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

Integrated signalling pathways for mast-cell activation

Mast-cell activation mediated by the high-affinity receptor for IgE (FcepsilonRI) is considered to be a key event in the allergic inflammatory response. However, in a physiological setting, other receptors, such as KIT, might also markedly influence the release of mediators by mast cells. Recent studies have provided evidence that FcepsilonRI-dependent degranulation is regulated by two complementary signalling pathways, one of which activates phospholipase Cgamma and the other of which activates phosphatidylinositol 3-kinase, using specific transmembrane and cytosolic adaptor molecules. In this Review, we discuss the evidence for these interacting pathways and describe how the capacity of KIT, and other receptors, to influence FcepsilonRI-dependent mast-cell-mediator release might be a function of the relative abilities of these receptors to activate these alternative pathways.

Role of non-raft cholesterol in lymphocytic choriomeningitis virus infection via alpha-dystroglycan

Dystroglycan (DG) is an extracellular matrix receptor necessary for the development of metazoans from flies to humans and is also an entry route for various pathogens. Lymphocytic choriomeningitis virus (LCMV), a member of the family Arenaviridae, infects by binding to alpha-DG. Here, the role of cholesterol lipid rafts in infection by LCMV via alpha-DG was investigated. The cholesterol-sequestering drugs methyl-beta-cyclodextrin (MbetaCD), filipin and nystatin inhibited the infectivity of LCMV selectively, but did not affect infection by vesicular stomatitis virus. Cholesterol loading after depletion with MbetaCD restored infectivity to control levels. DG was not found in lipid rafts identified with the raft marker ganglioside GM1. Treatment with MbetaCD, however, enhanced the solubility of DG. This may reflect the association of DG with cholesterol outside lipid rafts and suggests that association of DG with non-raft cholesterol is critical for infection by LCMV through alpha-DG.

Effects of various statins on cytokine-dependent growth and IgE-dependent release of histamine in human mast cells

BACKGROUND

Statins are inhibitors of hydroxymethylglutaryl coenzyme A (HMG CoA) reductase, a key enzyme in mevalonic acid (MVA)-dependent signaling. Recent data suggest that statins exhibit profound inhibitory effects on growth and function of various immune cells. In the present study, we examined the in vitro effects of five different statins on primary human mast cells (MCs), MC progenitors, and the human MC line HMC-1.

METHODS

Histamine release experiments were conducted on isolated MCs using statins and an anti-immunoglobulin E (IgE) antibody. Culture experiments were performed with stem cell factor (SCF) and interleukin (IL)-6, and cord blood-derived progenitors.

RESULTS

Preincubation of primary lung MCs with cerivastatin or atorvastatin (1-50 microM) for 24 h resulted in inhibition of anti-IgE-induced release of histamine. The effects of both statins were dose-dependent. Moreover, both statins, and to a lesser degree lovastatin, were found to inhibit the SCF-induced differentiation of MCs from their progenitors. The other statins tested (simvastatin, pravastatin) did not affect mediator release or growth of MCs.

CONCLUSIONS

Cerivastatin and atorvastatin act as inhibitors of growth and function of human MCs.

Bacterial invasion via lipid rafts

Accumulating reports document the use by pathogens of cholesterol-enriched lipid microdomains, often called lipid rafts, as cell surface platforms to interact, bind and possibly enter into host cells. The challenge is now to understand what could be the functional role of these domains during pathogen invasion. Are they hijacked as general clustering devices for cellular binding sites and/or do they have other roles? In particular, is their cell signalling capacity activated and used by pathogens? In reverse, could lipid rafts activate bacterial mechanisms required for invasion? These issues will be discussed after an introduction on the current view on lipid rafts.

Negative Signaling in Fc Receptor Complexes

Cell activation results from the transient displacement of an active balance between positive and negative signaling. This displacement depends in part on the engagement of cell surface receptors by extracellular ligands. Among these are receptors for the Fc portion of immunoglobulins (FcRs). FcRs are widely expressed by cells of hematopoietic origin. When binding antibodies, FcRs provide these cells with immunoreceptors capable of triggering numerous biological responses in response to a specific antigen. FcR-dependent cell activation is regulated by negative signals which are generated together with positive signals within signalosomes that form upon FcR engagement. Many molecules involved in positive signaling, including the FcRbeta subunit, the src kinase lyn, the cytosolic adapter Grb2, and the transmembrane adapters LAT and NTAL, are indeed also involved in negative signaling. A major player in negative regulation of FcR signaling is the inositol 5-phosphatase SHIP1. Several layers of negative regulation operate sequentially as FcRs are engaged by extracellular ligands with an increasing valency. A background protein tyrosine phosphatase-dependent negative regulation maintains cells in a "resting" state. SHIP1-dependent negative regulation can be detected as soon as high-affinity FcRs are occupied by antibodies in the absence of antigen. It increases when activating FcRs are engaged by multivalent ligands and, further, when FcR aggregation increases, accounting for the bell-shaped dose-response curve observed in excess of ligand. Finally, F-actin skeleton-associated high-molecular weight SHIP1, recruited to phosphorylated ITIMs, concentrates in signaling complexes when activating FcRs are coengaged with inhibitory FcRs by immune complexes. Based on these data, activating and inhibitory FcRs could be used for new therapeutic approaches to immune disorders.

Lipid segregation and IgE receptor signaling: A decade of progress

Recent work to characterize the roles of lipid segregation in IgE receptor signaling has revealed a mechanism by which segregation of liquid ordered regions from disordered regions of the plasma membrane results in protection of the Src family kinase Lyn from inactivating dephosphorylation by a transmembrane tyrosine phosphatase. Antigen-mediated crosslinking of IgE receptors drives their association with the liquid ordered regions, commonly called lipid rafts, and this facilitates receptor phosphorylation by active Lyn in the raft environment. Previous work showed that the membrane skeleton coupled to F-actin regulates stimulated receptor phosphorylation and downstream signaling processes, and more recent work implicates cytoskeletal interactions with ordered lipid rafts in this regulation. These and other results provide an emerging view of the complex role of membrane structure in orchestrating signal transduction mediated by immune and other cell surface receptors.

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.

Spying on IgE receptor signaling: simply complex, or not?

Plasma membrane organization and the potential role, or not, of lipid raft microdomains in signal transduction is a controversial topic. Cross-correlation fluorescent correlation spectroscopy (CC-FCS) shows promise as a new approach to rapidly probe protein–protein interactions in living cells during signal transduction. CC-FCS data from studies of IgE receptor signaling challenge models of large stable lipid raft signaling domains and reveal a new complexity in the dynamic (re)organization of signaling complexes.

Dynamic interactions of Fc gamma receptor IIB with filamin-bound SHIP1 amplify filamentous actin-dependent negative regulation of Fc epsilon receptor I signaling

The engagement of high affinity receptors for IgE (FcepsilonRI) generates both positive and negative signals whose integration determines the intensity of mast cell responses. FcepsilonRI-positive signals are also negatively regulated by low affinity receptors for IgG (FcgammaRIIB). Although the constitutive negative regulation of FcepsilonRI signaling was shown to depend on the submembranous F-actin skeleton, the role of this compartment in FcgammaRIIB-dependent inhibition is unknown. We show in this study that the F-actin skeleton is essential for FcgammaRIIB-dependent negative regulation. It contains SHIP1, the phosphatase responsible for inhibition, which is constitutively associated with the actin-binding protein, filamin-1. After coaggregation, FcgammaRIIB and FcepsilonRI rapidly interact with the F-actin skeleton and engage SHIP1 and filamin-1. Later, filamin-1 and F-actin dissociate from FcR complexes, whereas SHIP1 remains associated with FcgammaRIIB. Based on these results, we propose a dynamic model in which the submembranous F-actin skeleton forms an inhibitory compartment where filamin-1 functions as a donor of SHIP1 for FcgammaRIIB, which concentrate this phosphatase in the vicinity of FcepsilonRI and thereby extinguish activation signals.

Rethinking the role of Src family protein tyrosine kinases in the allergic response: new insights on the functional coupling of the high affinity IgE receptor

Antigen-induced cross-linking of immunoglobulin E (IgE) antibodies bound to the high-affinity IgE receptor (FcepsilonRI), on mast cells results in the release of mediators that initiate an inflammatory response. This normal immune response has been abducted by immunological adaptation, through the production of IgE antibodies to normally innocuous substances, to cause allergic disease. Therefore, understanding the molecular requirements in IgE-dependent mast-cell activation holds promise for therapeutic intervention in disease. Recent investigation on the functional coupling of FcepsilonRI to the intracellular signaling apparatus has provided paradigm-altering insights on the importance and function of Src family protein tyrosine kinases (Src PTK) in mast-cell activation. In this synopsis, we review the current knowledge on the role of the Src PTKs, Fyn and Lyn, in mast-cell activation and discuss the implications of our findings on allergic disease.

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.

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.

Developmental partitioning of myelin basic protein into membrane microdomains

Specific membrane microdomains (including lipid rafts) exist in myelin but have not been fully characterized. Myelin basic protein (MBP) maintains the compactness of the myelin sheath and is highly posttranslationally modified. Thus, it has been suggested that MBP might also have other functions, e.g., in signal transduction. Here, the distribution of MBP and its modified forms was studied, spatially and temporally, by detailed characterization of membrane microdomains from developing and mature bovine myelin. Myelin membranes were extracted with three different detergents (Brij 96V, CHAPS, or Triton X-100) at 4 degrees C. The detergent-resistant membranes (DRMs), representing coalesced lipid rafts, were isolated as low-buoyant-density fractions on a sucrose density gradient. These myelin rafts were disrupted when cholesterol was depleted with methyl-beta-cyclodextrin. The use of CHAPS detergent led to enrichment of several myelin proteins, including phospho-Thr97-MBP, in the DRMs from mature myelin. Citrullinated and methylated MBP remained in "nonraft" microdomains. In contrast, the DRMs from early myelin were enriched in Golli-MBP, Fyn, Lyn, and CNP. The localization of various proteins in DRMs was further supported by the colocalization of these lipid raft components in cultured mouse oligodendrocytes. Thus, there is a developmental regulation of posttranslationally modified forms of MBP into specific membrane microdomains.

Bombesin and angiotensin II rapidly stimulate Src phosphorylation at Tyr-418 in fibroblasts and intestinal epithelial cells through a PP2-insensitive pathway

Src is activated in response to a variety of growth factors and hormones that bind G protein-coupled receptors (GPCRs), and its activity is regulated by phosphorylation at key sites, including the autophosphorylation site Tyr-418 and the inhibitory site Tyr-529. To better understand the mechanisms controlling Src activation, we examined Src phosphorylation in Swiss 3T3 fibroblasts stimulated with bombesin and in IEC-18 intestinal epithelial cells stimulated with angiotensin II (Ang II). Phosphorylation at Src Tyr-418, the activation loop site, was rapidly and markedly increased after GPCR agonist addition in both cell types. However, treatment of intact cells with the selective Src family kinase inhibitor PP2, at concentrations which abolished Src-mediated phosphorylation of focal adhesion kinase (FAK) at Tyr-577, unexpectedly led to increased phosphorylation at Src Tyr-418 and diminished phosphorylation at Tyr-529. In Swiss 3T3 cells, PP2 enhanced Tyr-418 phosphorylation after 1 min of bombesin stimulation, while in IEC-18 cells, PP2 increased Ang II-stimulated Tyr-418 phosphorylation at all times tested. These results imply that a distinct, non-Src family kinase may be responsible for phosphorylating Src at Tyr-418 in intact fibroblasts and epithelial cells stimulated by GPCR agonists.

The proto-oncogene Fgr regulates cell migration and this requires its plasma membrane localization

Fgr participates in integrin signaling in myeloid leukocytes. To examine the role of its specific domains in regulating cell migration, we expressed various Fgr molecules in COS-7 cells. Full-length, membrane-bound Fgr, but not an N-terminal truncation mutant that distributed to an intracellular compartment, increased cell migration on fibronectin and enhanced phosphorylation of the p85 subunit of phosphatidylinositol 3-kinase (PI3K), cortactin and focal adhesion kinase (FAK) at Y397 and Y576. Fgr increased Rac GTP loading, and phosphorylation of the Rac GEF Vav2, and bound to a protein complex formed by the Rho inhibitor p190RhoGAP and FAK, increasing p190RhoGAP phosphorylation, in a manner absolutely dependent on membrane localization. A kinase-defective truncation mutant of Fgr increased cell migration, albeit to a much lower extent than full-length Fgr, and was found to associate with the plasma membrane, to activate Rac and to form complexes with p190RhoGAP/FAK. Formation of complexes between p190RhoGAP, Fgr, and the FAK-related protein Pyk2 were also detected in murine macrophages. These findings suggest that the proto-oncogene Fgr regulates cell migration impinging on a signaling pathway implicating FAK/Pyk2 and leading to activation of Rac and the Rho inhibitor p190RhoGAP.

Molecular dynamics and interactions for creation of stimulation-induced stabilized rafts from small unstable steady-state rafts

We have evaluated the sizes and lifetimes of rafts in the plasma membrane from the existing literature, with a special attention paid to their intrinsically broad distributions and the limited time and space scales that are covered by the observation methods used for these studies. Distinguishing the rafts in the steady state (reserve rafts) from those after stimulation or unintentional crosslinking of raft molecules (stabilized receptor-cluster rafts) is critically important. In resting cells, the rafts appear small and unstable, and the consensus now is that their sizes are smaller than the optical diffraction limit (250 nm). Upon stimulation, the raft-preferring receptors are clustered, inducing larger, stabilized rafts, probably by coalescing small, unstable rafts or cholesterol-glycosphingolipid complexes in the receptor clusters. This receptor-cluster-induced conversion of raft types may be caused by suppression of alkyl chain isomerization and the lipid lateral diffusion in the cluster, with the aid of exclusion of cholesterol from the bulk domain and the boundary region of the majority of transmembrane proteins. We critically inspected the possible analogy to the boundary lipid concept. Finally, we propose a hypothesis for the coupling of GPI-anchored receptor signals with lipid-anchored signaling molecules in the inner-leaflet raft.

Reconstitution of regulated phosphorylation of FcepsilonRI by a lipid raft-excluded protein-tyrosine phosphatase

To examine the exquisite regulation of IgE-FcepsilonRI tyrosine phosphorylation by Lyn kinase that is stimulated by antigen-mediated cross-linking, we utilized co-expression of FcepsilonRI and Lyn in Chinese hamster ovary cells, which results in high basal levels of Lyn kinase activity and spontaneous phosphorylation of FcepsilonRI. We found that co-expression of a lipid raft-excluded transmembrane tyrosine phosphatase, PTPalpha, suppresses Lyn kinase activity and markedly reduces the level of spontaneous phosphorylation of FcepsilonRI, while facilitating its antigen-stimulated phosphorylation. Other tyrosine phosphatases, including SHP-1, CD45, and a lipid raft-preferring chimeric version of PTPalpha fail to reconstitute antigen-dependent FcepsilonRI phosphorylation. We concluded that both substrate specificity and submembrane location are critical to phosphatase-mediated regulation of Lyn kinase activity that supports activation of FcepsilonRI.

Early activation of sphingosine kinase in mast cells and recruitment to FcepsilonRI are mediated by its interaction with Lyn kinase

Sphingosine kinase has been recognized as an essential signaling molecule that mediates the intracellular conversion of sphingosine to sphingosine-1-phosphate. In mast cells, induction of sphingosine kinase and generation of sphingosine-1-phosphate have been linked to the initial rise in Ca(2+), released from internal stores, and to degranulation. These events either precede or are concomitant with the activation of phospholipase C-gamma and the generation of inositol trisphosphate. Here we show that sphingosine kinase type 1 (SPHK1) interacts directly with the tyrosine kinase Lyn and that this interaction leads to the recruitment of this lipid kinase to the high-affinity receptor for immunoglobulin E (FcepsilonRI). The interaction of SPHK1 with Lyn caused enhanced lipid and tyrosine kinase activity. After FcepsilonRI triggering, enhanced sphingosine kinase activity was associated with FcepsilonRI in sphingolipid-enriched rafts of mast cells. Bone marrow-derived mast cells from Lyn(-/)(-) mice, compared to syngeneic wild-type cells, were defective in the initial induction of SPHK1 activity, and the defect was overcome by retroviral Lyn expression. These findings position the activation of SPHK1 as an FcepsilonRI proximal event.

Isolation and characterization of lipid rafts with different properties from RBL-2H3 (rat basophilic leukaemia) cells

Lipid rafts are plasma-membrane microdomains that are enriched in certain lipids (sphingolipids, glycosphingolipids and cholesterol), as well as in lipid-modified proteins. Rafts appear to exist in the liquid-ordered phase, which contributes to their partitioning from the surrounding liquid-disordered glycerophospholipid environment. DRM (detergent-resistant membrane) fractions isolated from cells are believed to represent coalesced lipid rafts. We have employed extraction using two different non-ionic detergents, Brij-96 and Triton X-100, to isolate detergent-resistant lipid rafts from rat basophilic leukaemia cell line RBL-2H3, and compared their properties with each other and with plasma-membrane vesicles. DRM fractions were isolated as sealed unilamellar vesicles of similar size (135-170 nm diameter), using either sucrose-density-gradient sedimentation or gel-filtration chromatography. Lipid rafts isolated using Brij-96 and Triton X-100 differed in density, protein content and the distribution between high- and low-density fractions of the known raft constituents, Thy-1, and the non-receptor protein tyrosine kinases, Yes and Lyn. Lyn was found in the raft microdomains in predominantly phosphorylated form. The level of enrichment of the protein constituents of the isolated lipid rafts seemed to depend on the ratio of cell lipid/protein to detergent. As indicated by reactivity with anti-Thy-1 antibodies, lipid rafts prepared using Brij-96 appeared to consist of vesicles with primarily right-side-out orientation. Both Brij-96 and Triton X-100 appear to isolate detergent-insoluble raft microdomains from the rat basophilic leukaemia cell line RBL-2H3, but the observed differences suggest that either the detergents themselves play a role in determining the physicochemical characteristics of the resulting DRM fractions, or different subsets of rafts are isolated by the two detergents.

Molecular interdiction of Src-family kinase signaling in hematopoietic cells

The ability of Src-family kinases (SFKs) to mediate signaling from cell surface receptors in hematopoietic cells is a function of their catalytic activity, location and binding partners. Kinase activity is regulated in the cell by kinases and phosphatases that alter the state of phosphorylation of key tyrosine residues and by protein binding partners that stabilize the kinase in active or inactive conformations or localize the enzyme to specific subcellular or submembrane domains. Kinase activity and function can be modulated experimentally through the use of small molecule inhibitors designed to directly target catalytic or binding domains or regulate the location of the protein by altering its state of acylation.

Protein kinase C-alpha and -delta are required for FcalphaR (CD89) trafficking to MHC class II compartments and FcalphaR-mediated antigen presentation

Studies have demonstrated that receptor-mediated signaling, receptor/antigen complex trafficking, and major histocompatibility complex class II compartments (MIIC) are critically related to antigen presentation to CD4+ T cells. In this study, we investigated the role of protein kinase C (PKC) in FcalphaR/gammagamma (CD89, human IgA receptor)-mediated internalization of immune complexes and subsequent antigen presentation. The classical and novel PKC inhibitor, Calphostin C, inhibits FcalphaR-mediated antigen presentation and interaction of MIIC and cargo vesicle (receptor and antigen). PKC-alpha, PKC-delta, and PKC-epsilon were recruited to lipid rafts following FcalphaR crosslinking, the extent of which was determined by the phenotype of the gamma chain. Mutant gamma chain with an FcgammaRIIA ITAM (immunoreceptor tyrosine-based activation motif) insert was less able to recruit PKC and trigger antigen presentation. Both PKC isoform-specific peptide inhibitors and short interfering RNA (siRNA) showed that PKC-alpha and PKC-delta, but not PKC-epsilon, were required for association of cargo vesicle and MIIC and for FcalphaR-mediated and soluble antigen presentation. Inhibition of PKC (classical and novel) did not alter major histocompatibility class II biosynthesis, assembly, transport, or plasma membrane stability. PKC's role in facilitating interaction of cargo vesicle and MIIC is likely due to regulation of vesicle biology required for fusion of cargo vesicles to MIIC.

Lipid rafts-protein association and the regulation of protein activity

Lipid rafts are membrane microdomains enriched in saturated phospholipids, sphingolipids, and cholesterol. They have a varied but distinct protein composition and have been implicated in diverse cellular processes including polarized traffic, signal transduction, endo- and exo-cytoses, entrance of obligate intracellular pathogens, and generation of pathological forms of proteins associated with Alzheimer's and prion diseases. Raft proteins can be permanently or temporarily associated to lipid rafts. Here, we review recent advances on the biochemical and cell biological characterization of rafts, and on the emerging concept of the temporary residency of proteins in rafts as a regulatory mechanism of their biological activity.

The 2004 Biophysical Society-Avanti Award in Lipids address: roles of bilayer structure and elastic properties in peptide localization in membranes

This review details how bilayer structural/elastic properties impact three distinct areas of biological significance. First, the partitioning of melittin into bilayers and melittin-induced bilayer leakage depended strongly on bilayer composition. The incorporation of cholesterol into phosphatidylcholine bilayers decreased melittin-induced leakage from 73 to 3%, and bilayers composed of lipopolysaccharide (LPS), the main lipid on the surface of Gram-negative bacteria, also had low (3%) melittin-induced leakage. Second, transbilayer peptides of different hydrophobic lengths were largely excluded from bilayer microdomains ("rafts") enriched in sphingomyelin (SM) and cholesterol, even when the length of the transbilayer peptide domain matched the hydrocarbon thickness of the raft bilayer. This is likely due to the large area compressibility modulus of SM:cholesterol bilayers. Third, the major water barrier of skin, the extracellular lamellae of the stratum corneum, was found to contain tightly packed asymmetric lipid bilayers with cholesterol located preferentially on one side of the bilayer and a unique skin ceramide containing an unsaturated acyl chain on the opposite side. We argue that, in each of these three areas, key factors are differences in lipid hydrocarbon chain packing for different lipids, particularly the tight hydrocarbon chain packing caused by cholesterol's strong interaction with saturated chains.

Negative regulation of immunoglobulin E-dependent allergic responses by Lyn kinase

A role for Lyn kinase as a positive regulator of immunoglobulin (Ig)E-dependent allergy has long been accepted. Contrary to this belief, Lyn kinase was found to have an important role as a negative regulator of the allergic response. This became apparent from the hyperresponsive degranulation of lyn-/- bone marrow-derived mast cells, which is driven by hyperactivation of Fyn kinase that occurs, in part, through the loss of negative regulation by COOH-terminal Src kinase (Csk) and the adaptor, Csk-binding protein. This phenotype is recapitulated in vivo as young lyn-/- mice showed an enhanced anaphylactic response. In vivo studies also demonstrated that as lyn-/- mice aged, their serum IgE increased as well as occupancy of the high affinity IgE receptor (FcepsilonRI). This was mirrored by increased circulating histamine, increased mast cell numbers, increased cell surface expression of the high affinity IgE receptor (FcepsilonRI), and eosinophilia. The increased IgE production was not a consequence of increased Fyn kinase activity in lyn-/- mice because both lyn-/- and lyn-/- fyn-/- mice showed high IgE levels. Thus, lyn-/- mice and mast cells thereof show multiple allergy-associated traits, causing reconsideration of the possible efficacy in therapeutic targeting of Lyn in allergic disease.

Lipid-dependent recruitment of neuronal Src to lipid rafts in the brain

Although most Src family tyrosine kinases are modified by palmitoylation as well as myristoylation, Src itself is only myristoylated. Dual acylation is important for attachment to liquid-ordered microdomains or lipid rafts. Accordingly, Src is excluded from lipid rafts in fibroblasts. Evidence of partial genetic redundancy between Src and Fyn for brain-specific targets suggests that these two kinases may occupy overlapping subcellular locations. Neuronal Src (NSrc), an alternative isoform of Src with a 6-amino acid insert in the Src homology 3 domain, is highly expressed in neurons. We investigated whether this structural difference in NSrc allows it to associate with lipid rafts. We found that perinatal mouse brains express predominantly NSrc, which is partly (10-20%) in a lipid raft fraction from brain but not fibroblasts. The association of Src with brain lipid rafts does not depend on the NSrc insert but depends on the amino-terminal myristoylation signal. A crude lipid fraction from brain promotes NSrc entry into rafts in vitro. Moreover, lipid raft-localized NSrc is more catalytically active than NSrc from the soluble fraction, possibly because raft localization alters access to other tyrosine kinases and phosphatases. These findings suggest that NSrc may be involved in signaling from lipid rafts in mouse brain.

Specific phospholipid oxidation products inhibit ligand activation of toll-like receptors 4 and 2

OBJECTIVE

We have previously shown that phospholipid oxidation products of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (ox-PAPC) inhibit lipopolysaccharide (LPS)-induced E-selectin expression and neutrophil binding in human aortic endothelial cells (HAECs). The current studies identify specific phospholipids that inhibit chemokine induction by Toll-like receptor-4 (TLR4) and -2 (TLR2) ligands inECs and macrophages.

METHODS AND RESULTS

Measurements of interleukin (IL)-8 and monocyte chemotactic protein-1 levels secreted from ox-PAPC- and LPS-cotreated ECs indicate that ox-PAPC inhibits activation of TLR4 by LPS. The effects of IL-1beta and tumor necrosis factor-alpha, which utilize the same intracellular signaling molecules, were not inhibited. Cell fractionation and immunofluorescence analyses demonstrate that LPS induces membrane translocation of the LPS receptor complex to a lipid raft/caveolar fraction in ECs. Ox-PAPC inhibits this translocation and alters caveolin-1 distribution. Supporting an important role for caveolae in LPS action, overexpression of caveolin-1 enhanced LPS-induced IL-8 synthesis. Ox-PAPC also inhibits the effect of TLR2 and TLR4 ligands in human macrophages.

CONCLUSIONS

These studies report a novel mechanism that involves alterations to lipid raft/caveolar processing, by which specific phospholipid oxidation products inhibit activation by TLR4 and TLR2 ligands. These studies have broader implications for the role of ox-PAPC as a regulator of specific lipid raft/caveolar function.