T cell glycolipid-enriched membrane domains are constitutively assembled as membrane patches that translocate to immune synapses

Dysregulation of cellular membrane homeostasis as a crucial modulator of cancer risk

Cellular membranes serve as an epicentre combining extracellular and cytosolic components with membranous effectors, which together support numerous fundamental cellular signalling pathways that mediate biological responses. To execute their functions, membrane proteins, lipids and carbohydrates arrange, in a highly coordinated manner, into well-defined assemblies displaying diverse biological and biophysical characteristics that modulate several signalling events. The loss of membrane homeostasis can trigger oncogenic signalling. More recently, it has been documented that select membrane active dietaries (MADs) can reshape biological membranes and subsequently decrease cancer risk. In this review, we emphasize the significance of membrane domain structure, organization and their signalling functionalities as well as how loss of membrane homeostasis can steer aberrant signalling. Moreover, we describe in detail the complexities associated with the examination of these membrane domains and their association with cancer. Finally, we summarize the current literature on MADs and their effects on cellular membranes, including various mechanisms of dietary chemoprevention/interception and the functional links between nutritional bioactives, membrane homeostasis and cancer biology.

A sensitive S-Trap-based approach to the analysis of T cell lipid raft proteome

The analysis of T cell lipid raft proteome is challenging due to the highly dynamic nature of rafts and the hydrophobic character of raft-resident proteins. We explored an innovative strategy for bottom-up lipid raftomics based on suspension-trapping (S-Trap) sample preparation. Mouse T cells were prepared from splenocytes by negative immunoselection, and rafts were isolated by a detergent-free method and OptiPrep gradient ultracentrifugation. Microdomains enriched in flotillin-1, LAT, and cholesterol were subjected to proteomic analysis through an optimized protocol based on S-Trap and high pH fractionation, followed by nano-LC-MS/MS. Using this method, we identified 2,680 proteins in the raft-rich fraction and established a database of 894 T cell raft proteins. We then performed a differential analysis on the raft-rich fraction from nonstimulated versus anti-CD3/CD28 T cell receptor (TCR)-stimulated T cells. Our results revealed 42 proteins present in one condition and absent in the other. For the first time, we performed a proteomic analysis on rafts from ex vivo T cells obtained from individual mice, before and after TCR activation. This work demonstrates that the proposed method utilizing an S-Trap-based approach for sample preparation increases the specificity and sensitivity of lipid raftomics.

Conformational States Control Lck Switching between Free and Confined Diffusion Modes in T Cells

T cell receptor phosphorylation by Lck is an essential step in T cell activation. It is known that the conformational states of Lck control enzymatic activity; however, the underlying principles of how Lck finds its substrate over the plasma membrane remain elusive. Here, single-particle tracking is paired with photoactivatable localization microscopy to observe the diffusive modes of Lck in the plasma membrane. Individual Lck molecules switched between free and confined diffusion in both resting and stimulated T cells. Lck mutants locked in the open conformation were more confined than Lck mutants in the closed conformation. Further confinement of kinase-dead versions of Lck suggests that Lck confinement was not caused by phosphorylated substrates. Our data support a model in which confined diffusion of open Lck results in high local phosphorylation rates, and inactive, closed Lck diffuses freely to enable long-range distribution over the plasma membrane.

Functional link between plasma membrane spatiotemporal dynamics, cancer biology, and dietary membrane-altering agents

The cell plasma membrane serves as a nexus integrating extra- and intracellular components, which together enable many of the fundamental cellular signaling processes that sustain life. In order to perform this key function, plasma membrane components assemble into well-defined domains exhibiting distinct biochemical and biophysical properties that modulate various signaling events. Dysregulation of these highly dynamic membrane domains can promote oncogenic signaling. Recently, it has been demonstrated that select membrane-targeted dietary bioactives (MTDBs) have the ability to remodel plasma membrane domains and subsequently reduce cancer risk. In this review, we focus on the importance of plasma membrane domain structural and signaling functionalities as well as how loss of membrane homeostasis can drive aberrant signaling. Additionally, we discuss the intricacies associated with the investigation of these membrane domain features and their associations with cancer biology. Lastly, we describe the current literature focusing on MTDBs, including mechanisms of chemoprevention and therapeutics in order to establish a functional link between these membrane-altering biomolecules, tuning of plasma membrane hierarchal organization, and their implications in cancer prevention.

The Actin Cytoskeleton Is Involved in Glial Cell Line-Derived Neurotrophic Factor (GDNF)-Induced Ret Translocation into Lipid Rafts in Dopaminergic Neuronal Cells

Glial cell line-derived neurotrophic factor (GDNF), a potential therapeutic factor for Parkinson’s disease (PD), exerts its biological effects through the Ret receptor tyrosine kinase. The redistribution of Ret into lipid rafts substantially influences Ret signaling, but the mechanisms underlying Ret translocation remain unclear. The purpose of our study was to further explore the signaling mechanisms of GDNF and to determine whether the actin cytoskeleton is involved in the GDNF-induced Ret translocation into lipid rafts. In MN9D dopaminergic neuronal cells, we used density gradient centrifugation and immunofluorescence confocal microscopy to separate and visualize lipid rafts, co-immunoprecipitation to analyze protein-protein interactions, and latrunculin B (Lat B) and jasplakinolide (Jas) to disrupt and enhance the polymerization of the actin cytoskeleton, respectively. The results showed that Ret translocated into lipid rafts and coimmunoprecipitated with actin in response to GDNF treatment. After Lat B or Jas treatment, the Ret–F-actin association induced by GDNF was impaired or enhanced respectively and then the levels of Ret translocated into lipid rafts were correspondingly inhibited or promoted. These data indicate that actin polymerization and cytoskeletal remodeling are integral to GDNF-induced cell signaling in dopaminergic cells and define a new role of the actin cytoskeleton in promoting Ret redistribution into lipid rafts.

Dances with Membranes: Breakthroughs from Super-resolution Imaging

Biological membrane organization mediates numerous cellular functions and has also been connected with an immense number of human diseases. However, until recently, experimental methodologies have been unable to directly visualize the nanoscale details of biological membranes, particularly in intact living cells. Numerous models explaining membrane organization have been proposed, but testing those models has required indirect methods; the desire to directly image proteins and lipids in living cell membranes is a strong motivation for the advancement of technology. The development of super-resolution microscopy has provided powerful tools for quantification of membrane organization at the level of individual proteins and lipids, and many of these tools are compatible with living cells. Previously inaccessible questions are now being addressed, and the field of membrane biology is developing rapidly. This chapter discusses how the development of super-resolution microscopy has led to fundamental advances in the field of biological membrane organization. We summarize the history and some models explaining how proteins are organized in cell membranes, and give an overview of various super-resolution techniques and methods of quantifying super-resolution data. We discuss the application of super-resolution techniques to membrane biology in general, and also with specific reference to the fields of actin and actin-binding proteins, virus infection, mitochondria, immune cell biology, and phosphoinositide signaling. Finally, we present our hopes and expectations for the future of super-resolution microscopy in the field of membrane biology.

The central role of the cytoskeleton in mechanisms and functions of the NK cell immune synapse

Natural killer (NK) cells discriminate between healthy and unhealthy target cells through a balance of activating and inhibitory signals at direct intercellular contacts called immune synapses. Rearrangements in the cellular cytoskeleton have long been known to be critical in assembly of immune synapses. Here, through bringing together the vast literature on this subject, the number of different ways in which the cytoskeleton is important becomes evident. The dynamics of filamentous actin are critical in (i) creating the nanometer-scale organization of NK cell receptors, (ii) establishing cellular polarity, (iii) coordinating immune receptor and integrin-mediated signaling, and (iv) directing secretion of lytic granules and cytokines. The microtubule network also is important in the delivery of lytic granules and vesicles containing cytokines to the immune synapse. Together, these data establish that the cytoskeleton acts as a central regulator of this complex and dynamic process - and an enormous amount of NK cell biology is controlled through the cytoskeleton.

Actin mediates the nanoscale membrane organization of the clustered membrane protein influenza hemagglutinin

The influenza viral membrane protein hemagglutinin (HA) is required at high concentrations on virion and host-cell membranes for infectivity. Because the role of actin in membrane organization is not completely understood, we quantified the relationship between HA and host-cell actin at the nanoscale. Results obtained using superresolution fluorescence photoactivation localization microscopy (FPALM) in nonpolarized cells show that HA clusters colocalize with actin-rich membrane regions (ARMRs). Individual molecular trajectories in live cells indicate restricted HA mobility in ARMRs, and actin disruption caused specific changes to HA clustering. Surprisingly, the actin-binding protein cofilin was excluded from some regions within several hundred nanometers of HA clusters, suggesting that HA clusters or adjacent proteins within the same clusters influence local actin structure. Thus, with the use of imaging, we demonstrate a dynamic relationship between glycoprotein membrane organization and the actin cytoskeleton at the nanoscale.

CD28 sensitizes TCR Ca²⁺ signaling during Ag-independent polarization of plasma membrane rafts

T cells become polarized during initial interactions with an APC to form an Ag-independent synapse (AIS) composed of membrane rafts, TCR, and TCR-proximal signaling molecules. AISs occur temporally before TCR triggering, but their role in downstream TCR signaling is not understood. Using both human and murine model systems, we studied the signals that activate AIS formation and the effect of these signals on TCR-dependent responses. We show that CD28 produces AISs detectable by spinning disc confocal microscopy seconds following initial interactions between the T cell and APC. AIS formation by CD28 coincided with costimulatory signaling, evidenced by a cholesterol-sensitive activation of the MAPK ERK that potentiated Ca²⁺ signaling in response to CD3 cross-linking. CD45 also enriched in AISs but to modulate Src kinase activity, because localization of CD45 at the cell interface reduced the activation of proximal Lck. In summary, we show that signaling by CD28 during first encounters between the T cell and APC both sensitizes TCR Ca²⁺ signaling by an Erk-dependent mechanism and drives formation of an AIS that modulates the early signaling until TCR triggering occurs. Thus, early Ag-independent encounters are an important window for optimizing T cell responses to Ag by CD28.

Trans-sialidase stimulates eat me response from epithelial cells

Epithelial cell invasion by the protozoan parasite Trypanosoma cruzi is enhanced by the presence of an enzyme expressed on its cell surface during the trypomastigote life cycle stage. The enzyme, trans-sialidase (TS), is a member of one of the largest gene families expressed by the parasite and the role of its activity in mediating epithelial cell entry has not hitherto been understood. Here we show that the T. cruzi TS generates an eat me signal which is capable of enabling epithelial cell entry. We have utilized purified, recombinant, active (TcTS) and inactive (TcTS2V0) TS coated onto beads to challenge an epithelial cell line. We find that TS activity acts upon G protein coupled receptors present at the epithelial cell synapse with the coated bead, thereby enhancing cell entry. By so doing, we provide evidence that TS proteins bind glycans, mediate the formation of distinct synaptic domains and promote macropinocytotic uptake of microparticles into a perinuclear compartment in a manner which may emulate entosis.

How does the kinase Lck phosphorylate the T cell receptor? Spatial organization as a regulatory mechanism

T cell signaling begins with the ligation of the T cell antigen receptor (TCR) by a cognate peptide and the phosphorylation of the receptor’s immunoreceptor tyrosine-based activation motif domains by the kinase Lck. However, the canonical receptor model is insufficient to explain how the constitutively active kinase Lck can discriminate between non-ligated and ligated TCRs. Here, we discuss the factors that are thought to regulate the spatial distribution of the TCR and Lck, and therefore critically influence TCR signaling initiation.

Linker for activation of T cells is displaced from lipid rafts and decreases in lupus T cells after activation via the TCR/CD3 pathway

Systemic lupus erythematosus (SLE) is characterized by abnormal signal transduction mechanisms in T lymphocytes. Linker for activation of T cells (LAT) couples TCR/CD3 activation with downstream signaling pathways. We reported diminished ERK 1/2 kinase activity in TCR/CD3 stimulated lupus T cells. In this study we evaluated the expression, phosphorylation, lipid raft and immunological synapse (IS) localization and colocalization of LAT with key signalosome molecules. We observed a diminished expression and an abnormal localization of LAT in lipid rafts and at the IS in activated lupus T cells. LAT phosphorylation, capture by GST-Grb2 fusion protein, and coupling to Grb2 and PLCγ1, was similar in healthy control and lupus T cells. Our results suggest that an abnormal localization of LAT within lipid rafts and its accelerated degradation after TCR/CD3 activation may compromise the assembly of the LAT signalosome and downstream signaling pathways required for full MAPK activation in lupus T cells.

Simulations of the NK cell immune synapse reveal that activation thresholds can be established by inhibitory receptors acting locally

NK cell activation is regulated by a balance between activating and inhibitory signals. To address the question of how these signals are spatially integrated, we created a computer simulation of activating and inhibitory NK cell immunological synapse (NKIS) assembly, implementing either a "quantity-based" inhibition model or a "distance-based" inhibition model. The simulations mimicked the observed molecule distributions in inhibitory and activating NKIS and yielded several new insights. First, the total signal is highly influenced by activating complex dissociation rates but not by adhesion and inhibitory complex dissociation rates. Second, concerted motion of receptors in clusters significantly accelerates NKIS maturation. Third, when the potential of a cis interaction between Ly49 receptors and MHC class I on murine NK cells was added to the model, the integrated signal as a function of receptor and ligand numbers was only slightly increased, at least up to the level of 50% cis-bound Ly49 receptors reached in the model. Fourth, and perhaps most importantly, the integrated signal behavior obtained when using the distance-based inhibition signal model was closer to the experimentally observed behavior, with an inhibition radius of the order 3-10 molecules. Microscopy to visualize Vav activation in NK cells on micropatterned surfaces of activating and inhibitory strips revealed that Vav is only locally activated where activating receptors are ligated within a single NK cell contact. Taken together, these data are consistent with a model in which inhibitory receptors act locally; that is, that every bound inhibitory receptor acts on activating receptors within a certain radius around it.

Gangliosides are important for the preservation of the structure and organization of RBL-2H3 mast cells

Gangliosides are known to be important in many biological processes. However, details concerning the exact function of these glycosphingolipids in cell physiology are poorly understood. In this study, the role of gangliosides present on the surface of rodent mast cells in maintaining cell structure was examined using RBL-2H3 mast cells and two mutant cell lines (E5 and D1) deficient in the gangliosides, GM(1) and the alpha-galactosyl derivatives of the ganglioside GD(1b). The two deficient cell lines were morphologically different from each other as well as from the parental RBL-2H3 cells. Actin filaments in RBL-2H3 and E5 cells were under the plasma membrane following the spindle shape of the cells, whereas in D1 cells, they were concentrated in large membrane ruffles. Microtubules in RBL-2H3 and E5 cells radiated from the centrosome and were organized into long, straight bundles. The bundles in D1 cells were thicker and organized circumferentially under the plasma membrane. The endoplasmic reticulum, the Golgi complex, and the secretory granule matrix were also altered in the mutant cell lines. These results suggest that the mast cell-specific alpha-galactosyl derivatives of ganglioside GD(1b) and GM(1) are important in maintaining normal cell morphology.

Cytoskeleton-membrane interactions in membrane raft structure

Cell membranes are structurally heterogeneous, composed of discrete domains with unique physical and biological properties. Membrane domains can form through a number of mechanisms involving lipid-lipid and protein-lipid interactions. One type of membrane domain is the cholesterol-dependent membrane raft. How rafts form remains a current topic in membrane biology. We review here evidence of structuring of rafts by the cortical actin cytoskeleton. This includes evidence that the actin cytoskeleton associates with rafts, and that many of the structural and functional properties of rafts require an intact actin cytoskeleton. We discuss the mechanisms of the actin-dependent raft organization, and the properties of the actin cytoskeleton in regulating raft-associated signaling events. We end with a discussion of membrane rafts and the actin cytoskeleton in T cell activation, which function synergistically to initiate the adaptive immune response.

Dynamics of membrane trafficking downstream of B and T cell receptor engagement: impact on immune synapses

The onset of an adaptive immune response requires the activation of T and B lymphocytes by antigen-presenting cells, through a specialized form of intercellular communication, known as the immunological synapse (IS). In B lymphocytes the IS promotes efficient recognition and acquisition of membrane-bound Ags, while in T cells, it modulates the T cell response upon exposure to peptide-major histocompatibility complexes. In this review, we highlight the similarities that determine B and T cell activation, focusing on immune receptor downstream signaling events that lead to synapse formation. We stress the notion that polarization of T and B lymphocytes characterized by global changes in cytoskeleton and membrane trafficking modulates synapse structure and function, thus determining lymphocyte effector functions and fate.

Compartmentalization of phosphatidylinositol 4,5-bisphosphate signaling evidenced using targeted phosphatases

Phosphatidylinositol 4,5-bisphosphate (PIP(2)) is a prevalent phosphoinositide in cell membranes, with important functions in cell signaling and activation. A large fraction of PIP(2) associates with the detergent-resistant membrane "raft" fraction, but the functional significance of this association remains controversial. To measure the properties of raft and nonraft PIP(2) in cell signaling, we targeted the PIP(2)-specific phosphatase Inp54p to either the raft or nonraft membrane fraction using minimal membrane anchors. Interestingly, we observed that targeting Inp54p to the nonraft fraction resulted in an enrichment of raft-associated PIP(2) and striking changes in cell morphology, including a wortmannin-sensitive increase in cell filopodia and cell spreading. In contrast, raft-targeted Inp54p depleted the raft pool of PIP(2) and produced smooth T cells void of membrane ruffling and filopodia. Furthermore, raft-targeted Inp54p inhibited capping in T cells stimulated by cross-linking the T cell receptor, but without affecting the T cell receptor-dependent Ca(2+) flux. Altogether, these results provide evidence of compartmentalization of PIP(2)-dependent signaling in cell membranes such as predicted by the membrane raft model.

Agrin signalling contributes to cell activation and is overexpressed in T lymphocytes from lupus patients

It is shown in this study that the heparan sulfate proteoglycan agrin is overexpressed in T cells isolated from patients with the autoimmune disease systemic lupus erythematosus (SLE). Freshly isolated CD4(+) and CD8(+) subpopulations both exhibited higher expression over healthy controls, which however, gradually declined when cells were cultured in vitro. Agrin expression was induced following in vitro activation of cells via their Ag receptor, or after treatment with IFN-alpha, a cytokine shown to be pathogenic in lupus. Furthermore, serum from SLE patients with active disease was able to induce agrin expression when added to T cells from healthy donors, an increase that was partially blocked by neutralizing anti-IFN-alpha Abs. Cross-linking agrin with mAbs resulted in rapid reorganization of the actin cytoskeleton, activation of the ERK MAPK cascade, and augmentation of anti-CD3-induced proliferation and IL-10 production, indicating that agrin is a functional receptor in T cells. These results demonstrate that agrin expression in human T cells is regulated by cell activation and IFN-alpha, and may have an important function during cell activation with potential implications for autoimmunity.

HIV-1 Nef employs two distinct mechanisms to modulate Lck subcellular localization and TCR induced actin remodeling

The Nef protein acts as critical factor during HIV pathogenesis by increasing HIV replication in vivo via the modulation of host cell vesicle transport and signal transduction processes. Recent studies suggested that Nef alters formation and function of immunological synapses (IS), thereby modulating exogenous T-cell receptor (TCR) stimulation to balance between partial T cell activation required for HIV-1 spread and prevention of activation induced cell death. Alterations of IS function by Nef include interference with cell spreading and actin polymerization upon TCR engagement, a pronounced intracellular accumulation of the Src kinase Lck and its reduced IS recruitment. Here we use a combination of Nef mutagenesis and pharmacological inhibition to analyze the relative contribution of these effects to Nef mediated alterations of IS organization and function on TCR stimulatory surfaces. Inhibition of actin polymerization and IS recruitment of Lck were governed by identical Nef determinants and correlated well with Nef's association with Pak2 kinase activity. In contrast, Nef mediated Lck endosomal accumulation was separable from these effects, occurred independently of Pak2, required integrity of the microtubule rather than the actin filament system and thus represents a distinct Nef activity. Finally, reduction of TCR signal transmission by Nef was linked to altered actin remodeling and Lck IS recruitment but did not require endosomal Lck rerouting. Thus, Nef affects IS function via multiple independent mechanisms to optimize virus replication in the infected host.

Clustering of membrane raft proteins by the actin cytoskeleton

Cell membranes are laterally organized into functionally discrete domains that include the cholesterol-dependent membrane "rafts." However, how membrane domains are established and maintained remains unresolved and controversial but often requires the actin cytoskeleton. In this study, we used fluorescence resonance energy transfer to measure the role of the actin cytoskeleton in the co-clustering of membrane raft-associated fluorescent proteins (FPs) and FPs targeted to the nonraft membrane fraction. By fitting the fluorescence resonance energy transfer data to an isothermal binding equation, we observed a specific co-clustering of raft-associated donor and acceptor probes that was sensitive to latrunculin B (Lat B), which disrupts the actin cytoskeleton. Conversely, treating with jasplakinolide to enhance actin polymerization increased co-clustering of the raft-associated FPs over that of the nonraft probes. We also observed by immunoblotting experiments that the actin-dependent co-clustering coincided with regulation of the raft-associated Src family kinase Lck. Specifically, Lat B decreased the phosphorylation of the C-terminal regulatory tyrosine of Lck (Tyr505), and combining the Lat B with filipin further decreased the Tyr505 phosphorylation. Furthermore, the Lat B-dependent changes in Lck regulation required CD45 because no significant changes occurred in treated T cells lacking CD45 expression. These data define a role for the actin cytoskeleton in promoting co-clustering of raft-associated proteins and show that this property is important toward regulating raft-associated signaling proteins such as Lck.

Phase segregation on different length scales in a model cell membrane system

Lipid rafts are sphingolipid- and cholesterol-enriched domains on cell membranes that have been implicated in many biological functions, especially in T lymphocytes. We used a field theory to examine the forces underlying raft formation on resting living cell membranes. We find that it is difficult to reconcile the observed size of rafts on living cell membranes ( approximately 100 nm) with a mechanism that involves coupling between spontaneous curvature differences and concentration fluctuations. Such a mechanism seems to predict raft domain sizes that are larger and commensurate with those observed on synthetic membranes. Therefore, using a Poisson-Boltzmann approach, we explore whether electrostatic forces originating from transmembrane proteins and net negative charges on cell membranes could play a role in determining the raft size in living cell membranes. We find that a balance among the intrinsic tendency of raft components to segregate, the line tension, and the effective dipolar interactions among membrane constituents leads to a stable phase with a characteristic length scale commensurate with the observed size of rafts on living cell membranes. We calculate the phase diagram of a system in which these three types of forces are important. In a certain region of the parameter space, an interesting phase with mosaic-like morphology consisting of an intertwined pattern of raft and nonraft domains is predicted. Experiments that could further assess the importance of dipolar interactions for lateral organization of the components on multiple length scales in membranes are suggested.

Dynamics of lipid raft components during lymphocyte apoptosis: The paradigmatic role of GD3

Several investigations have been carried out since many years in order to precisely address the function of lipid rafts in cell life and death. On the basis of the biochemical nature of lipid rafts, composed by sphingolipids, including gangliosides, sphingomyelin, cholesterol and signaling proteins, a plethora of possible interactions with various subcellular structures has been suggested. Their structural and functional role at the plasma membrane as well as in cell organelles such as endoplasmic reticulum and Golgi apparatus has been analyzed in detail in several studies. In particular, a specific activity of lipid rafts has been hypothesized to contribute to cell death by apoptosis. Although detected in various cell types, the role of lipid rafts in apoptosis has however been mostly studied in lymphocytes where the physiological apoptotic program occurs after CD95/Fas triggering. In this review, the possible contribution of lipid rafts to the cascade of events leading to T cell apoptosis after CD95/Fas ligation are summarized. Particular attention has been given to the mitochondrial raft-like microdomains, which may represent preferential sites where some key reactions can take place and can be catalyzed, leading to either survival or death of T cells.

Tuning immune responses: diversity and adaptation of the immunological synapse

The onset and regulation of a specific immune response results from communication between T cells and antigen-presenting cells (APCs), which form molecular interactions at the site of cell-cell contact--and this is known as the immunological synapse. Initially, the immunological synapse was viewed as a stereotypical adhesion and signalling device with a defined molecular structure and signalling processes. However, as we discuss here, T-cell-APC interactions comprise a diverse range of contact modes and distinct molecular arrangements. These diverse interaction modes might define a molecular code, in which the differences in timing, spacing and molecular composition of the signalling platform determine the outcome of T-cell-APC interactions.

CD95 capping is ROCK-dependent and dispensable for apoptosis

Upon engagement, the CD95 receptor is rapidly clustered into cellular 'caps'. This receptor capping is one of the first events to take place following activation and it has been proposed to be important for the initiation of apoptotic signaling. As the biological roles of CD95 capping are still elusive, we explored in detail the role of capping in induction of apoptosis in lymphocytes. CD95 capping was shown to be uncoupled from apoptosis, as apoptosis could occur in the absence of CD95 capping and, vice versa, capping could occur without inducing apoptosis. CD95 capping occurred concomitantly with reorganization of the actin cytoskeleton and aggregation of lipid rafts. While inhibition of actin polymerization and caspase-8 activity had cell type-specific effects on capping in type I and type II cells, the rapid CD95-mediated cellular polarization, as visualized by the orchestrated reorganization of CD95, F-actin and lipid rafts, was shown to be dependent on signaling by Rho kinase (ROCK) in both cell types, however, by distinct activation mechanisms in the respective cell type. CD95 activated RhoA exclusively in the type II cell, whereas ROCK activation was caspase-dependent in the type I cell. Taken together, our results imply that CD95 capping and the subsequent cellular polarization is a ROCK signaling-regulated process that does not correlate with the induction of apoptosis, but is more likely to be involved in the emerging non-apoptotic functions of CD95.

Artificially lipid-anchored proteins can elicit clustering-induced intracellular signaling events in Jurkat T-lymphocytes independent of lipid raft association

We have incorporated artificial lipid-anchored streptavidin conjugates with fully saturated or polyunsaturated lipid anchors into the plasma membranes of Jurkat T-lymphocytes to assess previous conclusions that the activation of signaling processes induced in these cells by clustering of endogenous glycosylphosphatidylinositol-anchored proteins or ganglioside GM1 depends specifically on the association of these membrane components with lipid rafts. Lipid-anchored streptavidin conjugates could be incorporated into Jurkat or other mammalian cell surfaces by inserting biotinylated phosphatidylethanolamine-polyethyleneglycols (PE-PEGs) and subsequently binding streptavidin to the cell-incorporated PE-PEGs. Saturated dipalmitoyl-PE-PEG-streptavidin conjugates prepared in this manner partitioned substantially into the detergent-insoluble membrane fraction isolated from Jurkat or fibroblast cells, whereas polyunsaturated dilinoleoyl-PE-PEG-anchored conjugates were wholly excluded from this fraction, consistent with the differences in the affinities of the two types of lipid anchors for liquid-ordered membrane domains. Remarkably, however, antibody-mediated cross-linking of either dipalmitoyl- or dilinoleoyl-PE-PEG-anchored streptavidin conjugates in Jurkat cells induced elevation of cytoplasmic calcium levels and tyrosine phosphorylation of the scaf-folding protein linker of T-cell activation in a manner similar to that observed upon cross-linking of endogenous CD59 or ganglioside GM1. The amplitude of the cross-linking-stimulated elevation of cytoplasmic calcium moreover showed an essentially identical dependence on the level of incorporated streptavidin conjugate for either type of lipid anchor. Confocal fluorescence microscopy revealed that PE-PEG-streptavidin conjugates with saturated versus polyunsaturated anchors showed very similar surface distributions vis à vis GM1 or CD59 under conditions where one or both species were cross-linked. These results indicate that cross-linking of diverse proteins anchored only to the outer leaflet of the plasma membrane can induce activation of Jurkat T-cell-signaling responses, but they appear to contradict previous suggestions that this phenomenon rests specifically on the association of such species with lipid rafts.

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.

CD28 and lipid rafts coordinate recruitment of Lck to the immunological synapse of human T lymphocytes

In T lymphocytes, the Src family kinase Lck associates lipid rafts and accumulates at the immunological synapse (IS) during T cell stimulation by APCs. Using CD4- or CD28-deficient murine T cells, it was suggested that recruitment of Lck to the IS depends on CD4, whereas CD28 sustains Lck activation. However, in human resting T cells, CD28 is responsible for promoting recruitment of lipid rafts to the IS by an unknown mechanism. Thus, we performed a series of experiments to determine 1) whether Lck is recruited to the IS through lipid rafts; and 2) whether Lck recruitment to the IS of human resting T cells depends on CD4 or on CD28 engagement. We found that CD28, but not CD4, stimulation induced recruitment of Lck into detergent-resistant domains as well as its accumulation at the IS. We also found that Lck recruitment to the IS depends on the CD28 COOH-terminal PxxPP motif. Thus, the CD28-3A mutant, generated by substituting the prolines in positions 208, 211, and 212 with alanines, failed to induce Lck and lipid raft accumulation at the synapse. These results indicate that CD28 signaling orchestrates both Lck and lipid raft recruitment to the IS to amplify T cell activation.

Diversity in immune-cell interactions: states and functions of the immunological synapse

The contact-dependent exchange of signals between epithelial and neuronal cells results from close membrane-membrane appositions, which are stabilized for years by polarized adhesion, cytoskeletal assemblies and extracellular scaffold proteins. By contrast, owing to a lack of scaffold proteins, interactions between immune cells such as T lymphocytes and antigen-presenting cells (APCs) comprise a spectrum of structurally diverse and short-lived interaction modes that last from minutes to hours. Signals exchanged between T cells and APCs are generated in a specific contact region, termed the "immunological synapse", that coordinates cytoskeletal dynamics with the T-cell receptor (TCR), the engagement of accessory receptors and membrane-proximal signaling. Recent data shed light on the different physical and molecular interaction modes that occur between T cells and APCs, including their dynamics and transition stages, and their consequences for signaling, activation and T-cell effector function.

Visualization of antigen presentation by actin-mediated targeting of glycolipid-enriched membrane domains to the immune synapse of B cell APCs

Glycolipid-enriched membrane (GEM) domains, or lipid rafts, function in signaling in immune cells, but their properties during Ag presentation are less clear. To address this question, GEM domains were studied using fluorescence cell imaging of mouse CH27 B cells presenting Ag to D10 T cells. Our experiments showed that APCs were enriched with GEM domains in the immune synapse, and this occurred in an actin-dependent manner. This enrichment was specific to GEM domains, because a marker for non-GEM regions of the membrane was excluded from the immune synapse. Furthermore, fluorescence photobleaching experiments showed that protein in the immune synapse was dynamic and rapidly exchanged with that in other compartments of CH27 cells. To identify the signals for targeting GEM domains to the immune synapse in APCs, capping of the domains was measured in cells after cross-linking surface molecules. This showed that co-cross-linking CD48 with MHC class II was required for efficient capping and intracellular signaling. Capping of GEM domains by co-cross-linking CD48 and MHC class II occurred with co-capping of filamentous actin, and both domain capping and T cell-CH27 cell conjugation were inhibited by pretreating CH27 cells with latrunculin B. Furthermore, disruption of the actin cytoskeleton of the CH27 cells also inhibited formation of a mature immune synapse in those T cells that did conjugate to APCs. Thus, Ag presentation and efficient T cell stimulation occur by an actin-dependent targeting of GEM domains in the APC to the site of T cell engagement.

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.

T-lymphocyte signalling in systemic lupus erythematosus: a lipid raft perspective

In the last few years it has become clear that in cells of the immune system, specialized microdomains present in the plasma membrane, called lipid rafts, have been found to play a central role in regulating signalling by immune receptors. Recent studies have looked at whether lipid rafts may be connected to the abnormalities in signalling seen in T lymphocytes isolated from patients with systemic lupus erythematosus (SLE). These early findings show that in SLE T cells, the expression and protein composition of lipid rafts is different when compared with normal T cells. These results also demonstrate changes in the function and localization of critical signalling molecules such as the LCK tyrosine kinase and the CD45 tyrosine phosphatase.