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

Membrane/cytoskeleton communication

Accumulations of particular lipids in ordered arrays in the membrane (termed microdomains or lipid rafts) can attract proteins with specific targeting domains. Both the lipid and protein components of rafts communicate with the cytoskeleton directly thereby regulating cellular responses. Recent evidence implicating phosphoinositide 1,5 bisphosphate (PIP2) in cytoskeletal regulation shows that agonist sensitive regulation of PIP2 homoeostasis occurs specifically rafts, which appear to provide a major structural substrate for its function. The crucial role of PIP2 in generating cytoskeletal responses is chiefly achieved by regulating proteins that control actin dynamics directly. Many of these regulatory proteins are also specifically enriched in rafts either directly (by insertion into the lipid bilayer via acetylation motifs), or indirectly via interactions with other raft components. The notion that rafts form membrane platforms or modules that mediate signaling responses has been most extensively demonstrated in the immune synapse (IS) of T cells, a complex assemblage of rafts that integrates signaling cascades originating from the simultaneous activation of a wide variety of receptors. The IS is essential for both the amplification and maintenance of T-cell activation, and its assembly at the antigen presenting site depends on the interactions between rafts and the actin cytoskeleton that regulates coalescence of smaller raft components into the larger IS complex. Likewise the neuron, which represents the most highly polarized cell in the body, utilizes the regulation of actin dynamics in response to a plethora of extracellular signals to control axon pathfinding thereby sculpting nervous system cytoarchitecture with utmost precision. It is now becoming clear, that as in the T-cell, lipid rafts in the growing axon can assemble into highly specific, yet malleable and dynamic, signaling modules that regulate actin dynamics in a fashion that is also PIP2-dependent and that utilizes both familiar and novel regulatory mechanisms. It seems clear that raft mediated cytoskeletal regulation represents a highly conserved mechanism to integrate cellular responses to diverse signals.

Polarization of Plasma Membrane Microviscosity during Endothelial Cell Migration

Cell movement is characterized by anterior-posterior polarization of multiple cell structures. We show here that the plasma membrane is polarized in moving endothelial cells (EC); in particular, plasma membrane microviscosity (PMM) is increased at the cell leading edge. Our studies indicate that cholesterol has an important role in generation of this microviscosity gradient. In vitro studies using synthetic lipid vesicles show that membrane microviscosity has a substantial and biphasic influence on actin dynamics; a small amount of cholesterol increases actin-mediated vesicle deformation, whereas a large amount completely inhibits deformation. Experiments in migrating ECs confirm the important role of PMM on actin dynamics. Angiogenic growth factor-stimulated cells exhibit substantially increased membrane microviscosity at the cell front but, unexpectedly, show decreased rates of actin polymerization. Our results suggest that increased PMM in lamellipodia may permit more productive actin filament and meshwork formation, resulting in enhanced rates of cell movement.

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.

The amide class herbicide 3,4-dichloropropionanilide (DCPA) alters the mobility of hydrocarbon chains in T-lymphocyte but not macrophage membranes

Previous studies in our laboratory have demonstrated that the lipophilic herbicide 3,4-dicholoropropionanilide (DCPA) adversely affects cytokine production by activated macrophages and T lymphocytes. The purpose of this study was to test the hypothesis that DCPA alters the mobility of plasma membrane lipid hydrocarbon chains, which interferes with normal T-lymphocyte activation and macrophage function. Electron spin reasonance (ESR) spectroscopy of stearic acid spin labels incorporated into each cell type was used to test the effects of DCPA on lipid hydrocarbon chain mobility in the absence and presence of specific agents that activate each cell type. The results indicated that DCPA treatment had no significant effect on hydrocarbon chain mobility in either cell type per se. However, for T lymphocytes, but not macrophages, DCPA treatment increased a small population of lipid molecules that exhibited reduced hydrocarbon chain mobility near the bilayer hydrocarbon core following cell stimulation. In contrast, there were no significant effects of DCPA on hydrocarbon chain mobility near the head group region of the bilayer for either cell type. The identity of this subpopulation of lipids and its motional properties could not be elucidated from these studies. Nevertheless, data show that DCPA alters the distribution of lipids in distinct motional environments in the membrane of activated T lymphocytes.

T-cell integrins: more than just sticking points

T cells use integrins in essentially all of their functions. They use integrins to migrate in and out of lymph nodes and, following infection, to migrate into other tissues. At the beginning of an immune response, integrins also participate in the immunological synapse formed between T cells and antigen-presenting cells. Because the ligands for integrins are widely expressed, integrin activity on T cells must be tightly controlled. Integrins become active following signalling through other membrane receptors, which cause both affinity alteration and an increase in integrin clustering. Lipid raft localization may increase integrin activity. Signalling pathways involving ADAP, Vav-1 and SKAP-55, as well as Rap1 and RAPL, cause clustering of leukocyte function-associated antigen-1 (LFA-1; integrin αLβ2). T-cell integrins can also signal, and the pathways dedicated to the migratory activity of T cells have been the most investigated so far. Active LFA-1 causes T-cell attachment and lamellipodial movement induced by myosin light chain kinase at the leading edge, whereas RhoA and ROCK cause T-cell detachment at the trailing edge. Another important signalling pathway acts through CasL/Crk, which might regulate the activity of the GTPases Rac and Rap1 that have important roles in T-cell migration.

Human immunodeficiency virus type 1 enters primary human brain microvascular endothelial cells by a mechanism involving cell surface proteoglycans independent of lipid rafts

Several studies have reported a crucial role for cholesterol-enriched membrane lipid rafts and cell-associated heparan sulfate proteoglycans (HSPGs), a class of molecules that can localize in lipid rafts, in the entry of human immunodeficiency virus type 1 (HIV-1) into permissive cells. For the present study, we examined the role of these cell surface moieties in HIV-1 entry into primary human brain microvascular endothelial cells (BMVECs), which represent an important HIV-1 central nervous system-based cell reservoir and a portal for neuroinvasion. Cellular cholesterol was depleted by exposure to beta-cyclodextrins and 3-hydroxy-3-methylglutaryl (HMG)-coenzyme A reductase inhibitors (statins), the loss of cholesterol was quantitated, and disruption of membrane rafts was verified by immunofluorescence. Nevertheless, these treatments did not affect binding of several strains of HIV-1 virions to BMVECs at 4 degrees C or their infectivities at 37 degrees C. In contrast, we confirmed that cholesterol depletion and raft disruption strongly inhibited HIV-1 binding and infection of Jurkat T cells. Enzymatic digestion of cell-associated HSPGs on human BMVECs dramatically inhibited HIV-1 infection, and our data from quantitative HIV-1 DNA PCR analysis strongly suggest that cell-associated chondroitin sulfate proteoglycans greatly facilitate infective entry of HIV-1 into human BMVECs. These findings, in combination with our earlier work showing that human BMVECs lack CD4, indicate that the molecular mechanisms for HIV-1 entry into BMVECs are fundamentally different from that of viral entry into T cells, in which lipid rafts, CD4, and probably HSPGs play important roles.

F-actin and myosin II binding domains in supervillin

Detergent-resistant membranes contain signaling and integral membrane proteins that organize cholesterol-rich domains called lipid rafts. A subset of these detergent-resistant membranes (DRM-H) exhibits a higher buoyant density ( approximately 1.16 g/ml) because of association with membrane skeleton proteins, including actin, myosin II, myosin 1G, fodrin, and an actin- and membrane-binding protein called supervillin (Nebl, T., Pestonjamasp, K. N., Leszyk, J. D., Crowley, J. L., Oh, S. W., and Luna, E. J. (2002) J. Biol. Chem. 277, 43399-43409). To characterize interactions among DRM-H cytoskeletal proteins, we investigated the binding partners of the novel supervillin N terminus, specifically amino acids 1-830. We find that the supervillin N terminus binds directly to myosin II, as well as to F-actin. Three F-actin-binding sites were mapped to sequences within amino acids approximately 280-342, approximately 344-422, and approximately 700-830. Sequences with combinations of these sites promote F-actin cross-linking and/or bundling. Supervillin amino acids 1-174 specifically interact with the S2 domain in chicken gizzard myosin and nonmuscle myosin IIA (MYH-9) but exhibit little binding to skeletal muscle myosin II. Direct or indirect binding to filamin also was observed. Overexpression of supervillin amino acids 1-174 in COS7 cells disrupted the localization of myosin IIB without obviously affecting actin filaments. Taken together, these results suggest that supervillin may mediate actin and myosin II filament organization at cholesterol-rich membrane domains.

Localization and regulation of thyrotropin receptors within lipid rafts

The TSH receptor (TSHR) is a prototypic G protein-coupled receptor with a large extracellular domain. We have previously demonstrated homophilic interactions of TSHRs and their existence as constitutive oligomers. However, we have also shown that TSH itself promotes the formation of receptor monomers. We hypothesized, therefore, that TSHR monomers induced by TSH ligand may move into lipid rafts before effective TSH-induced signaling by bringing the cognate signaling molecules resident in such rafts together with the TSHRs. Thus, we aimed to determine whether the TSHRs would partition into these lipid rafts. The B subunit of cholera toxin (CTxB) binds to lipid raft-enriched GM1 ganglioside and has been widely exploited to visualize lipid rafts. Using such a method, we demonstrated the presence of these GM1-enriched lipid microdomains in Chinese hamster ovary cells by using CTxB labeled with a red dye (Alexa 594). To provide evidence for the presence of TSHRs in lipid rafts, we stained Chinese hamster ovary cells expressing TSHRGFP with labeled CTxB. Our results demonstrated that the TSHRGFP complexes localized to GM1-enriched lipid raft microdomains as evidenced by colocalization of the green fluorescent protein tag with the labeled CTxB. Hence, we concluded that a significant proportion of TSHRs were constitutively associated with lipid rafts. Furthermore, upon activation of these stained raft-receptor complexes with increasing concentrations of TSH, we observed that the raft-receptor complexes decreased significantly. The relevance of such receptor movement out of the rafts suggested that these may be the receptors critical in the initiation of signal transduction

Thiol/disulfide exchange is a prerequisite for CXCR4-tropic HIV-1 envelope-mediated T-cell fusion during viral entry

Attachment of gp120 to CD4 during HIV-1 entry triggers structural rearrangement in gp120 that enables binding to an appropriate coreceptor. Following coreceptor engagement, additional conformational changes occur in the envelope (Env), resulting in fusion of virion and cell membranes. Catalysts with redox-isomerase activity, such as protein disulfide isomerase (PDI), facilitate Env conversion from its inactive to its fusion-competent conformation. We report here that anti-PDI agents effectively block CXCR4 Env-mediated fusion and spread of virus infection. Exogenously added PDI, in turn, can rescue fusion from this blockade. We further find that PDI facilitates thiol/disulfide rearrangement in gp120 during conformational change, whereas inhibition of this redox shuffling prevents gp41 from assuming the fusogenic 6-helix bundle conformation. At the virus-cell contact site, gp120 induces assembly of PDI, CD4, and CXCR4 into a tetramolecular protein complex serving as a portal for viral entry. Our findings support the hypothesis that Env conformational change depends on a well-coordinated action of a tripartite system in which PDI works in concert with the receptor and the coreceptor to effectively lower the activation energy barrier required for Env conformational rearrangement.

Polarisation of T-cadherin to the leading edge of migrating vascular cells in vitro: a function in vascular cell motility?

Both histological and in vitro studies indicate a relationship between T-cadherin levels and acquisition of a modulated, migratory phenotype by vascular cells. This study further examines a role for T-cadherin in relation to cell migration and adhesion. Fluorescence microscopic examination of T-cadherin localisation in confluent cultures of human umbilical vein endothelial cells (HUVEC), human aortic smooth muscle cells and the human carcinoma cell line ECV-304 revealed global distribution over the entire cell body, and with only slight enrichment at cell borders. This contrasts with restricted cell-cell junction localisation of classical cadherin (for example, VE-cadherin in HUVEC). In wounded cultures, T-cadherin polarised to the leading edge of cells migrating into the wound area, again contrasting with classical VE-cadherin, which was undetectable in this region. Confocal microscopy demonstrated that potential signalling functions of T-cadherin at the leading edge are unrelated to physical interactions with caveolin. Adherence of HUVEC onto a monolayer of T-cadherin-transfected L929 cells is significantly reduced compared with adhesion onto control (T-cadherin-negative) L929. Thus T-cadherin is not required for maintenance of intercellular adhesion, but may rather function as a signalling molecule involved in cell-cell recognition and sensing of the environment in processes where cell detachment occurs.

CD4 receptor localized to non-raft membrane microdomains supports HIV-1 entry. Identification of a novel raft localization marker in CD4

Despite the preferential localization of CD4 to lipid rafts, the significance and role of these microdomains in HIV-1 entry is still controversial. The possibility that CD4, when localized to non-raft domains, might be able to support virus entry cannot be excluded. Because disintegration of rafts by extraction of cellular cholesterol with methyl-beta-cyclodextrin suffers from various adverse effects, we investigated molecular determinants controlling raft localization of the CD4 receptor. Extensive mutagenesis of the receptor showed that a raft-localizing marker, consisting of a short sequence of positively charged amino acid residues, RHRRR, was present in the membrane-proximal cytoplasmic domain of CD4. Substitution of the RHRRR sequence with alanine residues abolished raft localization of the CD4 mutant, RA5, as determined biochemically using solubilization in nonionic detergents and by confocal microscopy. The possible inhibitory effect of the introduced mutations on the adjacent CVRC palmitoylation site was ruled out because wild type (wt) CD4 and RA5, but not a palmitoylation-deficient mutant, were efficiently palmitoylated. Nonetheless, the RA5 mutant supported productive virus entry to levels equivalent to that of wild type (wt) CD4. Sucrose gradient analysis of Triton X-100 virus lysates showed that Gag and envelope gp120 proteins accumulated in low buoyant, high-density fractions. This pattern was changed after virus incubation with cells. Whereas Gag proteins localized to lipid rafts in cells expressing wt CD4 and RA5, gp120 accumulated in rafts in cells expressing wt CD4 but not RA5. We propose that raft localization of CD4 is not required for virus entry, however, post-binding fusion/entry steps may require lipid raft assembly.

Smooth muscle raft-like membranes

We developed a method for extracting raft-like, liquid-ordered membranes from the particulate fraction prepared from porcine trachealis smooth muscle. This fraction, which contains most of the plasma membrane in this tissue, was homogenized in the presence of cold 0.5% Triton X-100. After centrifugation, membranes containing high contents of sphingomyelin (SM) and cholesterol and low phosphatidylcholine (PC) contents remained in the pellet. Thirty-five millimolar octyl glucoside (OG) extracted 75% of these membranes from the Triton X-100-resistant pellet. These membranes had low buoyant densities and accounted for 28% of the particulate fraction lipid. Their lipid composition, 22% SM, 60% cholesterol, 11% phosphatidylethanolamine, 8% PC, <1% phosphatidylinositol, and coisolation with 5'-nucleotidase and caveolin-1 suggest that they are liquid-ordered membranes. We compared characteristics of OG and Triton X-100 extractions of the particulate fraction. In contrast to Triton X-100 extractions, membranes released from the particulate fraction by OG were mainly collected in low buoyant fractions at densities ranging from 1.05 to 1.11 g/ml and had phospholipid and cholesterol contents consistent with a mixture of liquid-ordered and liquid-disordered membranes. Thus, OG extraction of apparent liquid-ordered membranes from Triton X-100-resistant pellets was not due to selective extraction of these membranes. Low buoyant density appears not to be unique for liquid-ordered membranes.

Differential recruitment of Kv1.4 and Kv4.2 to lipid rafts by PSD-95

The activity of voltage-gated potassium (Kv) channels, and consequently their influence on cellular functions, can be substantially altered by phosphorylation. Several protein kinases that modulate Kv channel activity are found in membrane subdomains known as lipid rafts, which are thought to organize signaling complexes in the cell. Thus, we asked whether Kv1.4 and Kv4.2, two channels with critical roles in excitable cells, are found in lipid rafts. Acylation can target proteins to raft regions; however, Kv channels are not acylated, and therefore, a different mechanism must exist to bring them into these membrane subdomains. Because both Kv1.4 and Kv4.2 interact with postsynaptic density protein 95 (PSD-95), which is acylated (specifically, palmitoylated), we examined whether PSD-95 can recruit these channels to lipid rafts. We found that a portion of Kv1.4 and Kv4.2 protein in rat brain membranes is raft-associated. Lipid raft patching and immunostaining confirmed that some Kv4.2 is in Thy-1-containing rafts in rat hippocampal neurons. Using a heterologous expression system, we determined that palmitoylation of PSD-95 was crucial to its localization to lipid rafts. We then assessed the contribution of PSD-95 to the raft association of these channels. Co-expression of PSD-95 increased the amount of Kv1.4, but not Kv4.2, in lipid rafts. Deleting the PSD-95 binding motif of Kv1.4 eliminated this recruitment, as did substituting a palmitoylation-deficient PSD-95 mutant. This work represents the first evidence that PSD-95 binding can recruit Kv channels into lipid rafts, a process that could facilitate interactions with the protein kinases that affect channel activity.

Flotillas of lipid rafts in transit amplifying cell-like keratinocytes

Lipid rafts are dynamic membrane microdomains enriched in cholesterol and sphingolipids and are involved in the regulation of a variety of cellular processes, such as proliferation, apoptosis and cell motility. We have previously described that large lipid raft aggregates are readily detectable on cultured keratinocyte cell line HaCaT by staining with the fluorescein-tagged cholera toxin (CTx-FITC). In this paper we adopted this method for the detection of lipid rafts in human epidermis and keratinocytes in culture. Double labelling of showed the non-overlapping clusters of basal cells in human epidermis: CD29dimCTx-FITCbright cells in the deep rate ridges and CD29brightCTx-FITCdim cells at the tips of dermal papillae. A similar patchy, non-overlapping staining pattern was observed in cultured keratinocytes in vitro. CTx-FITCbright cells are mitotically active whereas a large proportion of CTx-FITCdim cells are quiescent. We conclude that the epidermal stem-like cells, previously shown to occupy the tips of dermal papillae and to exhibit high density of membrane beta1 integrin have a low content of lipid rafts. In contrast, the putative transit amplifying cells in deep rate ridges show enrichment in lipid rafts. Thus, lipid rafts may be a factor controlling the mitotic activity of epidermal keratinocytes and possibly the differentiation of stem cells into the transit amplifying cells.

Location is everything: lipid rafts and immune cell signaling

The cells of both the adaptive and innate immune systems express a dizzying array of receptors that transduce and integrate an enormous amount of information about the environment that allows the cells to mount effective immune responses. Over the past several years, significant advances have been made in elucidating the molecular details of signal cascades initiated by the engagement of immune cell receptors by their ligands. Recent evidence indicates that immune receptors and components of their signaling cascades are spatially organized and that this spatial organization plays a central role in the initiation and regulation of signaling. A key organizing element for signaling receptors appears to be cholesterol- and sphingolipid-rich plasma membrane microdomains termed lipid rafts. Research into the molecular basis of the spatial segregation and organization of signaling receptors provided by rafts is adding fundamentally to our understanding of the initiation and prolongation of signals in the immune system.

Selective inhibition of T cell activation via CD147 through novel modulation of lipid rafts

The plasma membrane is compartmentalized into microdomains and the association/dissociation of receptors and signaling molecules with/from these membrane domains is a major principle for regulation of signal transduction. By following the reorganization of microdomains on living cells and performing biochemical studies, we show that Ab targeting of the T cell activation-associated Ag CD147 prevents TCR stimulation-dependent reorganization and clustering of microdomains. Triggering CD147 induces a displacement of the GPI-anchored coreceptors CD48 and CD59 from microdomains in human T lymphocytes. This perturbation of microdomains is accompanied by a selective inhibition of TCR-mediated T cell proliferation. The CD147-inhibited cells secret normal levels of IL-2 but acquire reduced amounts of the IL-2 receptor alpha-chain CD25. These results indicate that negative regulating signals can modulate microdomains and suggest a general mechanism for inhibition of receptor signaling.

Leukocyte-endothelium interaction promotes SDF-1-dependent polarization of CXCR4

Chemokine-driven migration is accompanied by polarization of the cell body and of the intracellular signaling machinery. The extent to which chemokine receptors polarize during chemotaxis is currently unclear. To analyze the distribution of the chemokine receptor CXCR4 during SDF-1 (CXCL12)-induced chemotaxis, we retrovirally expressed a CXCR4-GFP fusion protein in the CXCR4-deficient human hematopoietic progenitor cell line KG1a. This KG1a CXCR4-GFP cell line showed full restoration of SDF-1 responsiveness in assays detecting activation of ERK1/2 phosphorylation, actin polymerization, adhesion to endothelium under conditions of physiological flow, and (transendothelial) chemotaxis. When adhered to cytokine-activated endothelium in the absence of SDF-1, CXCR4 did not localize to the leading edge of the cell but was uniformly distributed over the plasma membrane. In contrast, when SDF-1 was immobilized on cytokine-activated endothelium, the CXCR4-GFP receptors that were present on the cell surface markedly redistributed to the leading edge of migrating cells. In addition, CXCR4-GFP co-localized with lipid rafts in the leading edge of SDF-1-stimulated cells, at the sites of contact with the endothelial surface. Inhibition of lipid raft formation prevents SDF-1-dependent migration, internalization of CXCR4, and polarization to the leading edge of CXCR4, indicating that CXCR4 surface expression and signaling requires lipid rafts. These data show that SDF-1, immobilized on activated human endothelium, induces polarization of CXCR4 to the leading edge of migrating cells, revealing co-operativity between chemokine and substrate in the control of cell migration.

The flotillins are integral membrane proteins in lipid rafts that contain TCR-associated signaling components: implications for T-cell activation

Lipid rafts play an important role in signal integration and cellular activation by the T-cell antigen receptor (TCR). We demonstrate that flotillin-1 and flotillin-2 are important structural raft components, which redistribute to the site of TCR engagement. An antibody to flotillin-1 was able to immobilize other TCR-associated raft components. Although rafts purified from unstimulated cells demonstrated abundant Lck but inabundant LAT, rafts from stimulated cells include an abundance of both components. This suggests dynamic changes in lipid raft composition during CD3/CD28 costimulation. Stimulation of primary human CD4(+) T cells leads to increased GM1 and flotillin-1 expression in the surface membrane, where these components colocalize. This may reconstitute new signaling complexes required for T-cell activation. Altered lipid raft composition and function may play a role in the decline of antigen responsiveness in senescent T cells. In this regard, we observed an increase in the raft-associated gangliolipid, GM1, in resting human CD4(+) and CD8(+) lymphocytes with aging.

RAPL, a Rap1-binding molecule that mediates Rap1-induced adhesion through spatial regulation of LFA-1

The small GTPase Rap1 is a potent activator of leukocyte integrin. However, the regulatory mechanism involved is unknown. Here, we identify the Rap1 effector, RAPL, as an essential regulator in this activation. RAPL was enriched in mouse lymphoid tissues and associated with Rap1 after stimulation by the T cell receptor and with chemokine CXCL12. Human RAPL stimulated lymphocyte polarization and the patch-like redistribution of lymphocyte-function-associated antigen 1 (LFA-1) to the leading edge, resulting in enhanced adhesion to intercellular adhesion molecule 1 (ICAM-1). Triggered by activated Rap1, RAPL associated with LFA-1 and rapidly relocated to the leading edge and accumulated at immunological synapses. Thus, RAPL regulates lymphocyte adhesion through the spatial distribution of LFA-1.

Differential caveolin-1 polarization in endothelial cells during migration in two and three dimensions

Endothelial cell (EC) migration is a critical event during multiple physiological and pathological processes. ECs move in the plane of the endothelium to heal superficially injured blood vessels but migrate in three dimensions during angiogenesis. We herein investigate differences in these modes of movement focusing on caveolae and their defining protein caveolin-1. Using a novel approach for morphological analysis of transmigrating cells, we show that ECs exhibit a polarized distribution of caveolin-1 when traversing a filter pore. Strikingly, in these cells caveolin-1 seems to be released from caveolar structures in the cell rear and to relocalize at the cell front in a cytoplasmic form. In contrast, during planar movement caveolin-1 is concentrated at the rear of ECs, colocalizing with caveolae. The phosphorylatable Tyr14 residue of caveolin-1 is required for polarization of the protein during transmigration but does not alter polarization during planar movement. Palmitoylation of caveolin-1 is not essential for redistribution of the protein during either mode of movement. Thus, ECs migrating in three dimensions uniquely exhibit dissociation of caveolin-1 from caveolae and phosphorylation-dependent relocalization to the cell front.

Platelet endothelial cell adhesion molecule-1 modulates endothelial cell motility through the small G-protein Rho

Platelet endothelial cell adhesion molecule-1 (PECAM-1), an immunoglobulin family vascular adhesion molecule, is involved in endothelial cell migration and angiogenesis (1, 2). We found that endothelial cells lacking PECAM-1 exhibit increased single cell motility and extension formation but poor wound healing migration, reminiscent of cells in which Rho activity has been suppressed by overexpressing a GTPase-activating protein (3). The ability of PECAM-1 to restore wound healing migration to PECAM-1-deficient cells was independent of its extracellular domain or signaling via its immunoreceptor tyrosine-based inhibitory motif. PECAM-1-deficient endothelial cells had a selective defect in RhoGTP loading, and inhibition of Rho activity mimicked the PECAM-1-deficient phenotype of increased chemokinetic single cell motility at the expense of coordinated wound healing migration. The wound healing advantage of PECAM-1-positive endothelial cells was not only Rho mediated but pertussis toxin inhibitable, characteristic of migration mediated by heterotrimeric G-protein-linked seven-transmembrane receptor signaling such as signaling in response to the serum sphingolipid sphingosine-1-phosphate (S1P) (4, 5). Indeed, we found that the wound healing defect of PECAM-1 null endothelial cells is minimized in sphingolipid-depleted media; moreover, PECAM-1 null endothelial cells fail to increase their migration in response to S1P. We have also found that PECAM-1 localizes to rafts and that in its absence heterotrimeric G-protein components are differentially recruited to rafts, providing a potential mechanism for PECAM-1-mediated coordination of S1P signaling. PECAM-1 may thus support the effective S1P/RhoGTP signaling required for wound healing endothelial migration by allowing for the spatially directed, coordinated activation of Galpha signaling pathways.

Divergent signals and cytoskeletal assemblies regulate self-organizing polarity in neutrophils

Like neutrophilic leukocytes, differentiated HL-60 cells respond to chemoattractant by adopting a polarized morphology, with F-actin in a protruding pseudopod at the leading edge and contractile actin-myosin complexes at the back and sides. Experiments with pharmacological inhibitors, toxins, and mutant proteins show that this polarity depends on divergent, opposing "frontness" and "backness" signals generated by different receptor-activated trimeric G proteins. Frontness depends upon Gi-mediated production of 3'-phosphoinositol lipids (PI3Ps), the activated form of Rac, a small GTPase, and F-actin. G12 and G13 trigger backness signals, including activation of a second GTPase (Rho), a Rho-dependent kinase, and myosin II. Functional incompatibility causes the two resulting actin assemblies to aggregate into separate domains, making the leading edge more sensitive to attractant than the back. The latter effect explains both the neutrophil's ability to polarize in uniform concentrations of chemoattractant and its response to reversal of an attractant gradient by performing a U-turn.

Ganglioside GM3 inhibits matrix metalloproteinase-9 activation and disrupts its association with integrin

Gangliosides GM3 and GT1b both inhibit epithelial cell adhesion and migration on fibronectin. GT1b binds to integrin alpha5beta1 and blocks the integrin-fibronectin interaction; GM3 does not interact with integrin, and its effect is poorly understood. We evaluated the effects of endogenous modulation of GM3 expression on epithelial cell motility on several matrices and the mechanism of these effects. Endogenous accumulation of GM3 decreased cell migration on fibronectin, types I, IV, and VII collagen matrices; depletion of GM3 dramatically increased cell migration, regardless of matrix. GM3 overexpression and depletion in vitro correlated inversely with the expression and activity of matrix metalloproteinase-9; consistently, the cell migration stimulated by GM3 depletion is reversed by inhibition of matrix metalloproteinase-9 activity. Accumulation and depletion of GM3 in epithelial cells grown on fibronectin also correlated inversely with epidermal growth factor receptor and mitogen activated protein kinase phosphorylation and with Jun expression. Ganglioside depletion facilitated the co-immunoprecipitation of matrix metal-loproteinase-9 and integrin alpha5beta1, while endogenous accumulation of GM3, but not GT1b, blocked the co-immunoprecipitation. These data suggest modulation of epidermal growth factor receptor signaling and dissociation of integrin/matrix metalloproteinase-9 as mechanisms for the GM3-induced effects on matrix metalloproteinase-9 function.

Asymmetric localization of flotillins/reggies in preassembled platforms confers inherent polarity to hematopoietic cells

Hematopoietic cells have long been defined as round, nonpolar cells that show uniform distribution of cell surface-associated molecules. However, recent analyses of the immunological synapse and the importance of lipid microdomains in signaling have shed new light on the aspect of lymphocyte polarization during the activation processes, but none of the molecules implicated so far in either the activation process or the microdomain residency are known to have a preferential localization in nonactivated cells. Chemical crosslinking and fluorescence resonance energy transfer methods have allowed the visualization of certain glycosylphosphatidylinositol-anchored proteins in lipid rafts but so far no microdomain resident protein has been shown to exist as visible stable platforms in the membrane. We report here that two lipid microdomain resident proteins, flotillins/reggies, form preassembled platforms in hematopoietic cells. These platforms recruit signaling molecules upon activation through lipid rafts. The preassembled platforms significantly differ from the canonical cholesterol-dependent "lipid rafts," as they are resistant to cholesterol-disrupting agents. Most evidence for the functional relevance of microdomains in living cells remains indirect. Using laser scanning confocal microscopy, we show that these proteins exist as stable, microscopically patent domains localizing asymmetrically to one pole of the cell. We present evidence that the asymmetric concentration of these microdomain resident proteins is built up during cytokinesis.

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

In T cells, glycolipid-enriched membrane (GEM) domains, or lipid rafts, are assembled into immune synapses in response to Ag presentation. However, the properties of T cell GEM domains in the absence of stimulatory signals, such as their size and distribution in the plasma membrane, are less clear. To address this question, we used confocal microscopy to measure GEM domains in unstimulated T cells expressing a GEM-targeted green fluorescent protein molecule. Our experiments showed that the GEM domains were assembled into membrane patches that were micrometers in size, as evidenced by a specific enrichment of GEM-associated molecules and resistance of the patches to extraction by Triton X-100. However, treatment of cells with latrunculin B disrupted the patching of the GEM domains and their resistance to Triton X-100. Similarly, the patches were coenriched with F-actin, and actin occurred in the detergent-resistant GEM fraction of T cells. Live-cell imaging showed that the patches were mobile and underwent translocation in the plasma membrane to immune synapses in stimulated T cells. Targeting of GEM domains to immune synapses was found to be actin-dependent, and required phosphatidylinositol 3-kinase activity and myosin motor proteins. We conclude from our results that T cell GEM domains are constitutively assembled by the actin cytoskeleton into micrometer-sized membrane patches, and that GEM domains and the GEM-enriched patches can function as a vehicle for targeting molecules to immune synapses.

Pathogens: raft hijackers

Throughout evolution, organisms have developed immune-surveillance networks to protect themselves from potential pathogens. At the cellular level, the signalling events that regulate these defensive responses take place in membrane rafts--dynamic microdomains that are enriched in cholesterol and glycosphingolipids--that facilitate many protein-protein and lipid-protein interactions at the cell surface. Pathogens have evolved many strategies to ensure their own survival and to evade the host immune system, in some cases by hijacking rafts. However, understanding the means by which pathogens exploit rafts might lead to new therapeutic strategies to prevent or alleviate certain infectious diseases, such as those caused by HIV-1 or Ebola virus.

Membrane cholesterol regulates LFA-1 function and lipid raft heterogeneity

Many surface receptors and signaling molecules are thought to associate with unique membrane microdomains termed lipid rafts. We examined the involvement of lipid rafts in the activation of leukocyte function-associated antigen-1 (LFA-1). Depletion or sequestration of cholesterol with methyl-beta-cyclodextrin (MCD) or filipin, respectively, strongly inhibited LFA-1-mediated adhesion of T-cell lines and primary T cells. This inhibition was reversed by cholesterol reconstitution. LFA-1 on T-cell lines was detected in cold Triton X-100-insoluble lipid rafts, which were disrupted by MCD or filipin treatment. However, no LFA-1 on primary T cells was detected in lipid rafts isolated by the same procedures, and these rafts were resistant to cholesterol depletion or sequestration. Association of LFA-1 with lipid rafts of primary T cells could be detected only when they were isolated with another nonionic detergent, Brij 35. Upon treatment with MCD, LFA-1 in Brij 35-insoluble lipid rafts partially shifted to nonraft fractions. T-cell lines were found to have a high level of cholesterol and a low level of ganglioside GM1, a common marker for lipid rafts, whereas primary T cells have a much lower level of cholesterol and a very high amount of GM1. Cross-linking of LFA-1 on primary T cells induced cocapping of cholesterol but not GM1. These results suggest that lipid rafts of T cells are heterogenous, and LFA-1 associates with a subset of lipid rafts containing a high level of cholesterol. This association seems to regulate LFA-1 functions, possibly by facilitating LFA-1 clustering.

From rafts to crafts: membrane asymmetry in moving cells

Many important biological events, including the leukocyte-mediated immune response, wound repair, axon guidance and developmental patterning, involve persistent cell movement towards a directional signal, a process termed chemotaxis. Establishment of functional and spatial cell polarity is an absolute requirement for this response. We propose that redistribution of specific membrane microdomains, termed rafts, during cell migration is a pivotal step in achieving polarity. On the one hand, partitioning of molecules into rafts might help to localize proteins at the front or the rear of moving cells, and on the other hand, rafts might function as platforms for local activation and coordination of the signaling pathways involved in cell migration.

Chemokines: roles in leukocyte development, trafficking, and effector function

Chemokines, representing a large superfamily of 8- to 15-kd proteins, were originally discovered through their ability to recruit various cell types into sites of inflammation. It is now clear that these molecules play a much wider role in immune homeostasis, playing key roles in driving the maturation, homing, and activation of leukocytes. In this review we analyze the roles chemokines play in the development, recruitment, and activation of leukocytes. Because signaling from the receptors drives these processes, signal transduction from chemokine receptors will also be reviewed. Taken together, we highlight the various points at which chemokines contribute to allergic inflammation and at which their targeting might contribute to new therapies for type I hypersensitivity reactions.

Lipid rafts in the maintenance of synapses, dendritic spines, and surface AMPA receptor stability

Cholesterol/sphingolipid microdomains (lipid rafts) in the membrane are involved in protein trafficking, formation of signaling complexes, and regulation of actin cytoskeleton. Here, we show that lipid rafts exist abundantly in dendrites of cultured hippocampal neurons, in which they are associated with several postsynaptic proteins including surface AMPA receptors. Depletion of cholesterol/sphingolipid leads to instability of surface AMPA receptors and gradual loss of synapses (both inhibitory and excitatory) and dendritic spines. The remaining synapses and spines in raft-depleted neurons become greatly enlarged. The importance of lipid rafts for normal synapse density and morphology could explain why cholesterol promotes synapse maturation in retinal ganglion cells (Mauch et al., 2001) and offers a potential link between disordered cholesterol metabolism and the synapse loss seen in neurodegenerative disease.

The fate and function of glycosphingolipid glucosylceramide

In higher eukaryotes, glucosylceramide is the simplest member and precursor of a fascinating class of membrane lipids, the glycosphingolipids. These lipids display an astounding variation in their carbohydrate head groups, suggesting that glycosphingolipids serve specialized functions in recognition processes. It is now realized that they are organized in signalling domains on the cell surface. They are of vital importance as, in their absence, embryonal development is inhibited at an early stage. Remarkably, individual cells can live without glycolipids, perhaps because their survival does not depend on glycosphingolipid-mediated signalling mechanisms. Still, these cells suffer from defects in intracellular membrane transport. Various membrane proteins do not reach their intracellular destination, and, indeed, some intracellular organelles do not properly differentiate to their mature stage. The fact that glycosphingolipids are required for cellular differentiation suggests that there are human diseases resulting from defects in glycosphingolipid synthesis. In addition, the same cellular differentiation processes may be affected by defects in the degradation of glycosphingolipids. At the cellular level, the pathology of glycosphingolipid storage diseases is not completely understood. Cell biological studies on the intracellular fate and function of glycosphingolipids may open new ways to understand and defeat not only lipid storage diseases, but perhaps other diseases that have not been connected to glycosphingolipids so far.

Formation of functional cell membrane domains: the interplay of lipid- and protein-mediated interactions

Numerous cell membrane associated processes, including signal transduction, membrane sorting, protein processing and virus trafficking take place in membrane subdomains. Protein-protein interactions provide the frameworks necessary to generate biologically functional membrane domains. For example, coat proteins define membrane areas destined for sorting processes, viral proteins self-assemble to generate a budding virus, and adapter molecules organize multimolecular signalling assemblies, which catalyse downstream reactions. The concept of raft lipid-based membrane domains provides a different principle for compartmentalization and segregation of membrane constituents. Accordingly, rafts are defined by the physical properties of the lipid bilayer and function by selective partitioning of membrane lipids and proteins into membrane domains of specific phase behaviour and lipid packing. Here, I will discuss the interplay of these independent principles of protein scaffolds and raft lipid microdomains leading to the generation of biologically functional membrane domains.

Resistance of cell membranes to different detergents

Partial resistance of cell membranes to solubilization with mild detergents and the analysis of isolated detergent-resistant membranes (DRMs) have been used operationally to define membrane domains. Given the multitude of detergents used for this purpose, we sought to investigate whether extraction with different detergents might reflect the same underlying principle of domain formation. We therefore compared the protein and lipid content of DRMs prepared with a variety of detergents from two cell lines. We found that the detergents differ considerably in their ability to selectively solubilize membrane proteins and to enrich sphingolipids and cholesterol over glycerophospholipids as well as saturated over unsaturated phosphatidylcholine. In addition, we observed cell type-dependent variations of the molecular characteristics of DRMs and the effectiveness of particular detergents. These results make it unlikely that different detergents reflect the same aspects of membrane organization and underscore both the structural complexity of cell membranes and the need for more sophisticated analytical tools to understand their architecture.

Microvillar membrane microdomains exist at physiological temperature. Role of galectin-4 as lipid raft stabilizer revealed by "superrafts"

Lipid rafts (glycosphingolipid/cholesterol-enriched membrane microdomains) have been isolated as low temperature, detergent-resistant membranes from many cell types, but despite their presumed importance as lateral sorting and signaling platforms, fundamental questions persist concerning raft function and even existence in vivo. The nonionic detergent Brij 98 was used to isolate lipid rafts from microvillar membrane vesicles of intestinal brush borders at physiological temperature to compare with rafts, obtained by "conventional" extraction using Triton X-100 at low temperature. Microvillar rafts prepared by the two protocols were morphologically different but had essentially similar profiles of protein- and lipid components, showing that raft microdomains do exist at 37 degrees C and are not "low temperature artifacts." We also employed a novel method of sequential detergent extraction at increasing temperature to define a fraction of highly detergent-resistant "superrafts." These were enriched in galectin-4, a beta-galactoside-recognizing lectin residing on the extracellular side of the membrane. Superrafts also harbored the glycosylphosphatidylinositol-linked alkaline phosphatase and the transmembrane aminopeptidase N, whereas the peripheral lipid raft protein annexin 2 was essentially absent. In conclusion, in the microvillar membrane, galectin-4, functions as a core raft stabilizer/organizer for other, more loosely raft-associated proteins. The superraft analysis might be applicable to other membrane microdomain systems.

Membrane incorporation of 22-hydroxycholesterol inhibits chemokine receptor activity

Cell membrane exposure to oxysterols, such as 22-hydroxycholesterol (22-OHC), has previously been shown to induce a suppressive effect on lymphocyte activation. Based on our previous findings that chemokine binding was significantly inhibited by the extraction of membrane cholesterol, we sought to assess the effects of 22-OHC treatment on chemokine ligand-binding and receptor activity. Our results revealed that 22-OHC, but not nonoxidized cholesterol, significantly reduced the binding of both SDF-1alpha and MIP-1beta to human T-cell lines and PBMCs within 1 h of treatment. Incubating the treated cells at 37 degrees C for 1 h reversed a majority of the inhibitory effects on chemokine binding. 22-OHC also inhibited intracellular calcium mobilization and cell migration in response to SDF-1alpha treatment. Interestingly, while the presence of oxysterols in cell membranes significantly inhibits chemokine receptor function, this inhibitory effect does not involve alterations in receptor conformation, expression, or a direct antagonism of chemokine binding. We propose here a novel mechanism for oxysterol-mediated inhibition of chemokine receptor function and the implications for the presence of oxysterols on immune cells.

Membrane dynamics and cell polarity: the role of sphingolipids

In recent years, glycosphingolipids (GSLs) have attracted widespread attention due to the appreciation that this class of lipids has a major impact on biological life. Inhibition of the synthesis of glucosylceramide, which serves as a precursor for the generation of complex glycosphinglipids, is embryonic lethal. GSLs play a major role in growth and development. Metabolites of sphingolipids, such as ceramide, sphinganine, and sphingosine, may function as second messengers or regulators of signal transduction that affect events ranging from apoptosis to the (co)regulation of the cell cycle. In addition, GSLs can provide a molecular platform for clustering of signal transducers. The ability of sphingolipids, with or without cholesterol, to form microdomains or rafts is critical in sorting and membrane transport that underlies the biogenesis of polarized membrane domains. Here, a brief summary is presented of some recent developments in this field, with a particular emphasis on raft assembly and membrane transport in the establishment of membrane polarity.

Leukocyte uropod formation and membrane/cytoskeleton linkage in immune interactions

The acquisition of a cell polarity is a crucial requirement for migration, activation, and apoptosis of leukocytes. The polarization of leukocytes involves the formation of two distinct poles: the leading edge--the attachment cell site to the substrate allowing directional movements of the cell--and on the opposite side, the uropod--mostly involved in cell-to-cell interaction and in a variety of leukocyte activities including activation and apoptosis. However, the uropod takes shape in neutrophils, monocytes, and natural killer cells, and the formation of this cell protrusion seems to exert an important role in immune interactions. In fact, the polarization sites of leukocytes are involved in a complex cross-talk between cells and extracellular matrix components, and a number of receptors and counter-receptors crowd in the contact sites to allow efficient cell-to-cell or cell-substrate interaction. The membrane/cytoskeleton interaction plays a crucial role in tuning these activities and in "predisposing" leukocytes to their function through the acquisition of a polarized phenotype. This review is focused on the mechanisms underlying the formation of the leukocyte uropod, the role of cytoskeleton in defining its structure and function, and the involvement of the uropod in the complex interplay between immune cells.

Rap1 translates chemokine signals to integrin activation, cell polarization, and motility across vascular endothelium under flow

Chemokines arrest circulating lymphocytes within the vasculature through the rapid up-regulation of leukocyte integrin adhesive activity, promoting subsequent lymphocyte transmigration. However, the key regulatory molecules regulating this process have remained elusive. Here, we demonstrate that Rap1 plays a pivotal role in chemokine-induced integrin activation and migration. Rap1 was activated by secondary lymphoid tissue chemokine (SLC; CCL21) and stromal-derived factor 1 (CXCL4) treatment in lymphocytes within seconds. Inhibition of Rap1 by Spa1, a Rap1-specific GTPase-activating protein, abrogated chemokine-stimulated lymphocyte rapid adhesion to endothelial cells under flow via intercellular adhesion molecule 1. Expression of a dominant active Rap1V12 in lymphocytes stimulated shear-resistant adhesion, robust cell migration on immobilized intercellular adhesion molecule 1 and vascular cell adhesion molecule 1, and transendothelial migration under flow. We also demonstrated that Rap1V12 expression in lymphocytes induced a polarized morphology, accompanied by the redistribution of CXCR4 and CD44 to the leading edge and uropod, respectively. Spa1 effectively suppressed this polarization after SLC treatment. This unique characteristic of Rap1 may control chemokine-induced lymphocyte extravasation.

Lipid rafts in the maintenance of synapses, dendritic spines, and surface AMPA receptor stability

Cholesterol/sphingolipid microdomains (lipid rafts) in the membrane are involved in protein trafficking, formation of signaling complexes, and regulation of actin cytoskeleton. Here, we show that lipid rafts exist abundantly in dendrites of cultured hippocampal neurons, in which they are associated with several postsynaptic proteins including surface AMPA receptors. Depletion of cholesterol/sphingolipid leads to instability of surface AMPA receptors and gradual loss of synapses (both inhibitory and excitatory) and dendritic spines. The remaining synapses and spines in raft-depleted neurons become greatly enlarged. The importance of lipid rafts for normal synapse density and morphology could explain why cholesterol promotes synapse maturation in retinal ganglion cells (Mauch et al., 2001) and offers a potential link between disordered cholesterol metabolism and the synapse loss seen in neurodegenerative disease.

Lipid rafts: bringing order to chaos

Lipid rafts are subdomains of the plasma membrane that contain high concentrations of cholesterol and glycosphingolipids. They exist as distinct liquid-ordered regions of the membrane that are resistant to extraction with nonionic detergents. Rafts appear to be small in size, but may constitute a relatively large fraction of the plasma membrane. While rafts have a distinctive protein and lipid composition, all rafts do not appear to be identical in terms of either the proteins or the lipids that they contain. A variety of proteins, especially those involved in cell signaling, have been shown to partition into lipid rafts. As a result, lipid rafts are thought to be involved in the regulation of signal transduction. Experimental evidence suggests that there are probably several different mechanisms through which rafts control cell signaling. For example, rafts may contain incomplete signaling pathways that are activated when a receptor or other required molecule is recruited into the raft. Rafts may also be important in limiting signaling, either by physical sequestration of signaling components to block nonspecific interactions, or by suppressing the intrinsic activity of signaling proteins present within rafts. This review provides an overview of the physical characteristics of lipid rafts and summarizes studies that have helped to elucidate the role of lipid rafts in signaling via receptor tyrosine kinases and G protein-coupled receptors.

Storage diseases: new insights into sphingolipid functions

Studying human diseases can help us to uncover important processes in normal cells. Cell biologists have recently focused on inherited sphingolipid-storage diseases. Eukaryotic life is characterized by internal membranes of various compositions, and sphingolipids are a small but important part of these membranes. Compositional differences between cellular membranes are maintained by sorting and sphingolipids are thought to organize this process by forming ordered domains of increased thickness in the bilayer. Here, we describe the impact of sphingolipid accumulation on the sorting of endocytic membranes and discuss the proposed basis for the pathology of these diseases at the cellular level.

Membranes, trafficking, and signaling during animal development

Molecular genetics has been key in allowing developmental biologists to uncover many of the molecules that participate in pattern formation. Cell biology is now beginning to help developmental biologists in their quest to understand how these molecules interact within cells to direct tissue behavior. This is particularly true in the areas of membrane trafficking and cell motility. Recent work has shown that various trafficking events such as secretion, endocytosis, segregation in membrane microdomains, intracellular transport, and targeting to lysosomes regulate various signaling pathways. It is likely that within the context of an embryo, these trafficking events are integrated such that secreted factors reliably orchestrate many developmental decisions.

Membrane lipid organization is critical for human neutrophil polarization

In response to chemoattractants neutrophils extend an actin-rich pseudopod, which imparts morphological polarity and is required for migration. Even when stimulated by an isotropic bath of chemoattractant, neutrophils exhibit persistent polarization and continued lamellipod formation at the front, suggesting that the cells establish an internal polarity. In this report, we show that perturbing lipid organization by depleting plasma membrane cholesterol levels reversibly inhibits cell polarization and migration. Among other receptor-mediated responses, beta(2) integrin up-regulation was unaffected, and initial calcium mobilization was only partially reduced by cholesterol depletion, indicating that this treatment did not abrogate initial receptor-mediated signal transduction. Interestingly, cholesterol depletion did not prevent initial activation of the GTPase Rac or an initial burst of actin polymerization, but rather it inhibited prolonged activation of Rac and sustained actin polymerization. Collectively, these findings support a model in which the plasma membrane is organized into domains that aid in amplifying the chemoattractant gradient and maintaining cell polarization.

Distinct caveolae-mediated endocytic pathways target the Golgi apparatus and the endoplasmic reticulum

Internalization of autocrine motility factor (AMF) into the endoplasmic reticulum is sensitive to the cholesterol-extracting reagent methyl-beta-cyclodextrin, inhibited by the dynamin-1 K44A mutant and negatively regulated by caveolin-1. Thus, AMF internalization requires a caveolae-mediated endocytic pathway. Similarly, we show here that endocytosis of cholera toxin (CTX) in NIH-3T3 fibroblasts is inhibited by adenoviral expression of the dynamin-1 K44A mutant but only partially by expression of the clathrin hub. Treatment with methyl-beta-cyclodextrin and overexpression of caveolin-1, but not the clathrin hub, selectively diminishes CTX endocytosis to the Golgi apparatus but not to endosomes. CTX is therefore targeted via a caveolin-1-regulated caveolae-mediated pathway to the Golgi. Disruption of Golgi-, caveosome- or endosome-mediated trafficking with brefeldin A, nocodazole or a 20 degrees C temperature block, respectively, inhibit CTX endocytosis to the Golgi but do not affect AMF delivery to the endoplasmic reticulum. Following an incubation of only five minutes in the presence of the clathrin hub, AMF and CTX are not cointernalized, and AMF is delivered to the AMF-R-positive smooth ER. The internalization of both ligands is nevertheless sensitive to the tyrosine kinase inhibitor genistein, confirming that they are both internalized via caveolae/raft pathways. Two distinct caveolae-mediated endocytic pathways therefore exist, including a novel direct pathway to the ER from the plasma membrane.

Differential regulation of CXCR4-mediated T-cell chemotaxis and mitogen-activated protein kinase activation by the membrane tyrosine phosphatase, CD45

The chemokine receptor CXCR4 and its cognate ligand, stromal cell-derived factor-1alpha (CXCL12), regulate lymphocyte trafficking and play an important role in host immune surveillance. However, the molecular mechanisms involved in CXCL12-induced and CXCR4-mediated chemotaxis of T-lymphocytes are not completely elucidated. In the present study, we examined the role of the membrane tyrosine phosphatase CD45, which regulates antigen receptor signaling in CXCR4-mediated chemotaxis and mitogen-activated protein kinase (MAPK) activation in T-cells. We observed a significant reduction in CXCL12-induced chemotaxis in the CD45-negative Jurkat cell line (J45.01) as compared with the CD45-positive control (JE6.1) cells. Expression of a chimeric protein containing the intracellular phosphatase domain of CD45 was able to partially restore CXCL12-induced chemotaxis in the J45.01 cells. However, reconstitution of CD45 into the J45.01 cells restored the CXCL12-induced chemotaxis to about 90%. CD45 had no significant effect on CXCL12 or human immunodeficiency virus gp120-induced internalization of the CXCR4 receptor. Furthermore, J45.01 cells showed a slight enhancement in CXCL12-induced MAP kinase activity as compared with the JE6.1 cells. We also observed that CXCL12 treatment enhanced the tyrosine phosphorylation of CD45 and induced its association with the CXCR4 receptor. Pretreatment of T-cells with the lipid raft inhibitor, methyl-beta-cyclodextrin, blocked the association between CXCR4 and CD45 and markedly abolished CXCL12-induced chemotaxis. Comparisons of signaling pathways induced by CXCL12 in JE6.1 and J45.01 cells revealed that CD45 might moderately regulate the tyrosine phosphorylation of the focal adhesion components the related adhesion focal tyrosine kinase/Pyk2, focal adhesion kinase, p130Cas, and paxillin. CD45 has also been shown to regulate CXCR4-mediated activation and phosphorylation of T-cell receptor downstream effectors Lck, ZAP-70, and SLP-76. Our results show that CD45 differentially regulates CXCR4-mediated chemotactic activity and MAPK activation by modulating the activities of focal adhesion components and the downstream effectors of the T-cell receptor.

Sterol-rich plasma membrane domains in the fission yeast Schizosaccharomyces pombe

Sterol-rich membrane domains exist in unicellular and multicellular eukaryotes. They are thought to provide a structural framework for interactions among a subset of proteins by selectively incorporating some proteins while excluding others. Although most studies have focused on the biophysical and biochemical properties of sterol-rich membrane domains and incorporated proteins, relatively little is known about their intracellular distribution. Using a cytological approach we show here that in the fission yeast Schizosaccharomyces pombe, sterols are enriched in the plasma membrane at the growing cell tips and at the site of cytokinesis. The distribution of sterols is regulated in a cell-cycle-dependent manner and requires a functional secretory pathway. By manipulating the integrity of sterol-rich membrane domains using sterol sequestering agents and genetic means, we show that these domains are important for multiple processes regulating cytokinesis. In these cells, defects in proper maintenance of the actomyosin ring and/or its attachment to the overlying plasma membrane were observed. Furthermore, the stability of a plasma membrane protein that colocalises with sterol-rich membrane domains was compromised. Taken together, our studies establish S. pombe as a genetically tractable model organism in which to study the role(s) of sterol-rich membrane domains in cell polarity and cytokinesis.

The roles of membrane microdomains (rafts) in T cell activation

Detergent-resistant membrane microdomains enriched in sphingolipids, cholesterol and glycosylphosphatidylinositol-anchored proteins play essential roles in T cell receptor (TCR) signaling. These 'membrane rafts' accumulate several cytoplasmic lipid-modified molecules, including Src-family kinases, coreceptors CD4 and CD8 and transmembrane adapters LAT and PAG/Cbp, essential for either initiation or amplification of the signaling process, while most other abundant transmembrane proteins are excluded from these structures. TCRs in various T cell subpopulations may differ in their use of membrane rafts. Membrane rafts also seem to be involved in many other aspects of T cell biology, such as functioning of cytokine and chemokine receptors, adhesion molecules, antigen presentation, establishing cell polarity or interaction with important pathogens. Although the concept of membrane rafts explains several diverse biological phenomena, many basic issues, such as composition, size and heterogeneity, under native conditions, as well as the dynamics of their interactions with TCRs and other immunoreceptors, remain unclear, partially because of technical problems.

Phosphorylation of FcgammaRIIA is required for the receptor-induced actin rearrangement and capping: the role of membrane rafts

Activation of Fcgamma receptor II (FcgammaRII) induces rearrangement of the actin-based cytoskeleton that serves as a driving force for FcgammaRII-mediated phagocytosis and FcgammaRII capping. To get insight into the signaling events that lead to the actin reorganization we investigated the role of raft-associated Src family tyrosine kinases in capping of FcgammaRII in U937 cells. After crosslinking, FcgammaRII was found to be recruited to detergent-resistant membrane domains (DRMs), rafts, where it coexisted with Lyn kinase and underwent tyrosine phosphorylation. Lyn was displaced from DRMs under the influence of DL-alpha-hydroxymyristic acid and 2-bromopalmitic acid, agents blocking N-terminal myristoylation and palmitoylation of proteins, respectively, and after disruption of DRM integrity by depletion of plasma membrane cholesterol with beta-cyclodextrin. Under these conditions, phosphorylation of the crosslinked FcgammaRII was diminished and assembly of FcgammaRII caps was blocked. The similar reduction of FcgammaRII cap formation correlated with inhibition of receptor phosphorylation was achieved with the use of PP1 and herbimycin A, specific inhibitors of Src family tyrosine kinases. Phosphorylation of FcgammaRIIA expressed in BHK cells, lacking endogenous FcgammaRs, was abolished by substitution of tyrosine 298 by phenylalanine in the ITAM of the receptor. The mutant receptor did not undergo translocation towards cap-like structures and failed to promote the receptor-mediated spreading of the cells, as compared to BHK cells transfected with the wild-type FcgammaRIIA. On the basis of these data, we suggest that tyrosine phosphorylation of activated FcgammaRIIA by raft-residing tyrosine kinases of the Src family triggers signaling pathways that control the rearrangement of the actin cytoskeleton required for FcgammaRII-mediated motility.

Actin cytoskeletal dynamics in T lymphocyte activation and migration

Dynamic rearrangements of the actin cytoskeleton are crucial for the function of numerous cellular elements including T lymphocytes. They are required for migration of T lymphocytes through the body to scan for the presence of antigens, as well as for the formation and stabilization of the immunological synapse at the interface between antigen-presenting cells and T lymphocytes. Supramolecular activation clusters within the immunological synapse play an important role for the initiation of T cell responses and for the execution of T cell effector functions. In addition to the T cell receptor/CD3 induced actin nucleation via Wasp/Arp2/3-activation, signals through accessory receptors of the T cell (i.e., costimulation) regulate actin cytoskeletal dynamics. In this regard, the actin-binding proteins cofilin and L-plastin represent prominent candidates linking accessory receptor stimulation to the rearrangement of the actin cytoskeleton. Cofilin enhances actin polymerization via its actin-severing activity, and as a long-lasting effect, cofilin generates novel actin monomers through F-actin depolymerization. L-plastin stabilizes actin filament structures by means of its actin-bundling activity.

Sphingolipid partitioning into ordered domains in cholesterol-free and cholesterol-containing lipid bilayers

We have used fluorescence-quenching measurements to characterize the partitioning of a variety of indolyl-labeled phospho- and sphingolipids between gel or liquid-ordered and liquid-disordered lipid domains in several types of lipid bilayers where such domains coexist. In both cholesterol-free and cholesterol-containing lipid mixtures, sphingolipids with diverse polar headgroups (ranging from sphingomyelin and monoglycosylceramides to ganglioside GM1) show a net preference for partitioning into ordered domains, which varies modestly in magnitude with varying headgroup structure. The affinities of different sphingolipids for ordered lipid domains do not vary in a consistent manner with the size or other simple structural properties of the polar headgroup, such that for example ganglioside GM1 partitions between ordered and disordered lipid domains in a manner very similar to sphingomyelin. Ceramide exhibits a dramatically higher affinity for ordered lipid domains in both cholesterol-free and cholesterol-containing bilayers than do other sphingolipids. Our findings suggest that sphingolipids with a variety of headgroup structures will be enriched by substantial factors in liquid-ordered versus liquid-disordered regions of membranes, in a manner that is only modestly dependent on the nature of the polar headgroup. Ceramide is predicted to show a very strong enrichment in such domains, supporting previous suggestions that ceramide-mediated signaling may be compartmentalized to liquid-ordered (raft and raft-related) domains in the plasma membrane.