Segregation of leading-edge and uropod components into specific lipid rafts during T cell polarization

Molecular partitioning during host cell penetration by Toxoplasma gondii

During invasion by Toxoplasma gondii, host cell transmembrane proteins are excluded from the forming parasitophorous vacuole membrane (PVM) by the tight apposition of host and parasite cellular membranes. Previous studies suggested that the basis for the selective partitioning of membrane constituents may be a preference for membrane microdomains, and this hypothesis was herein tested. The partitioning of a diverse group of molecular reporters for raft and nonraft membrane subdomains was monitored during parasite invasion by time-lapse video or confocal microscopy. Unexpectedly, both raft and nonraft lipid probes, as well as both raft and nonraft cytosolic leaflet proteins, flowed unhindered past the host-parasite junction into the PVM. Moreover, neither a raft-associated type 1 transmembrane protein nor its raft-dissociated counterpart accessed the PVM, while a multispanning membrane raft protein readily did so. Considered together with previous data, these studies demonstrate that selective partitioning at the host-parasite interface is a highly complex process, in which raft association favors, but is neither necessary nor sufficient for, inclusion into the T. gondii PVM.

Cytoskeletal interactions regulate inducible L-selectin clustering

L-selectin (CD62L) amplifies neutrophil capture within the microvasculature at sites of inflammation. Activation by G protein-coupled stimuli or through ligation of L-selectin promotes clustering of L-selectin and serves to increase its adhesiveness, signaling, and colocalization with beta(2)-integrins. Currently, little is known about the molecular process regulating the lateral mobility of L-selectin. On neutrophil stimulation, a progressive change takes place in the organization of its plasma membrane, resulting in membrane domains that are characteristically enriched in glycosyl phosphatidylinositol (GPI)-anchored proteins and exclude the transmembrane protein CD45. Clustering of L-selectin, facilitated by E-selectin engagement or antibody cross-linking, resulted in its colocalization with GPI-anchored CD55, but not with CD45 or CD11c. Disrupting microfilaments in neutrophils or removing a conserved cationic motif in the cytoplasmic domain of L-selectin increased its mobility and membrane domain localization in the plasma membrane. In addition, the conserved element was critical for L-selectin-dependent tethering under shear flow. Our data indicate that L-selectin's lateral mobility is regulated by interactions with the actin cytoskeleton that in turn fortifies leukocyte tethering. We hypothesize that both membrane mobility and stabilization augment L-selectin's effector functions and are regulated by dynamic associations with membrane domains and the actin cytoskeleton.

Vaccinia virus penetration requires cholesterol and results in specific viral envelope proteins associated with lipid rafts

Vaccinia virus infects a wide variety of mammalian cells from different hosts, but the mechanism of virus entry is not clearly defined. The mature intracellular vaccinia virus contains several envelope proteins mediating virion adsorption to cell surface glycosaminoglycans; however, it is not known how the bound virions initiate virion penetration into cells. For this study, we investigated the importance of plasma membrane lipid rafts in the mature intracellular vaccinia virus infection process by using biochemical and fluorescence imaging techniques. A raft-disrupting drug, methyl-beta-cyclodextrin, inhibited vaccinia virus uncoating without affecting virion attachment, indicating that cholesterol-containing lipid rafts are essential for virion penetration into mammalian cells. To provide direct evidence of a virus and lipid raft association, we isolated detergent-insoluble glycolipid-enriched membranes from cells immediately after virus infection and demonstrated that several viral envelope proteins, A14, A17L, and D8L, were present in the cell membrane lipid raft fractions, whereas the envelope H3L protein was not. Such an association did not occur after virions attached to cells at 4 degrees C and was only observed when virion penetration occurred at 37 degrees C. Immunofluorescence microscopy also revealed that cell surface staining of viral envelope proteins was colocalized with GM1, a lipid raft marker on the plasma membrane, consistent with biochemical analyses. Finally, mutant viruses lacking the H3L, D8L, or A27L protein remained associated with lipid rafts, indicating that the initial attachment of vaccinia virions through glycosaminoglycans is not required for lipid raft formation.

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.

A role for caveolin-1 in post-injury reactive neuronal plasticity

Remodeling and plasticity in the adult brain require cholesterol redistribution and synthesis for the formation of new membrane components. Caveolin-1 is a cholesterol-binding membrane protein involved in cellular cholesterol transport and homeostasis. Evidence presented here demonstrates an up-regulation of caveolin-1 in the hippocampus, which was temporally correlated with an increase in synaptophysin during the reinnervation phase in a mouse model of hippocampal deafferentation. Using an in vitro model of neuronal reactive plasticity, we examined the effect of virally mediated overexpression of caveolin-1 on injured differentiated PC12 cells undergoing terminal remodeling. Three days post lesion, caveolin-1-overexpressing cells revealed increases in synaptophysin and GAP-43, two markers of neurite sprouting and synaptogenesis. Morphologically, caveolin-1-overexpressing cells showed a decrease in primary neurite outgrowth and branching as well as an increase in neurite density. Caveolin-1-overexpressing cells also revealed the presence of terminal swelling and beading along processes, consistent with a possible alteration of microtubules stability. Moreover, a focal enrichment of caveolin-1 immunofluorescence was observed at the bases of axonal and dendritic terminals of mouse primary hippocampal neurons. Altogether, these results indicate that caveolin-1 plays an active role in the regulation of injury-induced synaptic and terminal remodeling in the adult CNS.

The subapical compartment: a traffic center in membrane polarity development

Spatially separated apical and basolateral plasma membrane domains that have distinct functions and molecular compositions are a characteristic feature of epithelial cell polarity. The subapical compartment (SAC), also known as the common endosome (CE), where endocytic pathways from both surfaces merge, plays a crucial role in the maintenance and probably the biogenesis of these distinct membrane domains. Although differences in morphology are apparent, the same principal features of a SAC can be distinguished in different types of epithelial cells. As polarity develops, the compartment acquires several distinct machineries that, in conjunction with the cytoskeleton, are necessary for polarized trafficking. Disrupting trafficking via the SAC and hence bypassing its sorting machinery, as occurs upon actin depolymerization, leads to mis-sorting of apical and basolateral molecules, thereby compromising the development of polarity. The structural and functional integrity of the compartment in part depends on microtubules. Moreover, the acquisition of a particular set of Rab proteins, including Rab11 and Rab3, appears to be crucial in regulating molecular sorting and vesicular transport relevant both to recycling to either plasma membrane domain and to de novo assembly of the apical domain. Furthermore, subcompartmentalization of the SAC appears to be key to its various functions.

Lipid rafts and their roles in T-cell activation

Lipid rafts are defined as detergent-resistant membrane microdomains of specific lipid and protein composition. They are involved in many aspects of cell biology, including T-cell activation and immunoreceptor signaling. This review discusses current controversies around lipid rafts and summarizes recent developments in the area.

Secreted MMP9 promotes angiogenesis more efficiently than constitutive active MMP9 bound to the tumor cell surface

Association of matrix metalloprotease 9 (MMP9) to the cell membrane is considered important in tumor growth and angiogenesis. To dissect this regulatory mechanism, we generated raft and non-raft MMP9 chimeras to force membrane expression in the MCF-7 human breast carcinoma cell line. MMP9 targeting to non-raft cell surface domains rendered a constitutive active membrane MMP9 form, suggesting a contribution by the lipid environment in MMP activation. We generated human breast cancer xenograft models using MCF-7 cells overexpressing secreted and membrane-anchored MMP9. The non-raft MMP9 chimera was constitutively active at the cell membrane in xenografts, but this activation did not correlate with an increase in MMP9-induced angiogenesis. Capillary number and vessel perimeter were specifically increased only in tumors overexpressing wild-type MMP9 (the secreted form); this increase was inhibited when tumors were induced in doxycycline-treated mice. Xenografts from tumor cells overexpressing wild-type MMP9 showed increased vascular endothelial growth factor (VEGF)/VEGFR2 receptor association, which was also dependent on MMP9 activity. These observations indicate that membrane location can influence MMP9 activity in vitro and in vivo, and confirm the relevance of stromal-associated, but not tumor-bound MMP9 in mediating tumor-induced angiogenesis.

Membrane domains in lymphocytes - from lipid rafts to protein scaffolds

Lateral compartmentalization of the plasma membrane into domains is a key feature of immune cell activation and subsequent immune effector functions. Here, we will review the high diversity of membrane domains, ranging from elementary lipid rafts, envisioned as dynamic and small domains (in the tens of nm), to relatively stable microm-scale membrane domains, which form the immunologic synapse of T lymphocytes. We will discuss the relationship between these different types of plasma membrane domains and how raft lipid- and protein-controlled interactions and cell biological processes cooperate to generate functional domains that mediate lymphocyte activity.

Polarized sorting in epithelial cells: raft clustering and the biogenesis of the apical membrane

Polarized cells establish and maintain functionally distinct surface domains by an elaborate sorting process, which ensures accurate delivery of biosynthetic cargo to different parts of the plasma membrane. This is particularly evident in polarized epithelial cells, which have been used as a model system for studies of sorting mechanisms. The clustering of lipid rafts through the oligomerization of raft components could be utilized for segregating apical from basolateral cargo and for the generation of intracellular transport carriers. Besides functioning in polarized sorting in differentiated cells, raft clustering might also play an important role in the biogenesis of apical membrane domains during development.

A role for caveolae in cell migration

Caveolae are specialized plasma membrane subdomains capable of transport and sophisticated compartmentalization of cell signaling. Numerous cell functions, including cell type-specific functions, involve caveolae and require caveolin-1, the major protein component of these organelles. Caveolae are particularly abundant in endothelial cells and participate in endothelial transcytosis, vascular permeability, vasomotor tone control, and vascular reactivity. Caveolin-1 drives the formation of plasma membrane caveolae and anchors them to the actin cytoskeleton, modulates cell interaction with the extracellular matrix, pulls together and regulates signaling molecules, and transports cholesterol. Via these functions, caveolin-1 might play an important role in cell movement through control of cell membrane composition and membrane surface expansion, polarization of signaling molecules and matrix proteolysis, and/or cytoskeleton remodeling. Caveolae and caveolin-1 are polarized in migrating endothelial cells, indicating they may play a role in cell motility. Several studies have shown that manipulation of caveolin-1 expression affects cell migration in a complex way. We are reviewing the current data and hypotheses in favor of an essential role for caveolae in cell migration.

Regulation of T-cell receptor signalling by membrane microdomains

There is now considerable evidence suggesting that the plasma membrane of mammalian cells is compartmentalized by functional lipid raft microdomains. These structures are assemblies of specialized lipids and proteins and have been implicated in diverse biological functions. Analysis of their protein content using proteomics and other methods revealed enrichment of signalling proteins, suggesting a role for these domains in intracellular signalling. In T lymphocytes, structure/function experiments and complementary pharmacological studies have shown that raft microdomains control the localization and function of proteins which are components of signalling pathways regulated by the T-cell antigen receptor (TCR). Based on these studies, a model for TCR phosphorylation in lipid rafts is presented. However, despite substantial progress in the field, critical questions remain. For example, it is unclear if membrane rafts represent a homogeneous population and if their structure is modified upon TCR stimulation. In the future, proteomics and the parallel development of complementary analytical methods will undoubtedly contribute in further delineating the role of lipid rafts in signal transduction mechanisms.

Dynamic recruitment of the adaptor protein LAT: LAT exists in two distinct intracellular pools and controls its own recruitment

The integral membrane adaptor protein linker for activation of T cells (LAT) couples the T-cell receptor (TCR) with downstream signalling and is essential for T-cell development and activation. Here, we investigate the dynamic distribution of LAT-GFP fusion proteins by time-lapse video imaging of live T lymphocytes interacting with antigen-presenting cells. We show that LAT forms two distinct cellular pools, one at the plasma membrane and one that co-distributes with transferrin-labelled intracellular compartments also containing the TCR/CD3-associated zeta chain. The distribution of LAT between these two pools is dependent on LAT intracytoplasmic residues. Whereas plasma membrane-associated LAT is recruited to immune synapses after a few seconds of cell conjugate formation, the intracellular pool is first polarized and then recruited after a few minutes. We further show that LAT intracytoplasmic amino acid residues, particularly the Tyr136, 175, 195 and 235 residues, are required for its own recruitment to the immune synapse and that a herein-identified juxtamembrane LAT region (amino acids 32-104) is involved in the localization of LAT in intracellular pools and in T-cell signalling. Altogether, our results demonstrate that LAT controls its own recruitment at the immune synapse, where it is required as a scaffold protein for the signalling machinery. The results also suggest that the intracellular pool of LAT might be required for T-cell activation.

Cholesterol depletion increases membrane stiffness of aortic endothelial cells

This study has investigated the effect of cellular cholesterol on membrane deformability of bovine aortic endothelial cells. Cellular cholesterol content was depleted by exposing the cells to methyl-beta-cyclodextrin or enriched by exposing the cells to methyl-beta-cyclodextrin saturated with cholesterol. Control cells were treated with methyl-beta-cyclodextrin-cholesterol at a molar ratio that had no effect on the level of cellular cholesterol. Mechanical properties of the cells with different cholesterol contents were compared by measuring the degree of membrane deformation in response to a step in negative pressure applied to the membrane by a micropipette. The experiments were performed on substrate-attached cells that maintained normal morphology. The data were analyzed using a standard linear elastic half-space model to calculate Young elastic modulus. Our observations show that, in contrast to the known effect of cholesterol on membrane stiffness of lipid bilayers, cholesterol depletion of bovine aortic endothelial cells resulted in a significant decrease in membrane deformability and a corresponding increase in the value of the elastic coefficient of the membrane, indicating that cholesterol-depleted cells are stiffer than control cells. Repleting the cells with cholesterol reversed the effect. An increase in cellular cholesterol to a level higher than that of normal cells, however, had no effect on the elastic properties of bovine aortic endothelial cells. We also show that although cholesterol depletion had no apparent effect on the intensity of F-actin-specific fluorescence, disrupting F-actin with latrunculin A abrogated the stiffening effect. We suggest that cholesterol depletion increases the stiffness of the membrane by altering the properties of the submembrane F-actin and/or its attachment to the membrane.

Fluorescence imaging of two-photon linear dichroism: cholesterol depletion disrupts molecular orientation in cell membranes

The plasma membrane of cells is an ordered environment, giving rise to anisotropic orientation and restricted motion of molecules and proteins residing in the membrane. At the same time as being an organized matrix of defined structure, the cell membrane is heterogeneous and dynamic. Here we present a method where we use fluorescence imaging of linear dichroism to measure the orientation of molecules relative to the cell membrane. By detecting linear dichroism as well as fluorescence anisotropy, the orientation parameters are separated from dynamic properties such as rotational diffusion and homo energy transfer (energy migration). The sensitivity of the technique is enhanced by using two-photon excitation for higher photo-selection compared to single photon excitation. We show here that we can accurately image lipid organization in whole cell membranes and in delicate structures such as membrane nanotubes connecting two cells. The speed of our wide-field imaging system makes it possible to image changes in orientation and anisotropy occurring on a subsecond timescale. This is demonstrated by time-lapse studies showing that cholesterol depletion rapidly disrupts the orientation of a fluorophore located within the hydrophobic region of the cell membrane but not of a surface bound probe. This is consistent with cholesterol having an important role in stabilizing and ordering the lipid tails within the plasma membrane.

Shape changes and vesicle fission of giant unilamellar vesicles of liquid-ordered phase membrane induced by lysophosphatidylcholine

Liquid-ordered phase (lo phase) of lipid membranes has properties that are intermediate between those of liquid-crystalline phase and those of gel phase and has attracted much attention in both biological and biophysical aspects. Rafts in the lo phase in biomembranes play important roles in cell function of mammalian cells such as signal transduction. In this report, we have prepared giant unilamellar vesicles (GUVs) of lipid membranes in the lo phase and investigated their physical properties using phase-contrast microscopy and fluorescence microscopy. GUVs of dipalmitoyl-phosphatidylcholine (DPPC)/cholesterol membranes and also GUVs of sphingomyelin (SM)/cholesterol membranes in the lo phase in water were formed at 20-37 degrees C successfully, when these membranes contained >/=30 mol % cholesterol. The diameters of GUVs of DPPC/cholesterol and SM/cholesterol membranes did not change from 50 to 28 degrees C, supporting that the membranes of these GUVs were in the lo phase. To elucidate the interaction of a substance with a long hydrocarbon chain with the lo phase membrane, we investigated the interaction of low concentrations (less than critical micelle concentration) of lysophosphatidylcholine (lyso-PC) with DPPC/cholesterol GUVs and SM/cholesterol GUVs in the lo phase. We found that lyso-PC induced several shape changes and vesicle fission of these GUVs above their threshold concentrations in water. The analysis of these shape changes indicates that lyso-PC can be partitioned into the external monolayer in the lo phase of the GUV from the aqueous solution. Threshold concentrations of lyso-PC in water to induce the shape changes and vesicle fission increased greatly with a decrease in chain length of lyso-PC. Thermodynamic analysis of this result indicates that shape changes and vesicle fission occur at threshold concentrations of lyso-PC in the membrane. These new findings on GUVs of the lo phase membranes indicate that substances with a long hydrocarbon chain such as lyso-PC can enter into the lo phase membrane and also the raft in the cell membrane. We have also proposed a mechanism for the lyso-PC-induced vesicle fission of GUVs.

Spatial and temporal control of signaling through lipid rafts

Sphingolipid- and cholesterol-dependent microdomains (rafts) order proteins at biological membranes and have been implicated in most signaling processes at the cell surface, but the principles and mechanisms through which lipid rafts influence signaling are not well understood. Recent studies have revealed how lipid rafts are rapidly redistributed and assembled locally in response to extracellular signals, and how components of raft-based signaling domains undergo rapid and regulated rearrangements influencing signal quality, duration, and strength. These findings highlight the exquisitely dynamic properties of signaling domains based on lipid rafts, and suggest that processes of raft trafficking and assembly take central roles in mediating spatial and temporal control of signaling.

Glycosylation patterns are sexually dimorphic throughout development of the olfactory system in Manduca sexta

In the moth Manduca sexta, development of the adult olfactory system depends on complex interactions between olfactory receptor neurons in the antenna, antennal-lobe neurons in the brain, and several classes of glial cells. As one approach to characterizing molecules that may play roles in these interactions, we used lectins to screen antennae and antennal lobes at different stages of adult development. We find that each of the major neural cell types has a distinct pattern of labeling by lectins. Effects of enzymatic and other treatments on lectin labeling lead us to conclude that the predominant lectin ligands are: glycosphingolipids and an O-linked, fucose-containing glycoprotein on axons of olfactory receptor neurons, O-linked glycoproteins on antennal-lobe neurons, and N-linked glycoproteins on all classes of glial cells in the primary olfactory pathway. Wheat germ agglutinin labels all olfactory axons uniformly during much of development, but labeling becomes restricted to the pheromone-responsive olfactory receptor neurons in the adult male. Succinylated WGA reveals differences in these axon classes earlier, as glomerului develop from protoglomeruli. The adult female displays a less pronounced difference in labeling of axons targeting ordinary and sexually dimorphic glomeruli. Differences in labeling of receptor axons targeted to ordinary and sexually dimorphic glomeruli may be correlated with differences in function or connectivity in different regions of the antennal lobe.

Lipid rafts mediate association of LFA-1 and CD3 and formation of the immunological synapse of CTL

Lipid rafts accumulate in the immunological synapse formed by an organized assembly of the TCR/CD3, LFA-1, and signaling molecules. However, the precise role of lipid rafts in the formation of the immunological synapse is unclear. In this study, we show that LFA-1 on CTL is constitutively active and mediates Ag-independent binding of CTL to target cells expressing its ligands. LFA-1 and CD3 on CTL, but not resting T cells, colocalize in lipid rafts. Binding of LFA-1 on CTL to targets initiates the formation of the immunological synapse, which is formed by LFA-1, CD3, and ganglioside GM1 distributed in the periphery of the cell contact site and cholesterol is more widely distributed. The formation of this synapse is Ag independent, but the recognition of Ag by the TCR induces accumulation of tyrosine phosphorylated proteins in the synapse as well as redistribution of the microtubule organization center toward the cell contact site. Our results suggest that LFA-1 recruits lipid rafts and the TCR/CD3 to the synapse, and facilitates efficient and rapid activation of CTL.

PrPc capping in T cells promotes its association with the lipid raft proteins reggie-1 and reggie-2 and leads to signal transduction

The cellular prion protein (PrPc) resides in lipid rafts, yet the type of raft and the physiological function of PrPc are unclear. We show here that cross-linking of PrPc with specific antibodies leads to 1) PrPc capping in Jurkat and human peripheral blood T cells; 2) to cocapping with the intracellular lipid raft proteins reggie-1 and reggie-2; 3) to signal transduction as seen by MAP kinase phosphorylation and an elevation of the intracellular Ca2+ concentration; 4) to the recruitment of Thy-1, TCR/CD3, fyn, lck and LAT into the cap along with local tyrosine phosphorylation and F-actin polymerization, and later, internalization of PrPc together with the reggies into limp-2 positive lysosomes. Thus, PrPc association with reggie rafts triggers distinct transmembrane signal transduction events in T cells that promote the focal concentration of PrPc itself by guiding activated PrPc into preformed reggie caps and then to the recruitment of important interacting signaling molecules.

Differential Requirements for DOCK2 and Phosphoinositide-3-Kinase γ during T and B Lymphocyte Homing

Chemokines guide lymphocytes from blood to secondary lymphoid organs by triggering integrin-dependent firm adhesion under vascular flow and directed migration of T and B lymphocytes within lymphoid tissue. Here, we analyze the roles of DOCK2, a mammalian homolog of Caenorhabditis elegans CED-5 and Drosophila melanogaster Myoblast City, and phosphoinositide-3-kinase (PI3K) during lymphocyte recirculation. DOCK2 mediated efficient lymphocyte migration in a largely PI3K-independent manner, although a minor, PI3K-dependent pathway for migration was observed in wild-type and DOCK2-deficient lymphocytes. In T cells, this residual migration depended mainly on PI3Kgamma, whereas other PI3K isoforms were implicated in B cells. In vitro adhesion assays and intravital microscopy of lymphoid organ vasculature uncovered an unexpected defect in integrin activation in DOCK2-/- B cells, whereas lack of DOCK2 did not affect chemokine-triggered integrin activation in T cells. DOCK2 and PI3Kgamma thus play distinct roles during T and B cell integrin activation and migration.

A lipid-specific toxin reveals heterogeneity of sphingomyelin-containing membranes

Little is known about the heterogenous organization of lipids in biological membranes. Sphingomyelin (SM) is a major plasma membrane lipid that forms lipid domains together with cholesterol and glycolipids. Using SM-specific toxin, lysenin, we showed that in cultured epithelial cells the accessibility of the toxin to SM is different between apical and basolateral membranes. Apical membranes are highly enriched with glycolipids. The inhibitory role of glycolipids in the binding of lysenin to SM was confirmed by comparing the glycolipid-deficient mutant melanoma cell line with its parent cell. Model membrane experiments indicated that glycolipid altered the local density of SM so that the affinity of the lipid for lysenin was decreased. Our results indicate that lysenin recognizes the heterogenous organization of SM in biomembranes and that the organization of SM differs between different cell types and between different membrane domains within the same cell. Isothermal titration calorimetry suggests that lysenin binding to SM is presumably the result of a SM-lysenin complex formation of specific stoichiometry, thus supporting the idea of the existence of small condensed lipid complexes consisting of just a few lipid molecules in living cells.

CD157 is an important mediator of neutrophil adhesion and migration

CD157, a glycosylphosphatidylinositol (GPI)-anchored protein encoded by a member of the CD38 NADase/ADP-ribosyl cyclase gene family, is expressed on the surface of most human circulating neutrophils. This work demonstrates that CD157 is a receptor that induces reorganization of the cytoskeleton and significant changes in cell shape, and that signals mediated by CD157 act through modulation of cytosolic Ca(2+) concentration. These signals are independent of the products of CD157's enzymatic activities (ie, cyclic adenosine diphosphate [ADP]-ribose and ADP-ribose). Indeed, the enzymatic activities of CD157 in circulating neutrophils as well as in dimethyl sulfoxide (DMSO)-differentiated (CD157(+)/CD38(-)) HL-60 cells, are hardly detectable. This work also shows that the receptorial activity relies on cross-talk between CD157 and beta(2) integrin. CD157 localizes in GM1-enriched lipid rafts and, upon activation, it migrates to the uropod, a structure specialized in motility and adhesive functions. Indeed, CD157 is involved in adhesion to extracellular matrix proteins and in chemotaxis induced in vitro by formyl-methionyl-leucyl-phenylalanine (fMLP). These findings were consistent with the results obtained in neutrophils from patients with paroxysmal nocturnal hemoglobinuria (PNH), in which CD157 is deficient. These neutrophils showed constant defects in adhesion and migration. Our data attribute specific and crucial roles to CD157 in the regulation of innate immunity during inflammation.

Lipid rafts in lymphocyte activation

The existence of sphingolipid- and cholesterol-rich membrane microdomains called "lipid rafts", as well as their role in lymphocyte biology, has been widely debated during the last few years. Plasma membrane microdomains seem to be primarily involved in initiation and propagation of the signal transduction cascade associated with lymphocyte activation. In this review, we discuss the recent literature suggesting that, during lymphocyte activation and chemotaxis, lipid rafts act as platforms to compartmentalise signalling and facilitate specific protein-protein interactions.

Cutting edge: optical microspectrophotometry supports the existence of gel phase lipid rafts at the lamellipodium of neutrophils: apparent role in calcium signaling

Although much progress has been made in elucidating the biochemical properties of lipid rafts, there has been less success in identifying these structures within living cell membranes, which has led to some concern regarding their existence. One difficulty in analyzing lipid rafts using optical microscopy is their small size. We now test the existence of lipid rafts in polarized neutrophils, which redistribute lipid raft markers into comparatively large lamellipodia. Optical microspectrophotometry of Laurdan-labeled neutrophils revealed a large blue shift at lamellipodia relative to cell bodies. This blue shift disappeared after exposure to methyl-beta-cyclodextrin (m beta CD), which disrupts lipid rafts. The Ca(2+) channel transient receptor potential-like channel-1, a lipid raft marker, traffics to lamellipodia, but redistributes uniformly about cells after exposure to m beta CD. This is accompanied by disruption of Ca(2+) waves normally initiated at lamellipodia. Thus, m beta CD-sensitive lipid-ordered domains are present at and participate in signaling from the lamellipodia of living neutrophils.

Control of Immune Responses by Trafficking Cell Surface Proteins, Vesicles and Lipid Rafts to and from the Immunological Synapse

Supramolecular clusters at the immunological synapse provide a mechanism for structuring complex communication networks between cells of the immune system. Regulating intra- and intercellular trafficking of proteins and lipids to and from the immunological synapse provides an additional level of complexity in determining the functional outcome of immune cell interactions. An emergent principle is that molecules requiring tightly regulated cell surface expression, e.g. negative regulators of cell activation or molecules promoting cytotoxicity, are trafficked to the immunological synapse from intracellular secretory as required lysosomes. Many molecules required for the early stages of the intercellular communication are already present at the cell surface, sometimes in lipid rafts, and are rapidly translocated laterally to the intercellular contact. Our understanding of these events critically depends on utilizing appropriate technologies for probing supramolecular recognition in live cells. Thus, we also present here a critical discussion of the technologies used to study lipid rafts and, more broadly, a map of the spatial and temporal dimensions covered by current live cell physical techniques, highlighting where advances are needed to exceed current spatial and temporal boundaries.

Segregation of lipid raft markers including CD133 in polarized human hematopoietic stem and progenitor cells

During ontogenesis and the entire adult life hematopoietic stem and progenitor cells have the capability to migrate. In comparison to the process of peripheral leukocyte migration in inflammatory responses, the molecular and cellular mechanisms governing the migration of these cells remain poorly understood. A common feature of migrating cells is that they need to become polarized before they migrate. Here we have investigated the issue of cell polarity of hematopoietic stem/progenitor cells in detail. We found that human CD34(+) hematopoietic cells (1) acquire a polarized cell shape upon cultivation, with the formation of a leading edge at the front pole and a uropod at the rear pole; (2) exhibit an amoeboid movement, which is similar to the one described for migrating peripheral leukocytes; and (3) redistribute several lipid raft markers including cholesterol-binding protein prominin-1 (CD133) in specialized plasma membrane domains. Furthermore, polarization of CD34(+) cells is stimulated by early acting cytokines and requires the activity of phosphoinositol-3-kinase as previously reported for peripheral leukocyte polarization. Together, our data reveal a strong correlation between polarization and migration of peripheral leukocytes and hematopoietic stem/progenitor cells and suggest that they are governed by similar mechanisms.

The inner side of T cell lipid rafts

A key question in understanding the functional role of lipid rafts is whether lipid microdomains at the plasma membrane outer leaflet are coupled to lipid microdomains at the inner leaflet. By using a cyan-fluorescent protein (CFP) targeted to inner plasma membrane rafts of Jurkat T cells, we found that raft domains at the outer and inner leaflets are physically coupled and that this coupling requires cholesterol. Interestingly, TCR/CD3 cross-linking induces co-capping of the raft bilayer independently of cholesterol or signaling events, indicating that cholesterol-extracting drugs are unable to destroy TCR-lipid rafts interaction.

Impact of cholesterol depletion on shape changes, actin reorganization, and signal transduction in neutrophil-like HL-60 cells

Stimulation of neutrophils with chemotactic peptide induces actin reorganization, formation of actin-rich protrusions, and development of polarity. Shape changes and actin polymerization can also be induced by phorbol ester-mediated direct activation of protein kinase C (PKC). We have investigated the role of cholesterol in stimulus-dependent motile events and in activation of signaling pathways in neutrophil-like differentiated HL-60 cells. Depletion of plasma membrane cholesterol using methyl-beta-cyclodextrin (MbetaCD) prevented chemotactic peptide and phorbol ester-induced shape changes and increases in cytoskeletal actin. Cholesterol depletion almost completely suppressed chemotactic peptide-mediated activation of p42/44 mitogen-activated protein kinase (MAPK). Phosphorylation of protein kinase B on Thr-308, which is indicative of activation of phosphatidylinositol 3-kinase, was in contrast only partially inhibited. Stimulus-mediated membrane recruitment of different PKC isoforms was differentially affected by treatment of cells with MbetaCD. Membrane recruitment of PKCalpha induced by chemotactic peptide or phorbol ester was suppressed, whereas that of PKCbetaII was only partially affected. Membrane association of PKCdelta was almost insensitive to cholesterol depletion. In summary, our results implicate an important role of cholesterol-containing lipid microdomains (rafts) especially in chemotactic peptide-induced activation of MAPK pathways and in chemotactic peptide- and phorbol ester-mediated activation of PKCalpha.

Syk-mediated tyrosine phosphorylation is required for the association of hematopoietic lineage cell-specific protein 1 with lipid rafts and B cell antigen receptor signalosome complex

Hematopoietic lineage cell-specific protein 1 (HS1) is an F-actin- and actin-related proteins 2 and 3 (Arp2/3)-binding protein that undergoes a rapid tyrosine phosphorylation upon B cell antigen receptor (BCR) activation. Density gradient centrifugation of Triton X-100 lysates from B lymphocytes demonstrated that HS1 was translocated in response to BCR cross-linking into lipid raft microdomain along with Arp2/3 complex and Wiskott-Aldrich syndrome protein. HS1-green fluorescent protein was localized in membrane patches enriched with GM1 gangliosides and BCR in the cells treated with anti-IgM antibody. Colocalization of HS1-green fluorescent protein with BCR was also correlated with tyrosine phosphorylation of HS1. Interestingly a murine HS1 mutant at the tyrosine residues Tyr388 and Tyr405 targeted by Syk failed to respond to BCR cross-linking for either translocation into lipid rafts or colocalization with BCR within cells. Furthermore HS1 was unable to translocate into lipid rafts in a chicken B cell line deficient in Syk. Reintroducing a Syk construct into the Syk knock-out cells recovered effectively both tyrosine phosphorylation and translocation of HS1 into lipid rafts. In contrast, translocation of HS1 into rafts was normal in a Lyn knock-out B cell line, and an HS1 mutant at the tyrosine residue Tyr222 targeted by Lyn maintained the ability to partition into rafts upon BCR cross-linking. These data indicate that Syk plays an important role in the translocation of HS1 into lipid rafts and may be responsible for actin assembly recruitment to rafts and subsequent antigen presentations.

Lipid rafts as organizing platforms for cell chemotaxis and axon guidance

Lipid rafts are thought to serve as plasma membrane platforms for localized trafficking and signaling. Recent findings reported by Guirland et al. in this issue of Neuron and by Gómez-Moutón in a recent issue of JCB support a direct role of lipid microdomains in organizing spatial signaling during axon guidance and cell chemotaxis by concentrating the gradient-sensing machinery at the leading edge.

Lipid rafts: heterogeneity on the high seas

Lipid rafts are membrane microdomains that are enriched in cholesterol and glycosphingolipids. They have been implicated in processes as diverse as signal transduction, endocytosis and cholesterol trafficking. Recent evidence suggests that this diversity of function is accompanied by a diversity in the composition of lipid rafts. The rafts in cells appear to be heterogeneous both in terms of their protein and their lipid content, and can be localized to different regions of the cell. This review summarizes the data supporting the concept of heterogeneity among lipid rafts and outlines the evidence for cross-talk between raft components. Based on differences in the ways in which proteins interact with rafts, the Induced-Fit Model of Raft Heterogeneity is proposed to explain the establishment and maintenance of heterogeneity within raft populations.

Co-localization of CD3 and prion protein in Jurkat lymphocytes after hypothermal stimulation

While long-term effects of temperature treatment in respect of, e.g., gene-expression and cellular function have already been studied in some detail, nothing is known on the physiological responses of lymphocytes during short-term hypothermal shifts. In this report, we characterized the effects of such a stimulation using the human lymphocyte cell line Jurkat E6.1 and present evidence that warming from 4 to 37 degrees C for only 2 min is sufficient to cause co-localization of CD3, prion protein and the lipid-raft ganglioside GM1 paralleling lymphocyte activation as observed by Ca(2+) mobilization and mitogen-activated protein kinase-phosphorylation.

Phospholipid Flip-Flop and Phospholipid Scramblase 1 (PLSCR1) Co-localize to Uropod Rafts in Formylated Met-Leu-Phe-stimulated Neutrophils

Movement of phosphatidylserine (PS) to the plasma membrane outer leaflet is a nearly universal marker of apoptosis and occurs during activation of many cells. Neutrophils stimulated with the chemotactic peptide formylated Met-Leu-Phe (fMLP) demonstrated transient PS exposure. Stimulated outward movement of PS was accompanied by enhanced inward movement of several phosphorylcholine lipid probes and was associated with enhanced FM 1-43 staining indicative of phospholipid packing changes. Unlike apoptosis, inward movement of exogenously added fluorescent PS did not decline, and DNA was not cleaved during fMLP stimulation. Movement of phospholipids occurred within minutes following stimulation, was independent of endocytosis/pinocytosis, and was consistent with bidirectional, transbilayer phospholipid flip-flop. While the role of phospholipid scramblase 1 (PLSCR1) is controversial in flip-flop, we sought evidence for its role in enhanced phospholipid movements during fMLP stimulation. Using antibodies to the carboxyl-terminal domain of PLSCR1, its presence in the plasma membranes of non-permeabilized neutrophils was confirmed by flow cytometry. Additionally subcellular fractionation demonstrated that PLSCR1 was also located in secretory vesicles and tertiary and secondary granules. Activation of neutrophils with fMLP, however, did not significantly alter surface labeling suggesting that stimulated phospholipid flip-flop does not require additional mobilization of PLSCR1 to the plasma membrane. As expected for palmitoylated proteins, PLSCR1 was enriched in detergent-insoluble membranes and co-localized with raft markers at the neutrophil uropod after stimulation. Of note, PS exposure, phospholipid uptake, and FM 1-43 staining also localized to the uropod following stimulation demonstrating that both PLSCR1 and phospholipid flip-flop characterize this specialized domain of polarized neutrophils.

Altered lipid raft-associated signaling and ganglioside expression in T lymphocytes from patients with systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is characterized by abnormalities in T lymphocyte receptor-mediated signal transduction pathways. Our previous studies have established that lymphocyte-specific protein tyrosine kinase (LCK) is reduced in T lymphocytes from patients with SLE and that this reduction is associated with disease activity and parallels an increase in LCK ubiquitination independent of T cell activation. This study investigated the expression of molecules that regulate LCK homeostasis, such as CD45, C-terminal Src kinase (CSK), and c-Cbl, in lipid raft domains from SLE T cells and investigated the localization of these proteins during T cell receptor (TCR) triggering. Our results indicate that the expression of raft-associated ganglioside, GM1, is increased in T cells from SLE patients and LCK may be differentially regulated due to an alteration in the association of CD45 with lipid raft domains. CD45 tyrosine phosphatase, which regulates LCK activity, was differentially expressed and its localization into lipid rafts was increased in T cells from patients with SLE. Furthermore, T cells allowed to "rest" in vitro showed a reversal of the changes in LCK, CD45, and GM1 expression. The results also revealed that alterations in the level of GM1 expression and lipid raft occupancy cannot be induced by serum factors from patients with SLE but indicated that cell-cell contact, activating aberrant proximal signaling pathways, may be important in influencing abnormalities in T cell signaling and, therefore, function in patients with SLE.

Transmembrane adaptor proteins in membrane microdomains: important regulators of immunoreceptor signaling

Membrane microdomains enriched in glycosphingolipids, cholesterol, glycosylphosphatidylinositol-anchored proteins and Src-family kinases (lipid rafts, GEMs) appear to play many important roles, especially in immunoreceptor signaling. Most transmembrane proteins are excluded from these specialized areas of membranes, notable exceptions being several palmitoylated proteins such as the T cell coreceptors CD4 and CD8, and several recently described transmembrane adaptor proteins, LAT, non-T cell activation linker (NTAL)/linker for activation of B cells (LAB), phosphoprotein associated with GEMs (PAG)/Csk-binding protein (Cbp) and LIME. All these molecules possess a very short N-terminal extracellular peptide (4-17 amino acids), transmembrane segment followed by a palmitoylation motif (CxxC) and cytoplasmic domain containing up to 10 tyrosine residues potentially phosphorylated by the Src- or Syk-family kinases. Tyrosine-phosphorylated transmembrane adaptors bind (directly via SH2 domains or indirectly) other signaling molecules such as several cytoplasmic adaptors and enzymes. LAT is indispensable for TCR signaling (and participates also at signal transduction initiated by some other receptors), NTAL/LAB appears to play a LAT-like role in signaling initiated by BCR and some Fc-receptors; PAG/Cbp cooperates with Csk, the cytoplasmic tyrosine kinase negatively regulating Src-family kinases. Additional transmembrane adaptors exist (TRIM, SIT, LAX) that are however not palmitoylated and therefore excluded from the lipid rafts; structurally and functionally, the zeta-chain family proteins tightly associated with immunoreceptors and activating NK-receptors may be also considered as transmembrane adaptors.

On the mechanism and significance of ligand-induced internalization of human neutrophil chemokine receptors CXCR1 and CXCR2

It is well established that leukocyte chemotactic receptors, a subset of G protein-coupled receptors, undergo endocytosis after stimulation by ligand. However, the significance of this phenomenon to cell motility and other important leukocyte functions induced by chemoattractants has not been clearly defined. Here we show that in primary human neutrophils, the threshold levels of agonist required for endocytosis of the chemotactic receptors CXCR1 and CXCR2 were approximately 10-fold or higher than those needed for maximal chemotactic and calcium flux responses. Moreover, when stimulated by agonists at concentrations that are high enough for chemotaxis but too low for receptor endocytosis, neutrophil CXCR1 and CXCR2 could be reactivated in response to repeated application of the same agonist. Both receptors were excluded from Triton X-100-insoluble lipid rafts, and at high agonist concentrations were rapidly endocytosed by a clathrin/rab5/dynamin-dependent pathway. These data support the conclusion that neutrophil migration in response to CXCR1 or CXCR2 agonists is not dependent on endocytosis of CXCR1 or CXCR2. Rather than being integral to the process of cell migration, receptor endocytosis may be a terminal stop signal when cells reach the focus of inflammation where the chemoattractant concentrations are the highest.

Integrins regulate Rac targeting by internalization of membrane domains

Translocation of the small GTP-binding protein Rac1 to the cell plasma membrane is essential for activating downstream effectors and requires integrin-mediated adhesion of cells to extracellular matrix. We report that active Rac1 binds preferentially to low-density, cholesterol-rich membranes, and specificity is determined at least in part by membrane lipids. Cell detachment triggered internalization of plasma membrane cholesterol and lipid raft markers. Preventing internalization maintained Rac1 membrane targeting and effector activation in nonadherent cells. Regulation of lipid rafts by integrin signals may regulate the location of membrane domains such as lipid rafts and thereby control domain-specific signaling events in anchorage-dependent cells.

The phagocyte NADPH oxidase depends on cholesterol-enriched membrane microdomains for assembly

The superoxide-producing phagocyte NADPH oxidase consists of a membrane-bound flavocytochrome b558 complex, and cytosolic factors p47phox, p67phox and the small GTPase Rac, which translocate to the membrane to assemble the active complex following cell activation. We here show that insolubility of NADPH oxidase subunits in nonionic detergents TX-100, Brij-58, and Brij-98 is a consequence of inclusion into cholesterol-enriched membrane microdomains (lipid rafts). Thus, flavocytochrome b558, in a cholesterol-dependent manner, segregated to the bouyant low-density detergent-resistant membrane (DRM) fraction, and the cytosolic NADPH oxidase factors associated dynamically with low-density DRM. Further, superoxide production following cholesterol depletion was severely compromised in intact cells or in a cell-free reconstituted system, correlating with a reduced translocation of cytosolic phox subunits to the membrane. In analogy with the widely accepted role of lipid rafts as signaling platforms, our data indicate that cholesterol-enriched microdomains act to recruit and/or organize the cytosolic NADPH oxidase factors in the assembly of the active NADPH oxidase.

Dynamic redistribution of raft domains as an organizing platform for signaling during cell chemotaxis

Spatially restricted activation of signaling molecules governs critical aspects of cell migration; the mechanism by which this is achieved nonetheless remains unknown. Using time-lapse confocal microscopy, we analyzed dynamic redistribution of lipid rafts in chemoattractant-stimulated leukocytes expressing glycosyl phosphatidylinositol-anchored green fluorescent protein (GFP-GPI). Chemoattractants induced persistent GFP-GPI redistribution to the leading edge raft (L raft) and uropod rafts of Jurkat, HL60, and dimethyl sulfoxide-differentiated HL60 cells in a pertussis toxin-sensitive, actin-dependent manner. A transmembrane, nonraft GFP protein was distributed homogeneously in moving cells. A GFP-CCR5 chimera, which partitions in L rafts, accumulated at the leading edge, and CCR5 redistribution coincided with recruitment and activation of phosphatidylinositol-3 kinase gamma in L rafts in polarized, moving cells. Membrane cholesterol depletion impeded raft redistribution and asymmetric recruitment of PI3K to the cell side facing the chemoattractant source. This is the first direct evidence that lipid rafts order spatial signaling in moving mammalian cells, by concentrating the gradient sensing machinery at the leading edge.

CTLA-4 and tolerance: the biochemical point of view

Potentially autoreactive T cells that escape negative selection in the thymus must be strictly controlled in the periphery to avoid autoimmune disease. The most robust regulatory process controlling autoreactivity is mediated by the CTLA-4-B7 pathway. The critical homeostasis mediated by CTLA-4 was proven using monoclonal antibodies and genetically disrupted CTLA-4 knockout mice that develop polyclonal lymphocyte activation and proliferation leading to massively enlarged lymph nodes and spleen and fatal multiorgan lymphocytic infiltrates. CTLA-4 ligation following T-cell activation downregulates cytokine production and cell-cycle progression, however, the proximal biochemical basis for robust T-cell regulation remains unclear. In this review, we summarize studies supporting a dynamic role for CTLA-4 at the immunological synapse leading to direct attenuation of early cell signals. A model is proposed based on these observations, which proposes that CTLA-4 may, in fact, function under some circumstances in a ligand-independent manner.

Do phosphoinositide 3-kinases direct lymphocyte navigation?

Phosphoinositide 3-kinase (PI3K)-dependent signalling pathways have been suggested to have pivotal roles in determining the polarity of leukocytes moving toward a chemotactic gradient, a process termed chemotaxis. Current perceptions concerning the role of PI3K in leukocyte migration are based predominantly around evidence derived from single-cell organisms and analysis of neutrophil migration from mice deficient in the gamma-isoform of the p110 catalytic subunit. With regard to directed T-lymphocyte migration, there is convincing evidence for the activation of PI3K isoforms in T lymphocytes by several chemokines. However, there is a growing body of evidence, which now indicates that in T lymphocytes at least, PI3K activation can be a dispensable signal for directed cell migration in certain settings. In fact, evidence is emerging that during directed cell migration, T lymphocytes use biochemical pathways distinct from those adopted by monocytes. The non-universal role of PI3K in directional cell migration and the existence of cell-specific signalling pathways for chemotactic responses has important implications for the validation of effective new targets for inflammation, where one aim is to block migration of leukocytes to the site of inflammatory lesion.

CD95/phosphorylated ezrin association underlies HIV-1 GP120/IL-2-induced susceptibility to CD95(APO-1/Fas)-mediated apoptosis of human resting CD4+T lymphocytes

CD95(APO-1/Fas)-mediated apoptosis of bystander uninfected T cells exerts a major role in the HIV-1-mediated CD4+ T-cell depletion. HIV-1 gp120 has a key role in the induction of sensitivity of human lymphocytes to CD95-mediated apoptosis through its interaction with the CD4 receptor. Recently, we have shown the importance of CD95/ezrin/actin association in CD95-mediated apoptosis. In this study, we explored the hypothesis that the gp120-mediated CD4 engagement could be involved in the induction of susceptibility of primary human T lymphocytes to CD95-mediated apoptosis through ezrin phosphorylation and ezrin-to-CD95 association. Here, we show that gp120/IL-2 combined stimuli, as well as the direct CD4 triggering, on human primary CD4(+)T lymphocytes induced an early and stable ezrin activation through phosphorylation, consistent with the induction of ezrin/CD95 association and susceptibility to CD95-mediated apoptosis. Our results provide a new mechanism through which HIV-1-gp120 may predispose resting CD4(+)T cell to bystander CD95-mediated apoptosis and support the key role of ezrin/CD95 linkage in regulating susceptibility to CD95-mediated apoptosis.

Dimerization controls the lipid raft partitioning of uPAR/CD87 and regulates its biological functions

The urokinase-type plasminogen activator receptor (uPAR/CD87) is a glycosylphosphatidylinositol-anchored membrane protein with multiple functions in extracellular proteolysis, cell adhesion, cell migration and proliferation. We now report that cell surface uPAR dimerizes and that dimeric uPAR partitions preferentially to detergent-resistant lipid rafts. Dimerization of uPAR did not require raft partitioning as the lowering of membrane cholesterol failed to reduce dimerization and as a transmembrane uPAR chimera, which does not partition to lipid rafts, also dimerized efficiently. While uPA bound to uPAR independently of its membrane localization and dimerization status, uPA-induced uPAR cleavage was strongly accelerated in lipid rafts. In contrast to uPA, the binding of Vn occurred preferentially to raft- associated dimeric uPAR and was completely blocked by cholesterol depletion.

Proteome Analysis of Lipid Rafts in Jurkat Cells Characterizes a Raft Subset That Is Involved in NF-κB Activation

Lipid rafts are detergent-insoluble membrane domains that play a key role in signal transduction by the T-cell antigen receptor. Proteome analysis revealed the presence of amidosulfobetaine-soluble signal transducing, integral membrane, cytoskeletal, heat shock, and GTP-binding proteins in rafts prepared from Jurkat cells. Several of these proteins were recruited to rafts by CD3/CD28 costimulation. Of particular interest is the inducible association of activated IkappaB kinase complexes with raft vesicles that could be captured with anti-flotillin-1 antibodies. Following amidosulfobetaine solubilization, flotillin-beta and IKKbeta underwent reciprocal co-immunoprecipitation. Treatment of Jurkat cells with methyl-beta-cyclodextrin disrupted the assembly and activation of this raft complex and also interfered in CD3/ CD28-induced activation of a NF-kappaB response element in the IL-2 promoter.

Ephrin signaling: One raft to rule them all? One raft to sort them? One raft to spread their call and in signaling bind them?

The Eph receptor tyrosine kinases (RTK) and their membrane-bound ligands, the ephrins, mediate cell-contact-dependent signaling events that control multiple aspects of metazoan embryonic development. The ephrins and their receptors regulate cell movement that is essential for forming and stabilizing the spatial organization of tissues and cell types. This includes the guidance of migrating cells or neuronal growth cones to specific targets. Although the biological responses mediated by the ephrin-Eph system were thought to be imparted by the Eph receptor via 'classical' RTK signaling pathways, there is now accumulating evidence that the ephrins are not merely ligands but have biological activity independent of the kinase activity of their cognate Eph receptor. This activity is commonly referred to as 'reverse' or 'bi-directional' signaling. Furthermore, ephrin-mediated signaling is restricted to specific membrane microdomains known as 'lipid rafts', which we believe imparts specificity to the extracellular signal. This review highlights the current data to support a role for lipid rafts in regulating aspects of ephrin-mediated signaling.

Co-clustering of galactosylceramide and membrane proteins in oligodendrocyte membranes on interaction with polyvalent carbohydrate and prevention by an intact cytoskeleton

We have shown previously that addition of liposomes containing the two major glycosphingolipids of myelin, galactosylceramide (GalC) and cerebroside sulfate (CBS), to cultured oligodendrocytes (OLs) caused clustering of GalC on the extracellular surface and myelin basic protein (MBP) on the cytosolic surface to the same membrane domains. It also caused depolymerization of actin microfilaments and microtubules, indicating that interaction of the liposomes with the OL surface induces transmembrane signal transmission. We show that a multivalent form of galactose conjugated to bovine serum albumin has a similar effect as the multivalent GalC/CBS-containing liposomes. Because GalC and CBS can interact with each other across apposed membranes and because anti-GalC and anti-CBS antibodies also cause redistribution of GalC/CBS and depolymerization of microtubules, we believe that the multivalent carbohydrate interacts with GalC and CBS in the OL membrane. Several myelin-specific transmembrane proteins could be involved in this transmembrane signal transmission from GalC/CBS. We looked at co-clustering of several myelin constituents by confocal microscopy to determine if they are located in or redistribute to GalC/MBP-containing domains. Myelin oligodendrocyte glycoprotein (MOG), proteolipid protein (PLP), MAPK, and some phosphotyrosine-containing proteins were found to co-cluster with GalC and MBP, but myelin-associated glycoprotein (MAG) and phosphatidylinositol-4,5-bisphosphate (PIP(2)) did not. These results suggest that MOG and PLP, but probably not MAG, are possible candidates for transmembrane transmission of the signal received by GalC/CBS. To determine if depolymerization of actin microfilaments was required for co-clustering, or was secondary to clustering, we stabilized F-actin with jasplakinolide. This also prevented depolymerization of the microtubules and prevented clustering of all constituents, including GalC. The prevention of clustering or redistribution of these glycolipids and proteins by an intact cytoskeleton is consistent with the picket fence model.