transmembrane CD58 in nonraft microdomains. Moreover, signaling through transmembrane CD58 does not depend on coexpression of the GPI-linked isoform. Thus, despite the residence of its GPI-anchored isoform in lipid rafts and the translocation of a fraction of its transmembrane isoform to lipid rafts, CD58 signaling is triggered by the transmembrane isoform outside lipid rafts. These findings corroborate signaling outside lipid rafts, as opposed to the established notion that rafts function as essential platforms for signaling.

Human basophils express the glycosylphosphatidylinositol-anchored low-affinity IgG receptor FcgammaRIIIB (CD16B)

Basophils express not only high-affinity IgE receptors, but also low-affinity IgG receptors. Which, among these receptors, are expressed by human basophils is poorly known. Low-affinity IgG receptors comprise CD32 (FcgammaRIIA, FcgammaRIIB, and FcgammaRIIC) and CD16 (FcgammaRIIIA and FcgammaRIIIB). FcgammaRIIA, FcgammaRIIC, and FcgammaRIIIA are activating receptors, FcgammaRIIB are inhibitory receptors, FcgammaRIIIB are GPI-anchored receptors whose function is poorly understood. Basophils were reported to express FcgammaRII, but not FcgammaRIII. We aimed at further identifying basophil IgG receptors. Basophils from normal donors and from patients suffering from an allergic skin disease (atopic dermatitis), allergic respiratory diseases (allergic rhinitis and asthma), or a nonallergic skin disease (chronic urticaria) were examined. We found that normal basophils contain FcgammaRIII transcripts and express FcgammaRIIIB, but not FcgammaRIIIA, which were detected on 24-81% basophils from normal donors and on 12-100% basophils from patients. Noticeably, the proportion of FcgammaRIIIB(+) basophils was significantly lower in atopic dermatitis patients than in other subjects. This decreased FcgammaRIII expression was not correlated with an activated phenotype of basophils in atopic dermatitis patients, although FcgammaRIIIB expression was down-regulated upon basophil activation by anti-IgE. Our results challenge the two dogmas 1) that basophils do not express FcgammaRIII and 2) that FcgammaRIIIB is exclusively expressed by neutrophils. They suggest that a proportion of basophils may be lost during enrichment procedures in which FcgammaRIII(+) cells are discarded by negative sorting using anti-CD16 Abs. They unravel an unexpected complexity of IgG receptors susceptible to modulate basophil activation. They identify a novel systemic alteration in atopic dermatitis.

Preparation and properties of asymmetric vesicles that mimic cell membranes: effect upon lipid raft formation and transmembrane helix orientation

A methyl-beta-cyclodextrin-induced lipid exchange technique was devised to prepare small unilamellar vesicles with stable asymmetric lipid compositions. Asymmetric vesicles that mimic biological membranes were prepared with sphingomyelin (SM) or SM mixed with 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) as the predominant lipids in the outer leaflet and dioleoylphosphatidylcholine (DOPC), POPC, 1-palmitoyl-2-oleoyl-phosphatidyl-L-serine (POPS), or POPS mixed with 1-palmitoyl-2-oleoyl-phosphatidylethanolamine (POPE) in the inner leaflet. Fluorescence-based assays were developed to confirm lipid asymmetry. Cholesterol was introduced into these vesicles using a second methyl-beta-cyclodextrin exchange step. In asymmetric vesicles composed of SM outside, DOPC inside (SMo/DOPCi) or SM outside, 2:1 mol:mol POPE:POPS inside (SMo/2:1 POPE:POPSi) the outer leaflet SM formed an ordered state with a thermal stability similar to that in pure SM vesicles and significantly greater than that in symmetric vesicles with the same overall lipid composition. Analogous behavior was observed in vesicles containing cholesterol. This shows that an asymmetric lipid distribution like that in eukaryotic plasma membranes can be conducive to ordered domain (raft) formation. Furthermore asymmetric vesicles containing approximately 25 mol % cholesterol formed ordered domains more thermally stable than those in asymmetric vesicles lacking cholesterol, showing that the crucial ability of cholesterol to stabilize ordered domain formation is likely to contribute to ordered domain formation in cell membranes. Additional studies demonstrated that hydrophobic helix orientation is affected by lipid asymmetry with asymmetry favoring formation of the transmembrane configuration. The ability to form asymmetric vesicles represents an important improvement in model membrane studies and should find many applications in the future.

Low substratum rigidity of collagen gel promotes ERK phosphorylation via lipid raft to augment cell migration

Previous study demonstrated that low substratum rigidity down-regulates focal adhesion proteins. In this study we found that cells cultured on collagen gel exhibited higher migration capacity than those cultured on collagen gel-coated dishes. Low rigidity of collagen gel induced delayed but persistent phosphorylation of ERK1/2. Inhibition of collagen gel-induced ERK1/2 phosphorylation by MEK inhibitors and ERK2 kinase mutant induced a rounding up of the cells and prevented collagen gel-induced cell migration. Interestingly, phosphorylated ERK1/2 induced by low rigidity was present in focal adhesion sites and the lipid raft. MbetaCD (Methyl-beta-cyclodextrin), a lipid raft inhibitor, inhibited collagen gel-induced ERK1/2 phosphorylation, and cell migration. Overexpression of FAK C-terminal fragment (FRNK) in MDCK cells triggered ERK phosphorylation. Meanwhile, low substratum rigidity induced degradation of FAK into a 35 kDa C-terminal fragment. A calpain inhibitor that partially rescued FAK degradation also prevented low rigidity-induced ERK phosphorylation. However, MbetaCD did not prevent low rigidity-induced FAK degradation. Taken together, we demonstrate that the degradation product of FAK induced by collagen gel triggers activation of ERK1/2, which in turn facilitates cell spreading and migration through the lipid raft.

Separate endocytic pathways regulate IL-5 receptor internalization and signaling

Eosinophils are critically dependent on IL-5 for their activation, differentiation, survival, and augmentation of cytotoxic activity. We previously showed that the cytoplasmic domain of the hematopoietic receptor, betac, which is shared by IL-5, IL-3, and GM-CSF, is directly ubiquitinated and degraded by the proteasomes in a JAK2-dependent manner. However, studies describing the spatial distribution, endocytic regulation, and trafficking of betac-sharing receptors in human eosinophils are currently lacking. Using deconvolution microscopy and biochemical methods, we clearly demonstrate that IL-5Rs reside in and are internalized by clathrin- and lipid raft-dependent endocytic pathways. Microscopy analyses in TF1 cells and human eosinophils revealed significant colocalization of betac, IL-5Ralpha, and Cy3-labeled IL-5 with transferrin- (clathrin) and cholera toxin-B- (lipid raft) positive vesicles. Moreover, whereas internalized IL-5Rs were detected in both clathrin- and lipid raft-positive vesicles, biochemical data revealed that tyrosine phosphorylated, ubiquitinated, and proteasome-degraded IL-5Rs partitioned to the soluble, nonraft fractions (clathrin-containing). Lastly, we show that optimal IL-5-induced signaling requires entry of activated IL-5Rs into the intracellular compartment, as coimmunoprecipitation of key signaling molecules with the IL-5R was completely blocked when either endocytic pathway was inhibited. These data provide the first evidence that IL-5Rs segregate and traffic into two distinct plasma membrane compartments, and they further establish that IL-5R endocytosis regulates signaling both positively and negatively.

A current understanding of Fc epsilon RI-dependent mast cell activation

Mast cell activation via the high-affinity immunoglobulin (Ig) E receptor Fc epsilon RI is a topic of extensive investigation with therapeutic potential in allergic disease. The protein tyrosine kinases Fyn, Lyn, and Syk are intimately linked with the early events initiated by allergen-mediated aggregation of IgE-occupied Fc epsilon RI. Fyn and Lyn initiate signaling events that are organized by adaptor molecules, which compartmentalize and coordinate the activity of activated protein and lipid kinases and phospholipases to generate lipid products essential for signal amplification and mast cell function. Fyn and Lyn counter-regulate phosphatidylinositol 3-OH kinase (PI3K), controlling the produced amount of phosphatidylinositol (3,4,5)-trisphosphate (PIP3), a key regulator of mast cell degranulation. Fyn and Lyn also activate sphingosine kinases (SphK), which generate sphingosine-1-phosphate (S1P), thus contributing to mast cell chemotaxis and degranulation. Here, we summarize the current knowledge and future challenges and directions.

Cholesterol-rich membrane rafts and Lyn are involved in phagocytosis during Pseudomonas aeruginosa infection

The mechanism of phagocytosis of pathogens remains to be fully characterized. We report a novel phagocytosis pathway for Pseudomonas aeruginosa, which is initiated by cholesterol-rich membrane rafts and is dependent on Lyn, primarily an immune regulator with both positive and negative roles. Blocking of Lyn or blocking of cholesterol synthesis significantly inhibited phagocytosis by alveolar macrophages. We found that Lyn, via Src homology 2 and 3 domains, bound to and then activated PI3K and Akt to regulate intracellular routing of the engulfed P. aeruginosa. Further analysis indicates that Lyn and raft components entered in phagosomes and late lysosomes. Finally, respiratory burst was dependent on Lyn and membrane rafts, as confirmed by small interfering RNA and dominant-negative strategies. Our investigations demonstrate that Lyn along with membrane rafts plays a fundamental role in phagocytosis by alveolar macrophages during infection.

Elasticity, strength, and water permeability of bilayers that contain raft microdomain-forming lipids

Bilayers composed of phosphatidylcholine (PC), sphingomyelin (SM), and cholesterol (CHOL) are commonly used as systems to model the raft-lipid domain structure believed to compartmentalize particular cell membrane proteins. In this work, micropipette aspiration of giant unilamellar vesicles was used to test the elasticities, water permeabilities, and rupture tensions of single-component PC, binary 1:1 PC/CHOL, and 1:1 SM/CHOL, and ternary 1:1:1 PC/SM/CHOL bilayers, one set of measurements with dioleoyl PC (DOPC; C18:1/C18:1 PC) and the other with stearoyloleoyl PC (SOPC; C18:0/C18:1 PC). Defining the elastic moduli (K(A)), the initial slopes of the increase in tension (sigma) versus stretch in lipid surface area (alpha(e)) were determined for all systems at low (15 degrees C) and high (32-33 degrees C) temperatures. The moduli for the single-component PC and binary phospholipid/CHOL bilayers followed a descending hierarchy of stretch resistance with SM/CHOL > SOPC/CHOL > DOPC/CHOL > PC. Although much more resistant to stretch than the single-component PC bilayers, the elastic response of vesicle bilayers made from the ternary phospholipid/CHOL mixtures showed an abrupt softening (discontinuity in slope), when immediately subjected to a steady ramp of tension at the low temperature (15 degrees C). However, the discontinuities in elastic stretch resistance at low temperature vanished when the bilayers were held at approximately 1 mN/m prestress for long times before a tension ramp and when tested at the higher temperature 32-33 degrees C. The elastic moduli of single-component PC and DOPC/CHOL bilayers changed very little with temperature, whereas the moduli of the binary SOPC/CHOL and SM/CHOL bilayers diminished markedly with increase in temperature, as did the ternary SOPC/SM/CHOL system. For all systems, increasing temperature increased the water permeability but decreased rupture tension. Concomitantly, the measurements of permeability exhibited a prominent correlation with the rupture tension across all the systems. Together, these micromechanical tests of binary and ternary phospholipid/CHOL bilayers demonstrate that PC hydrocarbon chain unsaturation and temperature are major determinants of the mechanical and permeation properties of membranes composed of raft microdomain-forming lipids.

CD66b regulates adhesion and activation of human eosinophils

Eosinophils and their products are likely important in the pathophysiology of allergic diseases, such as bronchial asthma, and in host immunity to parasitic organisms. However, the mechanisms for proinflammatory mediator release by eosinophils are poorly understood. CD66b (CEACAM8, CGM6, NCA-95) is a single chain, GPI-anchored, highly glycosylated protein belonging to the carcinoembryonic Ag supergene family. CD66b is an activation marker for human granulocytes; however, its biological functions are largely unknown in eosinophils. We found that CD66b is highly expressed on the surface of human peripheral blood eosinophils isolated from healthy individuals. Engagement of CD66b, but not CD66a, by mAb or a natural ligand, galectin-3, activated a Src kinase family molecule, hemopoietic cell kinase (Hck), and induced cellular adhesion, superoxide production, and degranulation of eosinophils. CD66b molecules were localized in lipid rafts, and disruption of lipid rafts or removal of the GPI anchor inhibited the adhesion and activation of eosinophils. Importantly, CD66b was constitutively and physically associated with a beta2 integrin, CD11b, and cross-linking of CD66b induced a striking clustering of CD11b molecules. Thus, CD66b molecules are involved in regulating adhesion and activation of eosinophils, possibly through their localization in lipid rafts and interaction with other cell surface molecules, such as CD11b. Binding of exogenous or endogenous carbohydrate ligands(s) to CD66b may be important in the release of proinflammatory mediators by human eosinophils.

New insights on mast cell activation via the high affinity receptor for IgE

Mast cells are innate immune cells that function as regulatory or effector cells and serve to amplify adaptive immunity. In adaptive immunity these cells function primarily through cell surface Fc receptors that bind immunoglobulin antibodies. The dysregulation of their adaptive role makes them central players in allergy and asthma. Upon encountering an allergen (antigen), which is recognized by immunoglobulin E (IgE) antibodies bound to the high affinity IgE receptor (FcepsilonRI) expressed on their cell surface, mast cells secrete both preformed and newly synthesized mediators of the allergic response. Blocking of these responses is an objective in therapeutic intervention of allergic diseases. Thus, understanding the mechanisms by which antigens elicit mast cell activation (via FcepsilonRI) holds promise toward identifying therapeutic targets. Here we review the most recent advances in understanding antigen-dependent mast cell activation. Specifically, we focus on the requirements for FcepsilonRI activation, the regulation of calcium responses, co-stimulatory signals in FcepsilonRI-mediated mast cell activation and function, and how genetics influences mast cell signaling and responses. These recent discoveries open new avenues of investigation with therapeutic potential.

Lipid raft specific knockdown of SRC family kinase activity inhibits cell adhesion and cell cycle progression of breast cancer cells

Src family kinase (SFK) is known to control various cell functions, but the significance of the location of its activation was largely unknown. We herein revealed that SFK activation occurs in lipid rafts. Based on this finding, we have developed a lipid raft-targeted SFK inhibitory fusion protein (LRT-SIFP) that inhibits the SFK activity in lipid rafts. LRT-SIFP has a peptide inhibitor of SFK and a lipid raft-targeting sequence in which two cysteine residues are palmitoylated for clustering in lipid rafts. LRT-SIFP was found to inhibit cell adhesion and cell cycle progression of human breast cancer cell lines MCF-7 and MDA-MB231. On the other hand, the cell functions of MCF-7 cells were found to be not affected with a previously developed peptide inhibitor of SFK that lacks the lipid raft-targeting sequence. In addition, when we replaced the targeting sequence of LRT-SIFP with the consensus sequence for geranylgeranylation to make LRT-SIFP unable to cluster in lipid rafts, this mutated LRT-SIFP did not show any effect on the above cell functions of MCF-7 cells. Furthermore, in contrast to the breast cancer cell lines, LRT-SIFP did not show any inhibitory effect on cell adhesion and cell cycle progression of human normal cell line HEK293. The present lipid raft-specific knockdown of SFK activity would potentially be useful for selective cancer therapy to prevent tumorigenesis and metastasis of breast cancer cells.

Signaling by committee: receptor clusters determine pathways of cellular activation

Receptor clustering is a common signaling mechanism for cell surface receptors. Exogenous ligands such as antibodies or synthetic analogues can be used to artificially induce clustering. New studies using defined synthetic ligands suggest that the spatial organization of these clusters attenuates signaling in one pathway but has no effect in another.

Kit- and Fc epsilonRI-induced differential phosphorylation of the transmembrane adaptor molecule NTAL/LAB/LAT2 allows flexibility in its scaffolding function in mast cells

The transmembrane adaptor protein (TRAP), NTAL, is phosphorylated in mast cells following FcvarepsilonRI aggregation whereby it cooperates with LAT to induce degranulation. The Kit ligand, stem cell factor (SCF), enhances antigen-induced degranulation and this also appears to be NTAL-dependent. However, Kit and FcvarepsilonRI appear to utilize different mechanisms to induce NTAL phosphorylation. Thus, we examined whether the responsible kinases selectively phosphorylated distinct tyrosines in NTAL and explored the implications for downstream signaling. Whereas FcvarepsilonRI required Lyn and Syk for NTAL phosphorylation, Kit appeared to directly phosphorylate NTAL. Furthermore, co-transfection studies with NTAL constructs revealed that Lyn, Syk, and Kit phosphorylate different tyrosines in NTAL. The tyrosines principally phosphorylated by Syk were recognized as Grb2-binding sites, whereas Lyn and Kit phosphorylated other tyrosines, both inside and outside of these motifs. Pull down studies revealed that PLCgamma1 associated with the two terminal Syk-phosphorylated Grb2-binding sites, which would help to explain the observed decrease in antigen-induced calcium signal and degranulation in NTAL-knock down-human mast cells. The observations reported herein support the conclusion that NTAL may be differentially utilized by specific receptors for relaying alternative signals and this suggests a flexibility in the function of TRAPs not previously appreciated.

Acid sphingomyelinase is required for lipid Raft TLR4 complex formation

BACKGROUND

Lipid rafts, composed of sphingolipids, are critical to Toll-like receptor 4 (TLR4) assembly during lipopolysaccharide (LPS) exposure as a result of phosphokinase C (PKC)-zeta activation. However, the mechanism responsible for these events remains unknown.

PURPOSE

We determined whether LPS-induced TLR4 assembly and activation are dependent on the sphingolipid metabolite ceramide, produced by acid sphingomyelinase following the initial binding of LPS to CD14.

METHODS

Cultured THP-1 cells were stimulated with LPS, exogenous C(2) ceramide, or both. Selected cells were pretreated with the acid sphingomyelinase inhibitor imipramine or CD14 neutralizing antibody.

RESULTS

Exposure to LPS led to activation of acid sphingomyelinase, production of ceramide, phosphorylation of PKCzeta, and assembly of the TLR4 receptor within lipid rafts. This was followed by activation of the MAPK family of products and the liberation of tumor necrosis factor-alpha. Pretreatment with imipramine or CD14 blockade was associated with attenuation of all of these LPS-induced events. Simultaneous treatment with C(2) ceramide and LPS reversed all the inhibitory effects induced by imipramine, but not those associated with CD14 blockade.

CONCLUSION

Assembly and activation of the TLR4 receptor following LPS binding to CD14 requires the production of ceramide by acid sphingomyelinase.

Oxidative stress, lipid rafts, and macrophage reprogramming

Oxidant stress, induced under a variety of conditions, is known to lead to the molecular reprogramming of the tissue-fixed macrophage. This reprogramming is associated with an altered response to subsequent inflammatory stimuli, such as lipopolysaccharide (LPS), leading to enhanced liberation of proinflammatory chemokines and cytokines. Due to this altered response, dysregulated immunity ensues, leading to the development of clinical syndromes such as multiple organ dysfunction syndrome (MODS). Although the mechanisms responsible for this altered macrophage activity by oxidant stress remains complex and poorly elucidated, it appears, based on recent research, that early and direct alterations within lipid rafts are responsible. This early and direct interaction with lipid rafts by oxidants leads to the mobilization of annexin VI from lipid raft constructs, leading to the release of calcium. This increased cytosolic concentration of this secondary messenger, in turn, results in the activation of calcium-dependent kinases, leading to further alterations in lipid raft lipids and eventually lipid raft proteins. Due to these lipid raft compositional changes, preassembly of receptor complexes occur, leading to enhanced proinflammatory activation. Within this review, the complexity of oxidant-induced reprogramming within the tissue fixed macrophage as currently understood is explained.

Insights into immunoglobulin E receptor signaling from structurally defined ligands

The asymmetrical structure of bent immunoglobulin E (IgE) bound to its high-affinity receptor, Fc epsilon RI, suggests a possible role for this configuration in the regulation of signaling mediated by cross-linking of Fc epsilon RI on the surface of mast cells and basophils. Indeed, the presence of bound IgE strongly influences the capacity of cross-linked Fc epsilon RI dimers to trigger mast cell degranulation, implicating orientational constraints by bound IgE. Bivalent ligands that cross-link by binding to bivalent IgE can form linear and cyclic chains of IgE/Fc epsilon RI complexes, and these exhibit only limited capacity to stimulate downstream signaling and degranulation, whereas structurally analogous trivalent ligands, which can form branched networks of cross-linked IgE/Fc epsilon RI complexes, are more effective at cell activation. Long bivalent ligands with flexible spacers can form intramolecular cross-links with IgE, and these stable 1:1 complexes are very potent inhibitors of mast cell degranulation stimulated by multivalent antigen. In contrast, trivalent ligands with rigid double-stranded DNA spacers effectively stimulate degranulation responses in a length-dependent manner, providing direct evidence for receptor transphosphorylation as a key step in the mechanism of signaling by Fc epsilon RI. Thus, studies with chemically defined oligovalent ligands show important features of IgE receptor cross-linking that regulate signaling, leading to mast cell activation.

Proteomic analysis of plasma membrane lipid rafts of HL-60 cells

Neutrophils acquire phagocytic activity as they differentiate. Recently, plasma membrane lipid rafts have been shown to play important roles in the process of phagocytosis in neutrophils. To characterize the proteins involved in phagocytosis and to elucidate the process by which they acquire phagocytic activity, we investigated by nano-LC-MS/MS analysis the changes in protein composition of plasma membrane lipid rafts during DMSO-induced differentiation of the human leukemia cell line HL-60 cells into neutrophilic lineage. Based on the spectrum counts of 147 proteins identified, 25 proteins were upregulated and 49 were downregulated by DMSO treatment. CD11b/CD18 subunits of beta2-integrin Mac-1, CD35, and GPI-80, which are known to be upregulated during differentiation, were dominantly detected in the lipid rafts of DMSO-treated cells. Many known membrane proteins, G proteins, and cytoskeletal proteins were also detected and they showed characteristic distributions. Absolute quantification of nine proteins in the lipid rafts using internal standard peptides labeled with stable isotopes showed that the amount of protein almost corresponded to the results obtained by spectrum count. Identified proteins, expression of which was altered by DMSO treatment, are expected to be candidate proteins involved in differentiation and functions of neutrophils.

Acute alcohol activates STAT3, AP-1, and Sp-1 transcription factors via the family of Src kinases to promote IL-10 production in human monocytes

Alcohol consumption is associated with an imbalance in pro- and anti-inflammatory cytokines and immunosuppression, partially as a result of enhanced IL-10 production. The mechanisms of IL-10 induction by alcohol remain poorly understood. We identified that increased IL-10 production in human monocytes after acute in vivo alcohol consumption or in vitro alcohol treatment was associated with increased STAT3 activation. Alcohol alone induced and in combination with LPS augmented STAT3 phosphorylation at tyrosine 705 (tyr705) and serine 727 (ser727) residues and increased STAT3 binding to DNA. Upstream, alcohol activated the Src kinases, as indicated by an increase in phosphorylated and a decrease in nonphosphorylated Src proteins. STAT3 activation by Src kinases occurred directly at the tyr705 residue and indirectly at the ser727 residue via JNK MAPKs. Using specific Src (PP2), JNK1/2 (SB600125), or p38 (SB203580) inhibitors, we determined that alcohol treatment alone induced and together with LPS, augmented the DNA-binding capacity of the specificity protein-1 (Sp-1) and AP-1 transcription factors involved in IL-10 production via Src-mediated activation of p38 MAPK and JNK, respectively. Our data suggest that acute alcohol activates Src/STAT3 and Src/MAPK/STAT3, AP-1, and Sp-1 pathways as important mechanisms for IL-10-mediated immunomodulation after acute alcohol use.

Lipid rafts of primary endothelial cells are essential for Kaposi's sarcoma-associated herpesvirus/human herpesvirus 8-induced phosphatidylinositol 3-kinase and RhoA-GTPases critical for microtubule dynamics and nuclear delivery of viral DNA but dispensable for binding and entry

Early during de novo infection of human microvascular dermal endothelial (HMVEC-d) cells, Kaposi's sarcoma-associated herpesvirus (KSHV) (human herpesvirus 8 [HHV-8]) induces the host cell's preexisting FAK, Src, phosphatidylinositol 3-kinase (PI3-K), Rho-GTPases, Diaphanous-2 (Dia-2), Ezrin, protein kinase C-zeta, extracellular signal-regulated kinase 1/2 (ERK1/2), and NF-kappaB signal pathways that are critical for virus entry, nuclear delivery of viral DNA, and initiation of viral gene expression. Since several of these signal molecules are known to be associated with lipid raft (LR) domains, we investigated the role of LR during KSHV infection of HMVEC-d cells. Pretreatment of cells with LR-disrupting agents methyl beta-cyclo dextrin (MbetaCD) or nystatin significantly inhibited the expression of viral latent (ORF73) and lytic (ORF50) genes. LR disruption did not affect KSHV binding but increased viral DNA internalization. In contrast, association of internalized viral capsids with microtubules (MTs) and the quantity of infected nucleus-associated viral DNA were significantly reduced. Disorganized and disrupted MTs and thick rounded plasma membranes were observed in MbetaCD-treated cells. LR disruption did not affect KSHV-induced FAK and ERK1/2 phosphorylation; in contrast, it increased the phosphorylation of Src, significantly reduced the KSHV-induced PI3-K and RhoA-GTPase and NF-kappaB activation, and reduced the colocalizations of PI3-K and RhoA-GTPase with LRs. Biochemical characterization demonstrated the association of activated PI3-K with LR fractions which was inhibited by MbetaCD treatment. RhoA-GTPase activation was inhibited by PI3-K inhibitors, demonstrating that PI3-K is upstream to RhoA-GTPase. In addition, colocalization of Dia-2, a RhoA-GTPase activated molecule involved in MT activation, with LR was reduced. KSHV-RhoA-GTPase mediated acetylation and aggregation of MTs were also reduced. Taken together, these studies suggest that LRs of endothelial cells play critical roles in KSHV infection and gene expression, probably due to their roles in modulating KSHV-induced PI3-K, RhoA-GTPase, and Dia-2 molecules essential for postbinding and entry stages of infection such as modulation of microtubular dynamics, movement of virus in the cytoplasm, and nuclear delivery of viral DNA.

Large-scale fluid/fluid phase separation of proteins and lipids in giant plasma membrane vesicles

The membrane raft hypothesis postulates the existence of lipid bilayer membrane heterogeneities, or domains, supposed to be important for cellular function, including lateral sorting, signaling, and trafficking. Characterization of membrane lipid heterogeneities in live cells has been challenging in part because inhomogeneity has not usually been definable by optical microscopy. Model membrane systems, including giant unilamellar vesicles, allow optical fluorescence discrimination of coexisting lipid phase types, but thus far have focused on coexisting optically resolvable fluid phases in simple lipid mixtures. Here we demonstrate that giant plasma membrane vesicles (GPMVs) or blebs formed from the plasma membranes of cultured mammalian cells can also segregate into micrometer-scale fluid phase domains. Phase segregation temperatures are widely spread, with the vast majority of GPMVs found to form optically resolvable domains only at temperatures below approximately 25 degrees C. At 37 degrees C, these GPMV membranes are almost exclusively optically homogenous. At room temperature, we find diagnostic lipid phase fluorophore partitioning preferences in GPMVs analogous to the partitioning behavior now established in model membrane systems with liquid-ordered and liquid-disordered fluid phase coexistence. We image these GPMVs for direct visual characterization of protein partitioning between coexisting liquid-ordered-like and liquid-disordered-like membrane phases in the absence of detergent perturbation. For example, we find that the transmembrane IgE receptor FcepsilonRI preferentially segregates into liquid-disordered-like phases, and we report the partitioning of additional well known membrane associated proteins. Thus, GPMVs now provide an effective approach to characterize biological membrane heterogeneities.

Adaptive and innate immune reactions regulating mast cell activation: from receptor-mediated signaling to responses

In this article, we have described studies that have demonstrated that mast cells can be activated as a consequence of adaptive and innate immune reactions and that these responses can be modified by ligands for other receptors expressed on the surface of mast cells. These various stimuli differentially activate multiple signaling pathways within the mast cells required for the generation and/or release of inflammatory mediators. Thus, the composition of the suite of mediators released and the physiologic ramifications of these responses are dependent on the stimuli and the microenvironment in which the mast cells are activated. Knowledge of the different signaling molecules used by cell surface receptors may allow selective pharmacologic targeting such that inhibiting the adverse effects of mast cell activation can be achieved without influencing the beneficial effects of mast cell activation. The exact interconnections between the signaling pathways initiated by the surface receptors described in this article remain to be completely worked out; thus, this remains a topic for future investigation.

Constitutively-active human LH receptors are self-associated and located in rafts

Several naturally occurring mutations in human luteinizing hormone receptors (LHR) at position 578 are associated with constitutive activation of the receptor. To determine whether human LHRs that signal in the absence of ligand are self-associated, fluorescence resonance energy transfer (FRET) between receptors was evaluated. Values for FRET between wild type LHR in the absence of ligand were less than 1% and increased significantly to over 11% after exposure to hCG. Constitutively active receptors exhibited 11-15% FRET efficiency in the absence of hormone and these values did not change with hCG treatment. A large fraction of constitutively active LHR-D578H receptors were also associated with so-called plasma membrane rafts. Disruption of these membrane microdomains reduced FRET efficiency but did not affect signalling through cAMP. Thus, in the absence of ligand, constitutively active receptors are self-associated and located in high buoyancy membrane fractions, both characteristics of the hormone-treated wild type receptor.

Membrane order and molecular dynamics associated with IgE receptor cross-linking in mast cells

Cholesterol-rich microdomains (or "lipid rafts") within the plasma membrane have been hypothesized to exist in a liquid-ordered phase and play functionally important roles in cell signaling; however, these microdomains defy detection using conventional imaging. To visualize domains and relate their nanostructure and dynamics to mast cell signaling, we use two-photon (760 nm and 960 nm) fluorescence lifetime imaging microscopy and fluorescence polarization anisotropy imaging, with comparative one-photon anisotropy imaging and single-point lifetime and anisotropy decay measurements. The inherent sensitivity of ultrafast excited-state dynamics and rotational diffusion to the immediate surroundings of a fluorophore allows for real-time monitoring of membrane structure and organization. When the high affinity receptor for IgE (FcepsilonRI) is extensively cross-linked with anti-IgE, molecules associated with cholesterol-rich microdomains (e.g., saturated lipids (the lipid analog diI-C(18) or glycosphingolipids)) and lipid-anchored proteins coredistribute with cross-linked IgE-FcepsilonRI. We find an enhancement in fluorescence lifetime and anisotropy of diI-C(18) and Alexa 488-labeled IgE-FcepsilonRI in the domains where these molecules colocalize. Our results suggest that fluorescence lifetime and, particularly, anisotropy permit us to correlate the recruitment of lipid molecules into more ordered domains that serve as platforms for IgE-mediated signaling.

Endotoxin tolerance attenuates LPS-induced TLR4 mobilization to lipid rafts: a condition reversed by PKC activation

Endotoxin tolerance is characterized by attenuated macrophage activation to subsequent LPS challenge and can be reversed through nonspecific protein kinase C (PKC) activation, and activation by LPS within naïve cells requires the activation of the cell surface receptors CD14 and TLR4 on lipid rafts. The effect of PKC activation and endotoxin tolerance on lipid raft receptor complex assembly is unknown and the focus of this study. Tolerance was induced in THP-1 cells through LPS pre-exposure. Naïve and tolerant cells were stimulated with LPS, with or without PMA pretreatment to activate PKC. TLR4 surface expression and LPS binding were determined by flow cytometry and immunohistochemistry. Cellular and lipid raft protein was analyzed for the presence and activation of the TLR4 complex components. Harvested supernatants were examined for TNF-alpha production. Total TLR4 surface expression and LPS binding were not affected by tolerance induction. LPS stimulation of naïve cells resulted in TLR4 and heat shock protein (HSP)70 lipid raft mobilization, MAPK activation, and TNF-alpha production. LPS stimulation of tolerant cells was associated with attenuation of all of these cellular events. Although PKC activation by PMA had no effect on naïve cells, it did result in reversal in tolerance-induced suppression of TLR4 and HSP70 lipid raft mobilization, MAPK activation, and TNF-alpha production. In addition, the effects associated with PMA were reversed with exposure to a myristoylated PKC-zeta pseudosubstrate. Thus, endotoxin tolerance appears to be induced through attenuated TLR4 formation following LPS stimulation. This complex formation appears to be PKC-dependent, and restoration of PKC activity reverses tolerance.

Phosphorylation-independent ubiquitylation and endocytosis of Fc gammaRIIA

Endocytosis of the Fc receptor Fc gammaRIIA depends on a functional ubiquitin conjugation system, and the receptor becomes ubiquitylated upon ligand binding. Phosphorylation of tyrosines in Fc gammaRIIA by Src family kinases is thought to be the initiating event in its signaling. However, although the Src family kinase inhibitor PP1 inhibited both ligand-induced phosphorylation of Fc gammaRIIA and phagocytosis in ts20 cells expressing Fc gammaRIIA, it did not inhibit receptor ubiquitylation or endocytosis of soluble ligands. Conversely, genistein and the proteasomal inhibitor MG132 did not inhibit receptor phosphorylation but strongly inhibited both receptor ubiquitylation and endocytosis. A region of the receptor lying within the immunoreceptor tyrosine-based activation motif was found to be necessary for both ubiquitylation and endocytosis. Ubiquitylation occurs at the plasma membrane before internalization. Endocytosis of Fc gammaRIIA is dependent on clathrin but independent of the adaptor protein AP-2. These findings point to a novel mechanism for ubiquitylation and endocytosis of this immunoreceptor.

Flotillin-1 regulates IgE receptor-mediated signaling in rat basophilic leukemia (RBL-2H3) cells

Cross-linking of high-affinity IgE receptors by multivalent Ag on mast cells (rat basophilic leukemia (RBL)-2H3) induces the phosphorylation of ITAM motifs of an IgE receptor by Src family tyrosine kinase, Lyn. The phosphorylation of IgE receptors is followed by a series of intracellular signals, such as Ca(2+) mobilization, MAPK activation, and degranulation. Therefore, Lyn is a key molecule in the activation of mast cells, but the molecular mechanisms for the activation of Lyn are still unclear. Recently, it is suggested that the localization of Lyn in lipid rafts is critical for its activation in several cell lines, although the precise mechanism is still unknown. In this study, we found that flotillin-1, which is localized in lipid rafts, is involved in the process of Lyn activation. We obtained flotillin-1 knockdown (KD)(2) rat basophilic leukemia (RBL)-2H3 cells, which express a low level of flotillin-1. In the flotillin-1 KD cells, we observed a significant decrease in Ca(2+) mobilization, the phosphorylation of ERKs, tyrosine phosphorylation of the gamma-subunit of IgE receptor, and IgE receptor-mediated degranulation. We also found that flotillin-1 is constitutively associated with Lyn in lipid rafts in RBL-2H3 cells, and Ag stimulation induced the augmentation of flotillin-1 binding to Lyn, resulting in enhancement of kinase activity of Lyn. These results suggest that flotillin-1 is an essential molecule in IgE receptor-mediated mast cell activation, and regulates the kinase activity of Lyn in lipid rafts.

The high-affinity immunoglobulin-E receptor (FcepsilonRI) is endocytosed by an AP-2/clathrin-independent, dynamin-dependent mechanism

Aggregation of the high-affinity immunoglobulin E (IgE) receptor (FcepsilonRI), expressed on mast cells and basophils, initiates the immediate hypersensitivity reaction. Aggregated FcepsilonRI has been reported to rapidly migrate to lipid rafts in RBL-2H3 cells. We confirmed that aggregated FcepsilonRI is found in the lipid raft fractions of cellular lysates. Furthermore, we show that the cross-linked FcepsilonRI remains associated with detergent-resistant structures upon internalization. Previous morphological studies have reported that aggregated FepsiloncRI is endocytosed via clathrin-coated pits, which in general are not lipid raft associated. To address this apparent discrepancy, we employed siRNA to suppress expression of components of the clathrin-mediated internalization machinery, namely, clathrin heavy chain, and the AP-2 (alpha-adaptin or mu2-subunit). Transferrin receptor (TfR) is endocytosed by a clathrin-mediated process and, as expected, each transfected siRNA caused a two to threefold elevation of TfR surface expression and almost completely inhibited its endocytosis. In contrast, there was no effect on surface expression levels of FcepsilonRI nor on the endocytosis of the dinitrophenyl-human serum albumin (DNP-HSA)/IgE/FcepsilonRI complex. On the contrary, internalization of DNP-HSA/IgE/FcepsilonRI was inhibited by overexpression of a dominant-negative dynamin mutant. We conclude that internalization of cross-linked FcRI does not require the AP-2/clathrin complex but is dynamin-dependent and may be lipid raft mediated.

Erlin-1 and erlin-2 are novel members of the prohibitin family of proteins that define lipid-raft-like domains of the ER

Our laboratory was interested in characterizing the molecular composition of non-caveolar lipid rafts. Thus, we generated monoclonal antibodies to lipid raft proteins of human myelomonocytic cells. Two of these proteins, KE04p and C8orf2, were found to be highly enriched in the detergent-insoluble, buoyant fraction of sucrose gradients in a cholesterol-dependent manner. They contain an evolutionarily conserved domain placing them in the prohibitin family of proteins. In contrast to other family members, these two proteins localized to the ER. Furthermore, the extreme N-termini of KE04p and C8orf2 were found to be sufficient for heterologous targeting of GFP to the ER in the absence of classical ER retrieval motifs. We also demonstrate that all prohibitin family members rely on sequences in their extreme N-termini for their distinctive subcellular distributions including the mitochondria, plasma membrane and Golgi vesicles. Owing to their subcellular localization and their presence in lipid rafts, we have named KE04p and C8orf2, ER lipid raft protein (erlin)-1 and erlin-2, respectively. Interestingly, the ER contains relatively low levels of cholesterol and sphingolipids compared with other organelles. Thus, our data support the existence of lipid-raft-like domains within the membranes of the ER.

Requirements for effective IgE cross-linking on mast cells and basophils

This article reviews the characteristics of high affinity IgE receptors (FcepsilonRI) and their role in the response to allergenic proteins. The requirements for successful cross-linking of FcepsilonRI on basophils and mast cells and subsequent degranulation by allergenic proteins will be explained in detail. Methods for in vitro analysis of allergen-induced mast cell and basophil degranulation will be described and issues/problems in applying these methods will be discussed. Finally, implications for manipulation of protein allergens will be discussed.

Temperature and composition dependence of the interaction of delta-lysin with ternary mixtures of sphingomyelin/cholesterol/POPC

The kinetics of carboxyfluorescein efflux induced by the amphipathic peptide delta-lysin from vesicles of porcine brain sphingomyelin (BSM), 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC), and cholesterol (Chol) were investigated as a function of temperature and composition. Sphingomyelin (SM)/Chol mixtures form a liquid-ordered (L(o)) phase whereas POPC exists in the liquid-disordered (L(d)) phase at ambient temperature. delta-Lysin binds strongly to L(d) and poorly to L(o) phase. In BSM/Chol/POPC vesicles the rate of carboxyfluorescein efflux induced by delta-lysin increases as the POPC content decreases. This is explained by the increase of delta-lysin concentration in L(d) domains, which enhances membrane perturbation by the peptide. Phase separations in the micrometer scale have been observed by fluorescence microscopy in SM/Chol/POPC mixtures for some SM, though not for BSM. Thus, delta-lysin must detect heterogeneities (domains) in BSM/Chol/POPC on a much smaller scale. Advantage was taken of the inverse variation of the efflux rate with the L(d) content of BSM/Chol/POPC vesicles to estimate the L(d) fraction in those mixtures. These results were combined with differential scanning calorimetry to obtain the BSM/Chol/POPC phase diagram as a function of temperature.

Coexisting domains in the plasma membranes of live cells characterized by spin-label ESR spectroscopy

The importance of membrane-based compartmentalization in eukaryotic cell function has become broadly appreciated, and a number of studies indicate that these eukaryotic cell membranes contain coexisting liquid-ordered (L(o)) and liquid-disordered (L(d)) lipid domains. However, the current evidence for such phase separation is indirect, and so far there has been no direct demonstration of differences in the ordering and dynamics for the lipids in these two types of regions or their relative amounts in the plasma membranes of live cells. In this study, we provide direct evidence for the presence of two different types of lipid populations in the plasma membranes of live cells from four different cell lines by electron spin resonance. Analysis of the electron spin resonance spectra recorded over a range of temperatures, from 5 to 37 degrees C, shows that the spin-labeled phospholipids incorporated experience two types of environments, L(o) and L(d), with distinct order parameters and rotational diffusion coefficients but with some differences among the four cell lines. These results suggest that coexistence of lipid domains that differ significantly in their dynamic order in the plasma membrane is a general phenomenon. The L(o) region is found to be a major component in contrast to a model in which small liquid-ordered lipid rafts exist in a 'sea' of disordered lipids. The results on ordering and dynamics for the live cells are also compared with those from model membranes exhibiting coexisting L(o) and L(d) phases.

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.

Cholesterol deficiency in a mouse model of Smith-Lemli-Opitz syndrome reveals increased mast cell responsiveness

Mutation of the 3β-hydroxysterol Δ⁷-reductase gene (Dhcr7^−/−) results in Smith-Lemli-Opitz syndrome (SLOS). Patients, and genetically altered mice, are unable to produce cholesterol and accumulate 7-dehydrocholesterol (DHC) in serum and tissue. This causes multiple growth and developmental abnormalities as well as immune system anomalies including allergy. Because cholesterol is a key component of liquid-ordered membranes (lipid rafts) and these domains have been implicated in regulating mast cell activation, we examined whether mast cell responsiveness is altered in this model. Mast cells derived from Dhcr7^−/− mice (DHCR KO) showed constitutive cytokine production and hyper-degranulation after stimulation of the high affinity IgE receptor (FcɛRI). DHCR KO mast cells, but not wild-type mast cells, accumulated DHC in lipid rafts. DHC partially disrupted lipid raft stability and displaced Lyn kinase protein and activity from lipid rafts. This led to down-regulation of some Lyn-dependent signaling events but increased Fyn kinase activity and Akt phosphorylation. The Lyn-dependent phosphorylation of Csk-binding protein, which negatively regulates Fyn activity, was decreased. This phenotype reproduces some of the characteristics of Lyn-null mast cells, which also demonstrate hyper-degranulation. These findings provide the first evidence of lipid raft dysfunction in SLOS and may explain the observed association of allergy with SLOS.

Ikaros has a crucial role in regulation of B cell receptor signaling

The transcription factor Ikaros, a key regulator of hematopoiesis, has an essential role in lymphocyte development. In mice, fetal lymphoid differentiation is blocked in the absence of Ikaros, and whereas T cells develop postnatally, B cells are totally absent. The significance of Ikaros in the B cell development is evident, but how Ikaros regulates B cell function has neither been established nor previously been studied with B cells that lack Ikaros expression. Here we show that disruption of Ikaros in the chicken B cell line DT40 induces a B cell receptor (BCR) signaling defect with reduced phospholipase Cgamma2 phosphorylation and impaired intracellular calcium mobilization, which is restored by Ikaros reintroduction. Furthermore, we show that lack of Ikaros induces hyperphosphorylation of Casitas B lymphoma protein subsequent to BCR activation. These results indicate that the absolute need of Ikaros for development, cell fate decisions and maintenance of B cells is due to the enhancement of BCR signaling.

Luteinizing hormone receptors translocate to plasma membrane microdomains after binding of human chorionic gonadotropin

Receptor-mediated signal transduction by G protein-coupled receptors can involve redistribution of plasma membrane receptors into membrane structures that are characterized by insolubility in Triton X-100 and low buoyant density in sucrose gradients. Here we describe the translocation of wild-type (wt) rat LH receptors (LHR-wt) from the bulk membrane into membrane microdomains (rafts) after the binding of human chorionic gonadotropin (hCG). In sucrose gradient ultracentrifugation of plasma membranes from cells stably expressing FLAG-tagged LHR-wt, receptors were located in high-density membrane fractions before binding of hormone and in low-density fractions after hCG treatment. Receptor translocation to low-density sucrose fractions did not occur when cells were pretreated with 1% methyl-beta-cyclodextrin, which reduces membrane cholesterol and disrupts rafts. Single-particle tracking of individual FLAG-LHR-wt receptors showed that hCG-treated receptors become confined in small compartments with a diameter of 86 +/- 36 nm, significantly smaller than 230 +/- 79 nm diameter regions accessed by the untreated receptor. Receptors were no longer confined in these small compartments after disruption of rafts by methyl-beta-cyclodextrin, a treatment that also decreased levels of cAMP in response to hCG. Finally, translocation of LHR into rafts required a functional hormone-receptor complex but did not occur after extensive receptor cross-linking that elevated cAMP levels. Thus, retention of LHR in rafts or small membrane compartments is a characteristic of functional, hormone-occupied LHR-wt. Although raft translocation was not essential for cAMP production, it may be necessary for optimizing hormone-mediated signaling.