Membrane raft microdomains mediate front-rear polarity in migrating cells

Role of cholesterol in human immunodeficiency virus type 1 envelope protein-mediated fusion with host cells

In this study we examined the effects of target membrane cholesterol depletion and cytoskeletal changes on human immunodeficiency virus type 1 (HIV-1) Env-mediated membrane fusion by dye redistribution assays. We found that treatment of peripheral blood lymphocytes (PBL) with methyl-beta-cyclodextrin (MbetaCD) or cytochalasin reduced their susceptibility to membrane fusion with cells expressing HIV-1 Env that utilize CXCR4 or CCR5. However, treatment of human osteosarcoma (HOS) cells expressing high levels of CD4 and coreceptors with these agents did not affect their susceptibility to HIV-1 Env-mediated membrane fusion. Removal of cholesterol inhibited stromal cell-derived factor-1alpha- and macrophage inflammatory protein 1beta-induced chemotaxis of both PBL and HOS cells expressing CD4 and coreceptors. The fusion activity as well as the chemotactic activity of PBL was recovered by adding back cholesterol to these cells. Confocal laser scanning microscopy analysis indicated that treatment of lymphocytes with MbetaCD reduced the colocalization of CD4 or of CXCR4 with actin presumably in microvilli. These findings indicate that, although cholesterol is not required for HIV-1 Env-mediated membrane fusion per se, its depletion from cells with relatively low coreceptor densities reduces the capacity of HIV-1 Env to engage coreceptor clusters required to trigger fusion. Furthermore, our results suggest that coreceptor clustering may occur in microvilli that are supported by actin polymerization.

Cell surface polarization during yeast mating

Exposure to mating pheromone in haploid Saccharomyces cerevisiae cells results in the arrest of the cell cycle, expression of mating-specific genes, and polarized growth toward the mating partner. Proteins involved in signaling, polarization, cell adhesion, and fusion are localized to the tip of the mating cell (shmoo) where fusion will eventually occur. The mechanisms ensuring the correct targeting and retention of these proteins are poorly understood. Here we show that in pheromone-treated cells, a reorganization of the plasma membrane involving lipid rafts results in the retention of proteins at the tip of the mating projection, segregated from the rest of the membrane. Sphingolipid and ergosterol biosynthetic mutants fail to polarize proteins to the tip of the shmoo and are deficient in mating. Our results show that membrane microdomain clustering at the mating projection is involved in the generation and maintenance of polarity during mating.

CD14 signalling in lipid rafts: new ligands and co-receptors

PURPOSE OF REVIEW

Lipid rafts on monocytes/macrophages provide a dynamic microenvironment for an integrated lipopolysaccharide receptor (CD14)-dependent clustering of a set of receptors involved in innate immunity and clearance of atherogenic lipoproteins. The purpose of this review is to summarize the recent advances in our understanding of CD14-dependent receptor clustering and its relevance in atherogenesis.

RECENT FINDINGS

Upon binding of various ligands, CD14 as a multiligand pattern recognition receptor induces specific coassembly of additional receptors present on circulating monocytes.

SUMMARY

The composition of the receptor cluster and thus the associated signalling pathways defines a ligand specific cellular response, linking endogenous and exogenous host defense to a common recognition platform in rafts.

Lipid rafts in protein sorting and cell polarity in budding yeast Saccharomyces cerevisiae

Cellular membranes contain many types and species of lipids. One of the most important functional consequences of this heterogeneity is the existence of microdomains within the plane of the membrane. Sphingolipid acyl chains have the ability of forming tightly packed platforms together with sterols. These platforms or lipid rafts constitute segregation and sorting devices into which proteins specifically associate. In budding yeast, Saccharomyces cerevisiae, lipid rafts serve as sorting platforms for proteins destined to the cell surface. The segregation capacity of rafts also provides the basis for the polarization of proteins at the cell surface during mating. Here we discuss some recent findings that stress the role of lipid rafts as key players in yeast protein sorting and cell polarity.

Properties of the Na+/H+ exchanger protein. Detergent-resistant aggregation and membrane microdistribution

The Na+/H+ exchanger is a ubiquitous membrane protein of bacteria, plants and mammals. The first isoform discovered (NHE1) is present on the mammalian plasma membrane and transports one H+ out of cells in exchange for one extracellular Na+. With solubilization in standard SDS/PAGE buffer, this protein had a high tendency to aggregate when subjected to elevated temperature. The aggregates were stable and did not dissociate in high concentrations of SDS or 2-mercaptoethanol. We examined the distribution of the Na+/H+ exchanger within membrane subfractions. The Na+/H+ exchanger was found both in caveolin-containing fractions and, in lesser amounts, in higher density membrane fractions where the bulk of proteins were contained. Treatment with cytochalasin D caused only a minor reduction of the amount of Na+/H+ exchanger present in caveolin-enriched fractions suggesting an intact cytoskeleton was not important for NHE1 localization to these microdomains. Treatment of cells with methyl beta-cyclodextrin had a small stimulatory effect on Na+/H+ exchanger activity and reduced the amount of Na+/H+ exchanger in low density membrane fractions. Our study demonstrates that SDS cannot maintain the protein in a monomeric state suggesting that strong hydrophobic interactions are responsible for this temperature dependent aggregation behavior. In addition a large proportion of the Na+/H+ exchanger protein is found to be enriched in low density caveolin-containing fractions.

Myristoylation as a target for inhibiting HIV assembly: unsaturated fatty acids block viral budding

Modification of HIV-1 Gag with myristic acid, a saturated 14-carbon fatty acid (14:0), is essential for HIV-1 assembly. We recently showed that exogenous treatment of cells with unsaturated 14-carbon fatty acids, 5-cis-tetradecenoic acid (14:1n-9) and 5-cis,8-cis-tetradecadienoic acid (14:2n-6), reduces the affinity of some myristoylated proteins for plasma membrane rafts, membrane subdomains that have been shown to be required for efficient assembly of HIV. Here we demonstrate that treatment of cells with 14:1n-9 and 14:2n-6 fatty acids reduced the affinity of Gag for rafts but not membranes in general. Furthermore, treatment of cells with 14-carbon unsaturated fatty acids inhibited Gag-driven particle assembly. These effects most likely reflect covalent modification of Gag with unsaturated fatty acids. Treatment with 14:1n-9 and 14:2n-6 fatty acids did not alter intracellular protein trafficking, nor did it reduce cell viability. These studies suggest a strategy to attack HIV assembly by selectively altering the patterns of fatty acid modification.

Upregulation of surface feline CXCR4 expression following ectopic expression of CCR5: implications for studies of the cell tropism of feline immunodeficiency virus

Feline CXCR4 and CCR5 were expressed in feline cells as fusion proteins with enhanced green fluorescent protein (EGFP). Expression of the EGFP fusion proteins was localized to the cell membrane, and surface expression of CXCR4 was confirmed by using a cross-species-reactive anti-CXCR4 monoclonal antibody. Ectopic expression of feline CCR5 enhanced expression of either endogenous feline CXCR4 or exogenous feline or human CXCR4 expressed from a retrovirus vector, indicating that experiments investigating the effect of CCR5 expression on feline immunodeficiency virus (FIV) infection must be interpreted with caution. Susceptibility to infection with cell culture-adapted strains of FIV or to syncytium formation following transfection with a eukaryotic vector expressing an env gene from a cell culture-adapted strain of virus correlated with expression of either human or feline CXCR4, whereas feline CCR5 had no effect. In contrast, neither CXCR4 nor CCR5 rendered cells permissive to either productive infection with primary strains of FIV or syncytium formation following transfection with primary env gene expression vectors. Screening a panel of Ghost cell lines expressing diverse human chemokine receptors confirmed that CXCR4 alone supported fusion mediated by the FIV Env from cell culture-adapted viruses. CXCR4 expression was upregulated in Ghost cells coexpressing CXCR4 and CCR5 or CXCR4, CCR5, and CCR3, and susceptibility to FIV infection could be correlated with the level of CXCR4 expression. The data suggest that beta-chemokine receptors may influence FIV infection by modulating the expression of CXCR4.

Blocking of HIV-1 infection by targeting CD4 to nonraft membrane domains

Human immunodeficiency virus (HIV)-1 infection depends on multiple lateral interactions between the viral envelope and host cell receptors. Previous studies have suggested that these interactions are possible because HIV-1 receptors CD4, CXCR4, and CCR5 partition in cholesterol-enriched membrane raft domains. We generated CD4 partitioning mutants by substituting or deleting CD4 transmembrane and cytoplasmic domains and the CD4 ectodomain was unaltered. We report that all CD4 mutants that retain raft partitioning mediate HIV-1 entry and CD4-induced Lck activation independently of their transmembrane and cytoplasmic domains. Conversely, CD4 ectodomain targeting to a nonraft membrane fraction results in a CD4 receptor with severely diminished capacity to mediate Lck activation or HIV-1 entry, although this mutant binds gp120 as well as CD4wt. In addition, the nonraft CD4 mutant inhibits HIV-1 X4 and R5 entry in a CD4(+) cell line. These results not only indicate that HIV-1 exploits host membrane raft domains as cell entry sites, but also suggest new strategies for preventing HIV-1 infection.

Sphingolipid metabolism and caveolin expression in gonadotropin-releasing hormone-expressing GN11 and gonadotropin-releasing hormone-secreting GT1-7 neuronal cells

In this paper, we show that caveolin-1 is abundantly present in a cell line of immortalized gonadotropin-releasing hormone-expressing neurons (GN11). In contrast to GN11, caveolin is undetectable in a cognate cell line of immortalized gonadotropin-releasing hormone-secreting neurons (GT1-7). These two cell lines are characterized by a radically different sphingolipid metabolism. After incubation in the presence of tracer amount of [1-(3)H]sphingosine, GN11 and GT1-7 neurons incorporated similar amounts of radioactivity. In GT1-7 neurons, [1-(3)H]sphingosine metabolism was markedly oriented toward the biosynthesis of complex sphingolipids. In fact, almost all the radioactivity in the lipid extracts from GT1-7 cells was associated with biosynthetic products (ceramide, sphingomyelin, and glycosphingolipids). In particular glycosphingolipids represented more than 65% of total lipid radioactivity in these cells, and the main glycosphingolipid was GM3 ganglioside (about 47% of total lipid radioactivity). In the case of GN11 neurons, a high portion of [1-(3)H]sphingosine underwent complete degradation, as indicated by the formation of high levels of radioactive phosphatidylethanolamine (about 23% of lipid radioactivity). Moreover, the main complex sphingolipid in GN11 neurons was not a glycolipid, but sphingomyelin (its level in these cells, about 54% of lipid radioactivity, was two-fold higher than in GT1-7). Glycolipids, gangliosides in particular, were present in low amount (9.5% of lipid radioactivity) if compared with the cognate GT1-7 cell line, and GM3 was almost absent in GN11 neurons. Despite the radical differences in ganglioside and caveolin content, from both cell types a membrane fraction similarly enriched in sphingolipids was prepared. In the case of GN11 cells, this fraction was also enriched in caveolin. The presence of caveolin or GM3 may correlate with different functional properties linked to the stage of neuronal maturation, since GN11 and GT1-7 are representative, respectively, of immature, migrating, and differentiated, postmigratory gonadotropin-releasing hormone-positive neurons.

Influence of non-volatile anesthetics on the migration behavior of the human breast cancer cell line MDA-MB-468

BACKGROUND

Anesthetic agents are known to influence functions of the immune system. Anesthetic drugs also support cancer progression by suppressing the activity of immune cells. In breast carcinoma an increase in expression of peripheral-type benzodiazepine receptors (PBR) and the gamma aminobutyl acid (GABA) level has been discovered. Therefore, an investigation of a direct influence of GABA-A agonist propofol, GABA-A and PBR-agonist etomidate, and the local anesthetic drug lidocaine, which can also bind to the GABA-A receptor and PBR, on migration of breast carcinoma cells was performed.

METHODS

MDA-MB-468 cells were incubated with anesthetic agents using clinically relevant concentrations (propofol 3, 6, 9 mg/l, etomidate 2, 3, 4 mg/l, and lidocaine 1.25, 2.5, 5 mg/l). Locomotion was investigated in a three-dimensional collagen matrix using time-lapse video microscopy and computer-assisted cell-tracking.

RESULTS

The percentage of migrating cells (57.4+/-1.9) as well as the velocity (0.22+/-0.09 microm/min) and distance migrated (89.4+/-66.8 microm/10 h) increased in the presence of propofol in a dose-dependent manner (up to 74.4+/-7.5, 0.30+/-0.09, 143.8+/-89.1, respectively) compared with the long chain triglyceride (LCT) control. In contrast, no influence of etomidate on the number of migrating cells could be observed. The velocity and distance migrated at 3 and 4 mg/l were found to be statistically significantly enhanced. Treatment with lidocaine caused an increase in the percentage of migrating cells (up to 75.0+/-5.6) in velocity dose dependently (up to 0.33+/-0.06) and in distance migrated (up to 151.5+/-92.9).

CONCLUSION

These results show that different anesthetic drugs are able to modulate the migratory machinery of human breast carcinoma cells in vitro.

Cholesterol is essential for macrophage inflammatory protein 1 beta binding and conformational integrity of CC chemokine receptor 5

The chemokine receptor, CCR5, is used as a human immunodeficiency virus coreceptor in combination with CD4 during transmission and early infection. CCR5 has been shown to be palmitoylated and targeted to cholesterol- and sphingolipid-rich membrane microdomains termed "lipid rafts." However, the role of cholesterol and lipid rafts on chemokine binding and signaling through CCR5 remains unknown. We found that cholesterol extraction by hydroxypropyl-beta-cyclodextrin (BCD) significantly reduced the binding and signaling of macrophage inflammatory protein 1 beta (MIP-1 beta) using CCR5-expressing CEM-NKR T cells. Reloading treated cells with cholesterol but not 4-cholesten-3-one, an oxidized form of cholesterol, restored MIP-1 beta binding to BCD-treated cells. Antibodies specific for distinct CCR5 epitopes lost their ability to bind to the cell surface after cholesterol extraction to varying degrees. Moreover, cells stained with fluorescently labeled MIP-1 beta extensively colocalized with the GM1 lipid raft marker while using anti-CCR5 antibodies; most of CCR5 on these cells only partially colocalized with GM1, suggesting that active ligand binding facilitates receptor association with lipid rafts or that raft association promotes a higher affinity conformation of CCR5. Together, these data demonstrate that cholesterol and lipid rafts are important for the maintenance of the CCR5 conformation and are necessary for both the binding and function of this chemokine receptor.

Membrane lipids, EGF receptors, and intracellular signals colocalize and are polarized in epithelial cells moving directionally in a physiological electric field

Directed cell migration is essential for tissue formation, inflammation, and wound healing. Chemotaxis plays a major role in these situations and is underpinned by asymmetric intracellular signaling. Endogenous electric fields (EFs) are common where cell movement occurs, such as in wound healing, and cells respond to electric field gradients by reorienting and migrating directionally (galvanotaxis/electrotaxis). We show that a physiological EF redistributed both EGF (epidermal growth factor) receptors and detergent-insoluble membrane lipids asymmetrically, leading to cathodal polarization and enhanced activation of the MAP kinase, ERK1/2. This induced leading-edge actin polymerization in directionally migrating mammalian epithelial cells. Inhibiting the EGF receptor-MAP kinase signaling pathway significantly decreased leading edge actin asymmetry and directional migration. We propose a model in which EF-polarized membrane lipid domains and EGF receptors cause asymmetric signaling through MAP kinase, which drives directional cell migration. A comparison is made with the mechanisms underpinning chemotaxis.

Molecular determinants of the sensory and motor neuron-derived factor insertion into plasma membrane

The sensory and motor neuron-derived factor (SMDF) is a type III neuregulin that regulates development and proliferation of Schwann cells. Although SMDF has been shown to be a type II protein, the molecular determinants of membrane biogenesis, insertion, and topology remain elusive. Here we used heterologous expression of a yellow fluorescent protein-SMDF fusion protein along with a stepwise deletion strategy to show that the apolar/uncharged segment (Ile(76)-Val(100)) acts as an internal, uncleaved membrane insertion signal that defines the topology of the protein. Unexpectedly, removal of the transmembrane segment (TM) did not eliminate completely membrane association of C-terminal fragments. TM-deleted fusion proteins, bearing the amino acid segment (Ser(283)-Glu(296)) located downstream to the epidermal growth factor-like motif, strongly interacted with plasma membrane fractions. However, synthetic peptides patterned after this segment did not insert into artificial lipid vesicles, suggesting that membrane interaction of the SMDF C terminus may be the result of a post-translational modification. Subcellular localization studies demonstrated that the 40-kDa form, but not the 83-kDa form, of SMDF was segregated into lipid rafts. Deletion of the N-terminal TM did not affect the interaction of the protein with these lipid microdomains. In contrast, association with membrane rafts was abolished completely by truncation of the protein C terminus. Collectively, these findings are consistent with a topological model for SMDF in which the protein associates with the plasma membrane through both the TM and the C-terminal end domains resembling the topology of other type III neuregulins. The TM defines its characteristic type II membrane topology, whereas the C terminus is a newly recognized anchoring motif that determines its compartmentalization into lipid rafts. The differential localization of the 40- and 83-kDa forms of the neuregulin into rafts and non-raft domains implies a central role in the protein biological activity.

Human immunodeficiency virus type 1 uses lipid raft-colocalized CD4 and chemokine receptors for productive entry into CD4(+) T cells

In this report, we describe a crucial role of lipid raft-colocalized receptors in the entry of human immunodeficiency virus type 1 (HIV-1) into CD4(+) T cells. We show that biochemically isolated detergent-resistant fractions have characteristics of lipid rafts. Lipid raft integrity was required for productive HIV-1 entry as determined by (i) semiquantitative PCR analysis and (ii) single-cycle infectivity assay using HIV-1 expressing the luciferase reporter gene and pseudotyped with HIV-1 HXB2 envelope or vesicular stomatitis virus envelope glycoprotein (VSV-G). Depletion of plasma membrane cholesterol with methyl-beta-cyclodextrin (MbetaCD) relocalized raft-resident markers to a nonraft environment but did not significantly change the surface expression of HIV-1 receptors. MbetaCD treatment inhibited productive infection of HIV-1 by 95% as determined by luciferase activity in cells infected with HXB2 envelope-pseudotyped virus. In contrast, infection with VSV-G-pseudotyped virus, which enters the cells through an endocytic pathway, was not suppressed. Biochemical fractionation and confocal imaging of HIV-1 receptor distribution in live cells demonstrated that CD4, CCR5, and CXCR4 colocalized with raft-resident markers, ganglioside GM1, and glycosylphosphatidylinositol-anchored CD48. While confocal microscopy analysis revealed that HIV-1 receptors localized most likely to the same lipid microdomains, sucrose gradient analysis of the receptor localization showed that, in contrast to CD4 and CCR5, CXCR4 was associated preferentially with the nonraft membrane fraction. The binding of HIV-1 envelope gp120 to lipid rafts in the presence, but not in the absence, of cholesterol strongly supports our hypothesis that raft-colocalized receptors are directly involved in virus entry. Dramatic changes in lipid raft and HIV-1 receptor redistribution were observed upon binding of HIV-1 NL4-3 to PM1 T cells. Colocalization of CCR5 with GM1 and gp120 upon engagement of CD4 and CXCR4 by HIV-1 further supports our observation that HIV-1 receptors localize to the same lipid rafts in PM1 T cells.

Specific SHP-2 partitioning in raft domains triggers integrin-mediated signaling via Rho activation

Cell signaling does not occur randomly over the cell surface, but is integrated within cholesterol-enriched membrane domains, termed rafts. By targeting SHP-2 to raft domains or to a non-raft plasma membrane fraction, we studied the functional role of rafts in signaling. Serum-depleted, nonattached cells expressing the raft SHP-2 form, but not non-raft SHP-2, display signaling events resembling those observed after fibronectin attachment, such as beta1 integrin clustering, 397Y-FAK phosphorylation, and ERK activation, and also increases Rho-GTP levels. Expression of the dominant negative N19Rho abrogates raft-SHP-2-induced signaling, suggesting that Rho activation is a downstream event in SHP-2 signaling. Expression of a catalytic inactive SHP-2 mutant abrogates the adhesion-induced feedback inhibition of Rho activity, suggesting that SHP-2 contributes to adhesion-induced suppression of Rho activity. Because raft recruitment of SHP-2 occurs physiologically after cell attachment, these results provide a mechanism by which SHP-2 may influence cell adhesion and migration by spatially regulating Rho activity.

Cholesterol-rich plasma membrane domains (lipid rafts) in keratinocytes: importance in the baseline and UVA-induced generation of reactive oxygen species

The biologic effects of ultraviolet radiation such as DNA damage, mutagenesis, cellular aging, and carcinogenesis are in part mediated by reactive oxygen species. In unirradiated cells the major known sources of reactive oxygen species are the mitochondrial respiratory chain and the membrane oxidases functionally coupled to several membrane growth factor receptors. There is evidence that mitochondria also play a role in oxidative stress after ultraviolet irradiation; however, it is unknown whether the biochemical processes at the level of the plasma membrane contribute to the regulation of reactive oxygen species synthesis. In order to elucidate this issue we examined here the importance of the microdomain plasma membrane organization in the regulation of oxidative stress in unirradiated and ultraviolet A (340-400 nm) irradiated HaCaT keratinocytes. Labeling of confluent HaCaT cultures with fluorescently tagged cholera toxin B subunit (FITC-CTx) revealed the presence of GM1 ganglioside and cholesterol-rich microdomains (lipid rafts) that formed junction-like structures in the membranes of adjacent cells and patchy microdomains elsewhere. There was a marked heterogeneity in the level of FITC-CTx labeling: there were groups of cells demonstrating prominent labeling (FITC-CTx(high)) whereas other cells were only weakly labeled (FITC-CTx(low)). When reactive oxygen species synthesis was measured with the fluorescent probe carboxy-2',7'-dichlorodihydrofluorescein diacetate, we found that (i) the baseline and ultraviolet-A-induced reactive oxygen species synthesis correlated with the magnitude of FITC-CTx labeling and was highest in the FITC-CTx(high) cells; (ii) reactive oxygen species synthesis was diminished in cells in which the integrity of membrane domains was disrupted by cholesterol sequestration with methyl-beta-cyclodextrin and filipin, or after treatment with GM1 ganglioside; (iii) reactive oxygen species synthesis in cholesterol-depleted cells was fully restored after cholesterol repletion. We conclude that the plasma membrane takes part in the regulation of oxidative stress in keratinocytes and disruption of its microdomain structure reduces reactive oxygen species synthesis both at the baseline and after ultraviolet A irradiation. We hypothesize that lipid-raft-associated protein(s) may be involved in the generation of reactive oxygen species and that pharmacologic modulation of membrane structure may provide a novel therapeutic approach relevant for photoprotection and cutaneous carcinogenesis.

Segregation of Bad from lipid rafts is implicated in the induction of apoptosis

Many molecules relocate subcellularly in cells undergoing apoptosis. Using coimmunoprecipitation experiments we demonstrate that Bad is not associated to 14-3-3 protein, suggesting a new mechanism for the control of the proapoptotic role of Bad. Here we show, by confocal microscopy and cellular fractionation, that Bad is attached to lipid rafts in IL-4-stimulated cells and thymocytes while associated with mitochondria in IL-4-deprived cells. Disruption of lipid rafts by methyl-beta-cyclodextrin treatment induces segregation of Bad from rafts, which correlates with apoptosis. Our results suggest that the interaction of Bad with rafts is a dynamic process regulated by IL-4 and involved in the control of apoptosis.

Potentiation of Fas-mediated apoptosis by an engineered glycosylphosphatidylinositol-linked Fas

FasL and TRAIL are apoptotic ligands of the TNF-like cytokines family, acting via activation of the transmembrane death domain containing receptors Fas for FasL, and DR4 or DR5 for TRAIL. A glycosylphosphatidylinositol-linked TRAIL receptor called DcR1 behaves as a decoy receptor inhibiting TRAIL-mediated cell death in several cellular systems. We engineered and stably expressed a chimeric GPI-linked Fas receptor (Fas-GPI) in T-lymphocyte cell lines constitutively expressing functional transmembrane Fas. Surprisingly, despite lacking the death domain region of functional Fas, Fas-GPI was able to significantly increase Fas-mediated cell death triggered by membrane bound or soluble FasL, whereas engagement of Fas-GPI alone did not trigger apoptosis. This potentiating effect, but not transmembrane Fas activation, was selectively inhibited by protein kinase C activation with phorbol esters, demonstrating that Fas-GPI activated a specific synergistic signal transduction pathway. Fas-GPI and transmembrane Fas were localized in distinct membrane compartments, since Fas-GPI, but not transmembrane Fas, was found in the glycolipid-rich membrane microdomains. These results suggest that apoptosis induced by members of this ligand/receptors family may be differentially modulated through other and parallel signalling pathways.

Purification of pseudopodia from polarized cells reveals redistribution and activation of Rac through assembly of a CAS/Crk scaffold

Initiation of cell migration requires morphological polarization with formation of a dominant leading pseudopodium and rear compartment. A molecular understanding of this process has been limited, due to the inability to biochemically separate the leading pseudopodium from the rear of the cell. Here we examine the spatio-temporal localization and activation of cytoskeletal-associated signals in purified pseudopodia directed to undergo growth or retraction. Pseudopodia growth requires assembly of a p130Crk-associated substrate (CAS)/c-CrkII (Crk) scaffold, which facilitates translocation and activation of Rac1. Interestingly, Rac1 activation then serves as a positive-feedback loop to maintain CAS/Crk coupling and pseudopodia extension. Conversely, disassembly of this molecular scaffold is critical for export and down regulation of Rac1 activity and induction of pseudopodia retraction. Surprisingly, the uncoupling of Crk from CAS during pseudopodium retraction is independent of changes in focal adhesion kinase activity and CAS tyrosine phosphorylation. These findings establish CAS/Crk as an essential scaffold for Rac1-mediated pseudopodia growth and retraction, and illustrate spatio-temporal segregation of cytoskeletal signals during cell polarization.

Sites of Ca2+ wave initiation move with caveolae to the trailing edge of migrating cells

The caveola is a membrane domain that compartmentalizes signal transduction at the cell surface. Normally in endothelial cells, groups of caveolae are found clustered along stress fibers or at the lateral margins in all regions of the cell. Subsets of these clusters appear to contain the signaling machinery for initiating Ca2+ wave formation. Here we report that induction of cell migration, either by wounding a cell monolayer or by exposing cells to laminar shear stress, causes caveolae to move to the trailing edge of the cell. Concomitant with the relocation of the caveolae,sites of Ca2+ wave initiation move to the same location. In as much as the relocated caveolae contain elements of the signaling machinery required for ATP-stimulated release of Ca2+ from the ER, these results suggest that caveolae function as containers that carry this machinery to different cellular locations.

Multiple active states and oligomerization of CCR5 revealed by functional properties of monoclonal antibodies

CC-chemokine receptor 5 (CCR5) is the principal coreceptor for macrophage-tropic strains of human immunodeficiency virus type 1 (HIV-1). We have generated a set of anti-CCR5 monoclonal antibodies and characterized them in terms of epitope recognition, competition with chemokine binding, receptor activation and trafficking, and coreceptor activity. MC-4, MC-5, and MC-7 mapped to the amino-terminal domain, MC-1 to the second extracellular loop, and MC-6 to a conformational epitope covering multiple extracellular domains. MC-1 and MC-6 inhibited regulated on activation normal T cell expressed and secreted (RANTES), macrophage inflammatory polypeptide-1beta, and Env binding, whereas MC-5 inhibited macrophage inflammatory polypeptide-1beta and Env but not RANTES binding. MC-6 induced signaling in different functional assays, suggesting that this monoclonal antibody stabilizes an active conformation of CCR5. Flow cytometry and real-time confocal microscopy showed that MC-1 promoted strong CCR5 endocytosis. MC-1 but not its monovalent isoforms induced an increase in the transfer of energy between CCR5 molecules. Also, its monovalent isoforms bound efficiently, but did not internalize the receptor. In contrast, MC-4 did not prevent RANTES binding or subsequent signaling, but inhibited its ability to promote CCR5 internalization. These results suggest the existence of multiple active conformations of CCR5 and indicate that CCR5 oligomers are involved in an internalization process that is distinct from that induced by the receptor's agonists.

Lipid rafts mediate biosynthetic transport to the T lymphocyte uropod subdomain and are necessary for uropod integrity and function

Polarized migrating T cells possess 2 poles, the uropod protrusion at the rear and the leading edge at the front, with specific protein composition and function. The influenza virus hemagglutinin (HA) is a prototypical molecule that uses lipid rafts for biosynthetic transport to the apical surface in polarized epithelial Madin-Darby canine kidney (MDCK) cells. In this study, HA was used as a tool to investigate the role of lipid rafts in vectorial protein traffic in polarized T lymphocytes. Results show that newly synthesized HA becomes selectively targeted to the uropod subdomain in polarized T lymphoblasts. HA incorporates into rafts soon after biosynthesis, suggesting that delivery of HA to the uropod occurs through a pathway of transport reminiscent of that used for its specific targeting to the apical surface. HA and the adhesion molecules, intercellular adhesion molecule 3 (ICAM-3), CD44, and CD43, 3 endogenous uropod markers, were detected in surface rafts of T lymphoblasts. Cholesterol, a major component of lipid rafts, was predominantly located in the uropod. Disruption of lipid raft integrity by cholesterol sequestration produced unclustering of ICAM-3 and the loss of uropodia and severely impaired processes that require a polarized phenotype such as intercellular aggregation and cell migration. Collectively, these results indicate that lipid rafts constitute a route for selective targeting of proteins to the uropod and that the rafts are essential for the generation, maintenance, and functionality of T-cell anteroposterior polarity.

Segregation of CD4 and CXCR4 into distinct lipid microdomains in T lymphocytes suggests a mechanism for membrane destabilization by human immunodeficiency virus

Recent evidence has suggested that plasma membrane sphingolipids and cholesterol spontaneously coalesce into raft-like microdomains and that specific proteins, including CD4 and some other T-cell signaling molecules, sequester into these rafts. In agreement with these results, we found that CD4 and the associated Lck tyrosine kinase of peripheral blood mononuclear cells and H9 leukemic T cells were selectively and highly enriched in a low-density lipid fraction that was resistant at 0 degrees C to the neutral detergent Triton X-100 but was disrupted by extraction of cholesterol with filipin or methyl-beta-cyclodextrin. In contrast, the CXCR4 chemokine receptor, a coreceptor for X4 strains of human immunodeficiency virus type 1 (HIV-1), was almost completely excluded from the detergent-resistant raft fraction. Accordingly, as determined by immunofluorescence with confocal microscopy, CD4 and CXCR4 did not coaggregate into antibody-induced cell surface patches or into patches of CXCR4 that formed naturally at the ruffled edges of adherent cells. The CXCR4 fluorescent patches were extracted with cold 1% Triton X-100, whereas the CD4 patches were resistant. In stringent support of these data, CD4 colocalized with patches of cholera toxin bound to the raft-associated sphingoglycolipid GM1, whereas CXCR4 did not. Addition of the CXCR4-activating chemokine SDF-1 alpha did not induce CXCR4 movement into rafts. Moreover, binding of purified monomeric gp120 envelope glycoproteins from strains of HIV-1 that use this coreceptor did not stimulate detectable redistributions of CD4 or CXCR4 between their separate membrane domains. However, adsorption of multivalent gp120-containing HIV-1 virion particles appeared to destabilize the local CD4-containing rafts. Indeed, adsorbed HIV-1 virions were detected by immunofluorescence microscopy and were almost all situated in nonraft regions of the cell surface. We conclude that HIV-1 initially binds to CD4 in a raft domain and that its secondary associations with CXCR4 require shifts of proteins and associated lipids away from their preferred lipid microenvironments. Our evidence suggests that these changes in protein-lipid interactions destabilize the plasma membrane microenvironment underlying the virus by at least several kilocalories per mole, and we propose that this makes an important contribution to fusion of the viral and cellular membranes during infection. Thus, binding of HIV-1 may be favored by the presence of CD4 in rafts, but the rafts may then disperse prior to the membrane fusion reaction.

Vaginal transmission of cell-associated HIV-1 in the mouse is blocked by a topical, membrane-modifying agent

Because both HIV-1 virions and HIV-infected cells are present in the semen and cervical mucus of infected individuals, HIV-1 prevention strategies must consider both cell-free and cell-associated virus. Antibodies that target HIV-1 virions have been shown to prevent vaginal transmission of cell-free virus in macaques, but since cell-associated transmission has not been reliably demonstrated in this model system, no strategies to prevent such transmission have been tested. We have employed a mouse model in which SCID mice carry human peripheral blood leukocytes (HuPBLs). In these mice, vaginal transmission of cell-associated, but not cell-free, HIV-1 transmission occurs, mediated by transepithelial migration of HIV-infected cells. Topical application of beta-cyclodextrin (beta-CD), a cholesterol-sequestering agent that interferes with cell migration and budding of virus from lipid rafts, blocks transmission of cell-associated HIV-1. The HuPBL-SCID model of vaginal HIV-1 transmission should prove useful for investigating cell-associated HIV-1 transmucosal HIV-1 transmission, as well as for screening reagents for their potential efficacy in preventing sexual HIV-1 transmission.

Open reading frame III of borna disease virus encodes a nonglycosylated matrix protein

The open reading frame III of Borna disease virus (BDV) codes for a protein with a mass of 16 kDa, named p16 or BDV-M. p16 was described as an N-glycosylated protein in several previous publications and therefore was termed gp18, although the amino acid sequence of p16 does not contain any regular consensus sequence for N glycosylation. We examined glycosylation of p16 and studied its membrane topology using antisera raised against peptides, which comprise the N and the C termini. Neither an N- nor a C-terminal peptide is cleaved from p16 during maturation. Neither deglycosylation of p16 by endoglycosidases nor binding of lectin to p16 was detectable. Introduction of typical N-glycosylation sites at the proposed sites of p16 failed in carbohydrate attachment. Flotation experiments with membranes of BDV-infected cells on density gradients revealed that p16 is not an integral membrane protein, since it can be dissociated from membranes. Our experimental data strongly suggest that p16 is a typical nonglycosylated matrix protein associated at the inner surface of the viral membrane, as is true for homologous proteins of other members of the Mononegavirales order.

Direct interaction of insulin-like growth factor-1 receptor with leukemia-associated RhoGEF

Insulin-like growth factor (IGF)-1 plays crucial roles in growth control and rearrangements of the cytoskeleton. IGF-1 binds to the IGF-1 receptor and thereby induces the autophosphorylation of this receptor at its tyrosine residues. The phosphorylation of the IGF-1 receptor is thought to initiate a cascade of events. Although various signaling molecules have been identified, they appear to interact with the tyrosine-phosphorylated IGF-1 receptor. Here, we identified leukemia-associated Rho guanine nucleotide exchange factor (GEF) (LARG), which contains the PSD-95/Dlg/ZO-1 (PDZ), regulator of G protein signaling (RGS), Dbl homology, and pleckstrin homology domains, as a nonphosphorylated IGF-1 receptor-interacting molecule. LARG formed a complex with the IGF-1 receptor in vivo, and the PDZ domain of LARG interacted directly with the COOH-terminal domain of IGF-1 receptor in vitro. LARG had an exchange activity for Rho in vitro and induced the formation of stress fibers in NIH 3T3 fibroblasts. When MDCKII epithelial cells were treated with IGF-1, Rho and its effector Rho-associated kinase (Rho-kinase) were activated and actin stress fibers were enhanced. Furthermore, the IGF-1-induced Rho-kinase activation and the enhancement of stress fibers were inhibited by ectopic expression of the PDZ and RGS domains of LARG. Taken together, these results indicate that IGF-1 activates the Rho/Rho-kinase pathway via a LARG/IGF-1 receptor complex and thereby regulates cytoskeletal rearrangements.

Cytoskeleton-dependent membrane domain segregation during neutrophil polarization

On treatment with chemoattractant, the neutrophil plasma membrane becomes organized into detergent-resistant membrane domains (DRMs), the distribution of which is intimately correlated with cell polarization. Plasma membrane at the front of polarized cells is susceptible to extraction by cold Triton X-100, whereas membrane at the rear is resistant to extraction. After cold Triton X-100 extraction, DRM components, including the transmembrane proteins CD44 and CD43, the GPI-linked CD16, and the lipid analog, DiIC(16), are retained within uropods and cell bodies. Furthermore, CD44 and CD43 interact concomitantly with DRMs and with the F-actin cytoskeleton, suggesting a mechanism for the formation and stabilization of DRMs. By tracking the distribution of DRMs during polarization, we demonstrate that DRMs progress from a uniform distribution in unstimulated cells to small, discrete patches immediately after activation. Within 1 min, DRMs form a large cap comprising the cell body and uropod. This process is dependent on myosin in that an inhibitor of myosin light chain kinase can arrest DRM reorganization and cell polarization. Colabeling DRMs and F-actin revealed a correlation between DRM distribution and F-actin remodeling, suggesting that plasma membrane organization may orient signaling events that control cytoskeletal rearrangements and, consequently, cell polarity.

Endocytosis without clathrin coats

Endocytosis is involved in an enormous variety of cellular processes. To date, most studies on endocytosis in mammalian cells have focused on pathways that start with uptake through clathrin-coated pits. Recently, new techniques and reagents have allowed a wider range of endocytic pathways to begin to be characterized. Various non-clathrin endocytic mechanisms have been identified, including uptake through caveolae, macropinosomes and via a separate constitutive pathway. Many markers for clathrin-independent endocytosis are found in detergent-resistant membrane fractions, or lipid rafts. We will discuss these emerging new findings and their implications for the nature of lipid rafts themselves, as well as for the potential roles of non-clathrin endocytic pathways in remodeling of the plasma membrane and in regulating the membrane composition of specific intracellular organelles.

Evidence for cell surface association between CXCR4 and ganglioside GM3 after gp120 binding in SupT1 lymphoblastoid cells

CXCR4 (fusin) is a chemokine receptor which is involved as a coreceptor in gp120 binding to the cell surface. In this study we provide evidence that binding of gp120 triggers CXCR4 recruitment to glycosphingolipid-enriched microdomains. Scanning confocal microscopy showed a nearly complete localization of CXCR4 within GM3-enriched plasma membrane domains of SupT1 cells and coimmunoprecipitation experiments revealed that CXCR4 was immunoprecipitated by IgG anti-GM3 after gp120 pretreatment. These findings reveal that gp120 binding induces a strict association between CXCR4 and ganglioside GM3, supporting the view that GM3 and CXCR4 are components of a functional multimolecular complex critical for HIV-1 entry.

Characterization of sequence determinants within the carboxyl-terminal domain of chemokine receptor CCR5 that regulate signaling and receptor internalization

The CC chemokine receptor CCR5 mediates chemotaxis of leukocytes and serves as a principal co-receptor for macrophage-tropic human immunodeficiency virus type 1. To identify determinants on the CCR5 carboxyl-terminal domain that regulate receptor signaling and internalization, we generated several CCR5 mutants, which were progressively shortened from the COOH terminus or had carboxyl-terminal serine, cysteine, or leucine residues substituted by alanine and expressed them in RBL-2H3 cells. Using fluorescence resonance energy transfer between beta-arrestin and CCR5 tagged with cyan and yellow variants of green fluorescent protein, we show that high affinity association of the two molecules in living cells requires intact carboxyl-terminal serine phosphorylation sites. Phosphorylation-deficient truncation or Ser/Ala replacement mutants of CCR5 mediated a sustained calcium response and enhanced granular enzyme release in RANTES-stimulated cells. Carboxyl-terminal serine residues are critically involved in CCR5 endocytosis and a dileucine motif, similar to that implicated in the regulation of CXCR2 and CXCR4, contributes to the internalization of CCR5 in a phosphorylation-independent manner. Despite their prominent role in receptor desensitization and internalization, beta-arrestins are dispensable for the CCR5-mediated stimulation of mitogen-activated protein kinase pathways in RBL-2H3 cells. We also show that CCR5 is palmitoylated on carboxyl-terminal cysteine residues. Inhibition of CCR5 palmitoylation by alanine mutagenesis of cysteines or treatment with a palmitate analogue inhibitor profoundly reduces phorbol 12-myristate 13-acetate- and RANTES-induced receptor phosphorylation, homologous desensitization, and internalization. Alanine mutagenesis of serine, cysteine, or leucine residues or the limited carboxyl-terminal truncation of CCR5 did not impair chemokine-stimulated migration of RBL-2H3 cells. Together these results indicate that post-translational modifications of carboxyl-terminal serine and cysteine residues have a significant impact on receptor deactivation and internalization.

Activation of mitogen-activated protein kinase by membrane-targeted Raf chimeras is independent of raft localization

Binding of proteins to the plasma membrane can be achieved with various membrane targeting motifs, including combinations of fatty acids, isoprenoids, and basic domains. In this study, we investigate whether attachment of different membrane targeting motifs influences the signaling capacity of membrane-bound signal transduction proteins by directing the proteins to different membrane microdomains. We used c-Raf-1 as a model for a signaling protein that is activated when membrane-bound. Three different membrane targeting motifs from K-Ras, Fyn, and Src proteins were fused to the N or C terminus of Raf-1. The ability of the modified Rafs to initiate MAPK signaling was then investigated. All three modified Raf-1 constructs activated MAPK to nearly equivalent levels. The extent of localization of the Raf-1 constructs to membrane microdomains known as rafts did not correlate with the level of MAPK activation. Moreover, treatment of cells with the raft disrupting drug methyl-beta-cyclodextrin (MbetaCD) caused activation of MAPK to levels equivalent to those achieved with membrane-targeted Raf constructs. The use of pharmacological agents as well as dominant negative mutants revealed that MAPK activation by MbetaCD proceeds via a phosphoinositide 3-kinase-dependent mechanism that is Ras/Raf-independent. We conclude that cholesterol depletion from the plasma membrane by MbetaCD constitutes an alternative pathway for activating MAPK.

Multimerization of human immunodeficiency virus type 1 Gag promotes its localization to barges, raft-like membrane microdomains

The Gag polyprotein of human immunodeficiency virus type 1 (HIV-1) organizes the assembly of nascent virions at the plasma membrane of infected cells. Here we demonstrate that a population of Gag is present in distinct raft-like membrane microdomains that we have termed "barges." Barges have a higher density than standard rafts, most likely due to the presence of oligomeric Gag-Gag assembly complexes. The regions of the Gag protein responsible for barge targeting were mapped by examining the flotation behavior of wild-type and mutant proteins on Optiprep density gradients. N-myristoylation of Gag was necessary for association with barges. Removal of the NC and p6 domains shifted much of the Gag from barges into typical raft fractions. These data are consistent with a model in which multimerization of myristoylated Gag proteins drives association of Gag oligomers into raft-like barges. The functional significance of barge association was revealed by several lines of evidence. First, Gag isolated from virus-like particles was almost entirely localized in barges. Moreover, a comparison of wild-type Gag with Fyn(10)Gag, a chimeric protein containing the N-terminal sequence of Fyn, revealed that Fyn(10)Gag exhibited increased affinity for barges and a two- to fourfold increase in particle production. These results imply that association of Gag with raft-like barge membrane microdomains plays an important role in the HIV-1 assembly process.

Heterogeneous fatty acylation of Src family kinases with polyunsaturated fatty acids regulates raft localization and signal transduction

Fatty acylation of Src family kinases is essential for localization of the modified proteins to the plasma membrane and to plasma membrane rafts. It has been suggested that the presence of saturated fatty acyl chains on proteins is conducive for their insertion into liquid ordered lipid domains present in rafts. The ability of unsaturated dietary fatty acids to be attached to Src family kinases has not been investigated. Here we demonstrate that heterogeneous fatty acylation of Src family kinases occurs and that the nature of the attached fatty acid influences raft-mediated signal transduction. By using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, we show that in addition to 14:0 (myristate), 14:1 and 14:2 fatty acids can be attached to the N-terminal glycine of the Src family kinase Fyn when the growth media are supplemented with these dietary fatty acids. Moreover, we synthesized novel iodinated analogs of oleate and stearate, and we showed that heterogeneous S-acylation can occur on cysteine residues within Fyn as well as Galpha, GAP43, and Ras. Modification of Fyn with unsaturated or polyunsaturated fatty acids reduced its raft localization and resulted in decreased T cell signal transduction. These studies establish that heterogeneous fatty acylation is a widespread occurrence that serves to regulate signal transduction by membrane-bound proteins.

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

Redistribution of specialized molecules in migrating cells develops asymmetry between two opposite cell poles, the leading edge and the uropod. We show that acquisition of a motile phenotype in T lymphocytes results in the asymmetric redistribution of ganglioside GM3- and GM1-enriched raft domains to the leading edge and to the uropod, respectively. This segregation to each cell pole parallels the specific redistribution of membrane proteins associated to each raft subfraction. Our data suggest that raft partitioning is a major determinant for protein redistribution in polarized T cells, as ectopic expression of raft-associated proteins results in their asymmetric redistribution, whereas non-raft-partitioned mutants of these proteins are distributed homogeneously in the polarized cell membrane. Both acquisition of a migratory phenotype and SDF-1alpha-induced chemotaxis are cholesterol depletion-sensitive. Finally, GM3 and GM1 raft redistribution requires an intact actin cytoskeleton, but is insensitive to microtubule disruption. We propose that membrane protein segregation not only between raft and nonraft domains but also between distinct raft subdomains may be an organizational principle that mediates redistribution of specialized molecules needed for T cell migration.

Chemokine signaling and functional responses: the role of receptor dimerization and TK pathway activation

A broad array of biological responses, including cell polarization, movement, immune and inflammatory responses, and prevention of HIV-1 infection, are triggered by the chemokines, a family of structurally related chemoattractant proteins that bind to specific seven-transmembrane receptors linked to G proteins. Here we discuss one of the early signaling pathways activated by chemokines, the JAK/STAT pathway. Through this pathway, and possibly in conjunction with other signaling pathways, the chemokines promote changes in cellular morphology, collectively known as polarization, required for chemotactic responses. The polarized cell expresses the chemokine receptors at the leading cell edge, to which they are conveyed by rafts, a cholesterol-enriched membrane fraction fundamental to the lateral organization of the plasma membrane. Finally, the mechanisms through which the chemokines promote their effect are discussed in the context of the prevention of HIV-1 infection.

'Reverse gear' cellular movement mediated by chemokines

We sought to model the mechanism by which leucocytes may be actively repulsed by a beta-chemokine signal. This model is used to interpret an apparent paradox in chemokine biology, whereby high levels of a T-cell chemoattractant, stromal cell derived factor-1 (SDF-1), are present in bone marrow and thymic tissues despite a paucity of mature T lymphocytes in these areas. We postulate the differential involvement in cell migration of the two binding sites on SDF-1 for its sole receptor, CXCR4, depending on whether high or low concentrations of SDF-1 are encountered by the cell. Site choice would be mediated by divergent affinities of the two binding interactions. We also propose differential signalling following SDF-1/CXCR4 interactions on the plasma membrane versus ligand/receptor complexes in endocytic vesicles. Preliminary data showing divergent susceptibility to kinase inhibitors depending on whether a cell is attracted to or repulsed by SDF-1, are consistent with this model. In terms of physical movement toward or away from a chemokine gradient, we compare the cycling of surface receptors during migration to the caterpillar drive of a tractor, which can change direction simply by altering the direction of rotation of its threads. Finally, the potential clinical implications of concentration-dependent, chemokine-based cell attraction and repulsion are discussed.

Lipid rafts and HIV pathogenesis: host membrane cholesterol is required for infection by HIV type 1

In a previous study we showed that budding of HIV-1 particles occurs at highly specialized membrane microdomains known as lipid rafts. These microdomains are characterized by a distinct lipid composition that includes high concentrations of cholesterol, sphingolipids, and glycolipids. Since cholesterol is known to play a key role in the entry of some other viruses, our observation of HIV budding from lipid rafts led us to investigate the role in HIV-1 entry of cholesterol and lipid rafts in the plasma membrane of susceptible cells. We have used 2-OH-propyl-beta-cyclodextrin (beta-cyclodextrin) to deplete cellular cholesterol and disperse lipid rafts. Our results show that removal of cellular cholesterol rendered primary cells and cell lines highly resistant to HIV-1-mediated syncytium formation and to infection by both CXCR4- and CCR5-specific viruses. beta-Cyclodextrin treatment of cells partially reduced HIV-1 binding, while rendering chemokine receptors highly sensitive to antibody-mediated internalization. There was no effect on CD4 expression. All of the above-described effects were readily reversed by incubating cholesterol-depleted cells with low concentrations of cholesterol-loaded beta-cyclodextrin to restore cholesterol levels. Cholesterol depletion made cells resistant to SDF-1-induced binding to ICAM-1 through LFA-1. Since LFA-1 contributes significantly to cell binding by HIV-1, this latter effect may have contributed to the observed reduction in HIV-1 binding to cells after treatment with beta-cyclodextrin. Our results indicate that cholesterol may be critical to the HIV-1 coreceptor function of chemokine receptors and is required for infection of cells by HIV-1.

Palmitoylation of CCR5 is critical for receptor trafficking and efficient activation of intracellular signaling pathways

CCR5 is a CC chemokine receptor expressed on memory lymphocytes, macrophages, and dendritic cells and also constitutes the main coreceptor for macrophage-tropic (or R5) strains of human immunodeficiency viruses. In the present study, we investigated whether CCR5 was palmitoylated in its carboxyl-terminal domain by generating alanine substitution mutants for the three cysteine residues present in this region, individually or in combination. We found that wild-type CCR5 was palmitoylated, but a mutant lacking all three Cys residues was not. Through the use of green fluorescent fusion proteins and immunofluorescence studies, we found that the absence of receptor palmitoylation resulted in sequestration of CCR5 in intracellular biosynthetic compartments. By using the fluorescence recovery after photobleaching technique, we showed that the non-palmitoylated mutant had impaired diffusion properties within the endoplasmic reticulum. We next studied the ability of the mutants to bind and signal in response to chemokines. Chemokines binding and activation of G(i)-mediated signaling pathways, such as calcium mobilization and inhibition of adenylate cyclase, were not affected. However, the duration of the functional response, as measured by a microphysiometer, and the ability to increase [(35)S]guanosine 5'-3-O-(thio)triphosphate binding to membranes were severely affected for the non-palmitoylated mutant. The ability of RANTES (regulated on activation normal T cell expressed and secreted) and aminooxypentane-RANTES to promote CCR5 endocytosis was not altered by cysteine replacements. Finally, we found that the absence of receptor palmitoylation reduced the human immunodeficiency viruses coreceptor function of CCR5, but this effect was secondary to the reduction in surface expression. In conclusion, we found that palmitoylated cysteines play an important role in the intracellular trafficking of CCR5 and are likely necessary for efficient coupling of the receptor to part of its repertoire of signaling cascades.

Human immunodeficiency virus type-1 and chemokines: beyond competition for common cellular receptors

The chemokines and their receptors have been receiving exceptional attention in recent years following the discoveries that some chemokines could specifically block human immunodeficiency virus type 1 (HIV-1) infection and that certain chemokine receptors were the long-sought coreceptors which, along with CD4, are required for the productive entry of HIV-1 and HIV-2 isolates. Several chemokine receptors or orphan chemokine receptor-like molecules can support the entry of various viral strains, but the clinical significance of the CXCR4 and CCR5 coreceptors appear to overshadow a critical role for any of the other coreceptors and all HIV-1 and HIV-2 strains best employ one or both of these coreceptors. Binding of the HIV-1 envelope glycoprotein gp120 subunit to CD4 and/or an appropriate chemokine receptor triggers conformational changes in the envelope glycoprotein oligomer that allow it to facilitate the fusion of the viral and host cell membranes. During these interactions, gp120 appears to be capable of inducing a variety of signaling events, all of which are still not defined in detail. In addition, the more recently observed dichotomous effects, of both inhibition and enhancement, that chemokines and their receptor signaling events elicit on the HIV-1 entry and replication processes has once again highlighted the intricate and complex balance of factors that govern the pathogenic process. Here, we will review and discuss these new observations summarizing the potential significance these processes may have in HIV-1 infection. Understanding the complexities and significance of the signaling processes that the chemokines and viral products induce may substantially enhance our understanding of HIV-1 pathogenesis, and perhaps facilitate the discovery of new ways for the prevention and treatment of HIV-1 disease.

Function and spatial distribution of ion channels and transporters in cell migration

Cell migration plays a central role in many physiological and pathophysiological processes, such as embryogenesis, immune defense, wound healing, or the formation of tumor metastases. Detailed models have been developed that describe cytoskeletal mechanisms of cell migration. However, evidence is emerging that ion channels and transporters also play an important role in cell migration. The purpose of this review is to examine the function and subcellular distribution of ion channels and transporters in cell migration. Topics covered will be a brief overview of cytoskeletal mechanisms of migration, the role of ion channels and transporters involved in cell migration, and ways by which a polarized distribution of ion channels and transporters can be achieved in migrating cells. Moreover, a model is proposed that combines ion transport with cytoskeletal mechanisms of migration.

Rantes activates Jak2 and Jak3 to regulate engagement of multiple signaling pathways in T cells

The chemokine RANTES (regulated on activation normal T cell expressed and secreted) and its cognate receptor CC chemokine receptor 5 (CCR5) have been implicated in regulating immune cell function. Previously we reported that in T cells, RANTES activation of CCR5 results in Stat1 and Stat3 phosphorylation-activation, leading to Stat1:1 and Stat1:3 dimers that exhibit DNA binding activity and the transcriptional induction of a Stat-inducible gene, c-fos. Given that RANTES and CCR5 have been implicated in T cell activation, we have studied RANTES-induced signaling events in a CCR5-expressing T cell line, PM1. RANTES treatment of PM1 T cells results in the rapid phosphorylation-activation of CCR5, Jak2, and Jak3. RANTES-inducible Jak phosphorylation is insensitive to pertussis toxin inhibition, indicating that RANTES-CCR5-mediated tyrosine phosphorylation events are not coupled directly to Galpha(i) protein-mediated events. In addition to Jaks, several other proteins are rapidly phosphorylated on tyrosine residues in a RANTES-dependent manner, including the Src kinase p56(lck), which associates with Jak3. Additionally our data confirm that the amino-terminally modified RANTES proteins, aminooxypentane-RANTES and Met-RANTES, are agonists for CCR5 and induce early tyrosine phosphorylation events that are indistinguishable from those inducible by RANTES with similar kinetics. Our data also demonstrate that RANTES activates the p38 mitogen-activated protein (MAP) kinase pathway. This is evidenced by the rapid RANTES-dependent phosphorylation and activation of p38 MAP kinase as well as the activation of the downstream effector of p38, MAP kinase-activated protein (MAPKAP) kinase-2. Pharmacological inhibition of RANTES-dependent p38 MAP kinase activation blocks MAPKAP kinase-2 activity. Thus, activation of Jak kinases and p38 MAP kinase by RANTES regulates the engagement of multiple signaling pathways.

Membrane raft microdomains in chemokine receptor function

Cell chemotaxis requires the acquisition and maintenance of both spatial and functional asymmetry between initially equivalent cell parts. In leukocytes one becomes the leading edge and the other, the rear edge or uropod. The acquisition of this cell polarity is controlled by an array of chemoattractants, including those of the chemokine family. We propose that chemokine receptor activation in highly organized lipid raft domains is a major determinant for the correct localization of the signaling pathways leading to the cell asymmetries required for migration. The lateral organization imposed by membrane raft microdomains is discussed in the context of other chemokine receptor activities, such as its role as a human immunodeficiency virus (HIV) coreceptor.

A role for chemokine receptor transactivation in growth factor signaling

Complex cell responses require the integration of signals delivered through different pathways. We show that insulin-like growth factor (IGF)-I induces specific transactivation of the Gi-coupled chemokine receptor CCR5, triggering its tyrosine phosphorylation and Galpha recruitment. This transactivation occurs via a mechanism involving transcriptional upregulation and secretion of RANTES, the natural CCR5 ligand. CCR5 transactivation is an essential downstream signal in IGF-I-induced cell chemotaxis, as abrogation of CCR5 function with a transdominant-negative KDELccr5A32 mutant abolishes IGF-I-induced migration. The relevance of this transactivation pathway was shown in vivo, as KDELccr5A32 overexpression prevents invasion by highly metastatic tumor cells; conversely, RANTES overexpression confers built-in invasive capacity on a non-invasive tumor cell line. Our results suggest that this extracellular growth factor-chemokine network represents a general mechanism connecting tumorigenesis and inflammation.

Segregation of gangliosides GM1 and GD3 on cell membranes, isolated membrane rafts, and defined supported lipid monolayers

Lateral assemblies of sphingolipids, glycosphingolipids and cholesterol, termed rafts, are postulated to be present in biological membranes and to function in important cellular phenomena. We probed whether rafts are heterogeneous by determining the relative distribution of two gangliosides, GM1 and GD3, in artificial supported monolayers, in intact rat primary cerebellar granule neurones, and in membrane rafts isolated from rat cerebellum. Fluorescence resonance energy transfer (FRET) using fluorophore-labelled cholera toxin B subunit (which binds GM1) and mAb R24 (which binds GD3) revealed that GM1 spontaneously self-associates but does not co-cluster with GD3 in supported monolayers and on intact neurones. Cholera toxin and immunocytochemical labelling of isolated membrane rafts from rat cerebellum further demonstrated that GM1 does not co-localise with GD3. Furthermore, whereas the membrane raft resident proteins Lyn and caveolin both co-localise with GD3 in isolated membrane rafts, GM1 appears in separate and distinct aggregates. These data support prior reports that membrane rafts are heterogeneous, although the mechanisms for establishing and maintaining such heterogeneity remain to be determined.

Stabilization of O-pyromellitylgramicidin channels in bilayer lipid membranes through electrostatic interaction with polylysines of different chain lengths

Functioning of membrane proteins, in particular ionic channels, can be modulated by alteration of their arrangement in membranes. We addressed this issue by studying the effect of different chain length polylysines on the kinetics of ionic channels formed in a bilayer lipid membrane (BLM) by O-pyromellitylgramicidin carrying three negative charges at the C-terminus. The method of sensitized photoinactivation was applied to the analysis of the channel association-dissociation kinetics (characterized by the exponential factor of the curve describing the time course of the flash-induced decrease in the transmembrane current, tau). Addition of polylysine to the bathing solutions of BLM led to the deceleration of the photoinactivation kinetics, i.e. to the increase in tau. It was shown here that for a series of polylysines differing in their chain lengths, the value of tau grew as their concentration increased above a threshold level until at a certain concentration of each polylysine tau reached maximum. At higher polylysine concentrations tau began to decrease and finally became close to the control level observed in the absence of polylysine. With lengthening of the polylysine chain the maximum value of tau increased, the concentration dependence became steeper, and the threshold concentration decreased. The increase in the ionic strength of the medium shifted the concentration dependence of tau to higher polylysine concentrations and decreased the maximum value of tau. It was concluded that the increase in tau was caused by the formation of domains of O-pyromellitylgramicidin molecules induced by binding of polylysines. This can be related to functional aspects of polycation-induced sequestering of negatively charged transmembrane peptides in neutral membranes.

Microbial pathogenesis: lipid rafts as pathogen portals

The route of initial entry influences how host cells respond to intracellular pathogens. Recent studies have demonstrated that a wide variety of pathogens target lipid microdomains in host cell membranes, known as lipid rafts, to enter host cells as an infectious strategy.

The integrins alpha3beta1 and alpha6beta1 physically and functionally associate with CD36 in human melanoma cells. Requirement for the extracellular domain OF CD36

Lateral association between different transmembrane glycoproteins can serve to modulate integrin function. Here we characterize a physical association between the integrins alpha(3)beta(1) and alpha(6)beta(1) and CD36 on the surface of melanoma cells and show that ectopic expression of CD36 by CD36-negative MV3 melanoma cells increases their haptotactic migration on extracellular matrix components. The association was demonstrated by co-immunoprecipitation, reimmunoprecipitation, and immunoblotting of surface-labeled cells lysed in Brij 96 detergent. Confocal microscopy illustrated the co-association of alpha(3) and CD36 in cell membrane projections and ruffles. A requirement for the extracellular domain of CD36 in this association was shown by co-immunoprecipitation experiments using surface-labeled MV3 melanoma or COS-7 cells that had been transiently transfected with chimeric constructs between CD36 and intercellular adhesion molecule 1 (ICAM-1) or with a truncation mutant of CD36. CD36 is known to engage in signal transduction and to localize to membrane microdomains or rafts in several cell types. Toward a mechanistic explanation for the functional effects of CD36 expression, we demonstrate that in fractionated Triton X-100 lysates of the MV3 cells stably transfected with CD36, CD36 was greatly enriched with the detergent-insoluble fractions that represent plasma membrane rafts. Significantly, when these fractionated lysates were reprobed for endogenous beta(1) integrin, it was found that a 4-fold increase in the proportion of the mature protein was contained within the detergent-insoluble fractions when extracted from the CD36-transfected cells compared with MV3 cells transfected with vector only. These results suggest that in melanoma cells CD36 expression may induce the sequestration of certain integrins into membrane microdomains and promote cell migration.

Presentation of chemokine SDF-1α by fibronectin mediates directed migration of T cells

The role of chemokine–matrix interactions in integrin-dependent T-cell migration was examined to address the critical question of how chemokines provide directional information. The chemokine SDF-1α binds fibronectin (Fn) with a low nanomolar Kd(equilibrium dissociation constant). SDF-1α presented by Fn induced directed migration. Spatial concentration gradients of chemokine were not required to maintain directed migration. Fn-presented chemokine induced the polarization of cells, including the redistribution of the SDF-1α receptor, to the basal surface and leading edge of the cell. A new model for directed migration is proposed in which the co-presentation of an adhesive matrix and chemokine provides the necessary positional information independent of a soluble spatial gradient.