Capping of cholera toxin-ganglioside GM1 complexes on mouse lymphocytes is accompanied by co-capping of alpha-actinin

Structural basis for acyl chain control over glycosphingolipid sorting and vesicular trafficking

The complex sphingolipids exhibit a diversity of ceramide acyl chain structures that influence their trafficking and intracellular distributions, but it remains unclear how the cell discerns among the different ceramides to affect such sorting. To address the mechanism, we synthesize a library of GM1 glycosphingolipids with naturally varied acyl chains and quantitatively assess their sorting among different endocytic pathways. We find that a stretch of at least 14 saturated carbons extending from C1 at the water-bilayer interface dictate lysosomal sorting by exclusion from endosome sorting tubules. Sorting to the lysosome by the C14^∗ motif is cholesterol dependent. Perturbations of the C14^∗ motif by unsaturation enable GM1 entry into endosomal sorting tubules of the recycling and retrograde pathways independent of cholesterol. Unsaturation occurring beyond the C14^∗ motif in very long acyl chains rescues lysosomal sorting. These results define a structural motif underlying the membrane organization of sphingolipids and implicate cholesterol-sphingolipid nanodomain formation in sorting mechanisms.

Sphingolipid Organization in the Plasma Membrane and the Mechanisms That Influence It

Sphingolipids are structural components in the plasma membranes of eukaryotic cells. Their metabolism produces bioactive signaling molecules that modulate fundamental cellular processes. The segregation of sphingolipids into distinct membrane domains is likely essential for cellular function. This review presents the early studies of sphingolipid distribution in the plasma membranes of mammalian cells that shaped the most popular current model of plasma membrane organization. The results of traditional imaging studies of sphingolipid distribution in stimulated and resting cells are described. These data are compared with recent results obtained with advanced imaging techniques, including super-resolution fluorescence detection and high-resolution secondary ion mass spectrometry (SIMS). Emphasis is placed on the new insight into the sphingolipid organization within the plasma membrane that has resulted from the direct imaging of stable isotope-labeled lipids in actual cell membranes with high-resolution SIMS. Super-resolution fluorescence techniques have recently revealed the biophysical behaviors of sphingolipids and the unhindered diffusion of cholesterol analogs in the membranes of living cells are ultimately in contrast to the prevailing hypothetical model of plasma membrane organization. High-resolution SIMS studies also conflicted with the prevailing hypothesis, showing sphingolipids are concentrated in micrometer-scale membrane domains, but cholesterol is evenly distributed within the plasma membrane. Reductions in cellular cholesterol decreased the number of sphingolipid domains in the plasma membrane, whereas disruption of the cytoskeleton eliminated them. In addition, hemagglutinin, a transmembrane protein that is thought to be a putative raft marker, did not cluster within sphingolipid-enriched regions in the plasma membrane. Thus, sphingolipid distribution in the plasma membrane is dependent on the cytoskeleton, but not on favorable interactions with cholesterol or hemagglutinin. The alternate views of plasma membrane organization suggested by these findings are discussed.

Dynamic Association between HIV-1 Gag and Membrane Domains

HIV-1 particle assembly is driven by the structural protein Gag. Gag binds to and multimerizes on the inner leaflet of the plasma membrane, eventually resulting in formation of spherical particles. During virus spread among T cells, Gag accumulates to the plasma membrane domain that, together with target cell membrane, forms a cell junction known as the virological synapse. While Gag association with plasma membrane microdomains has been implicated in virus assembly and cell-to-cell transmission, recent studies suggest that, rather than merely accumulating to pre-existing microdomains, Gag plays an active role in reorganizing the microdomains via its multimerization activity. In this paper, we will discuss this emerging view of Gag microdomain interactions. Relationships between Gag multimerization and microdomain association will be further discussed in the context of Gag localization to T-cell uropods and virological synapses.

EWI-2 association with α-actinin regulates T cell immune synapses and HIV viral infection

EWI motif-containing protein 2 (EWI-2) is a member of the Ig superfamily that links tetraspanin-enriched microdomains to the actin cytoskeleton. We found that EWI-2 colocalizes with CD3 and CD81 at the central supramolecular activation cluster of the T cell immune synapse. Silencing of the endogenous expression or overexpression of a cytoplasmic truncated mutant of EWI-2 in T cells increases IL-2 secretion upon Ag stimulation. Mass spectrometry experiments of pull-downs with the C-term intracellular domain of EWI-2 revealed the specific association of EWI-2 with the actin-binding protein α-actinin; this association was regulated by PIP2. α-Actinin regulates the immune synapse formation and is required for efficient T cell activation. We extended these observations to virological synapses induced by HIV and found that silencing of either EWI-2 or α-actinin-4 increased cell infectivity. Our data suggest that the EWI-2-α-actinin complex is involved in the regulation of the actin cytoskeleton at T cell immune and virological synapses, providing a link between membrane microdomains and the formation of polarized membrane structures involved in T cell recognition.

Alemtuzumab induces caspase-independent cell death in human chronic lymphocytic leukemia cells through a lipid raft-dependent mechanism

Alemtuzumab is a humanized IgG1 kappa antibody directed against CD52, a glycosyl-phosphatidylinositol linked cell-membrane protein of unknown function. Herein, we demonstrate that alemtuzumab promotes rapid death of chronic lymphocytic leukemia (CLL) cells in vitro, in a complement and accessory cell free system. Using minimal detergent solubilization of CLL membranes, we found that CD52 colocalizes with ganglioside GM-1, a marker of membrane rafts. Fluorescence microscopy revealed that upon crosslinking CD52 with alemtuzumab+anti-Fc IgG, large patches, and in many cases caps, enriched in CD52 and GM-1 formed upon the CLL cell plasma membrane. Depletion of membrane cholesterol or inhibition of actin polymerization significantly diminished the formation of alemtuzumab-induced caps and reduced alemtuzumab-mediated CLL cell death. We compared alemtuzumab-induced direct cytotoxicity, effector cell-mediated toxicity and complement-mediated cytotoxicity of CLL cells to normal T cells. The direct cytotoxicity and observed capping was significantly greater for CLL cells as compared to normal T cells. Cell-mediated and complement-mediated cytotoxicity did not significantly differ between the two cell types. In summary, our data support the hypothesis that alemtuzumab can initiate CLL cell death by crosslinking CD52-enriched lipid rafts. Furthermore, the differential direct cytotoxic effect suggests that CD52 directed antibodies could possibly be engineered to more specifically target CLL cells.

Detergent-resistant membranes in human erythrocytes and their connection to the membrane-skeleton

In cell membranes, local inhomogeneity in the lateral distribution of lipids and proteins is thought to exist in vivo in the form of lipid 'rafts', microdomains enriched in cholesterol and sphingolipids, and in specific classes of proteins, that appear to play specialized roles for signal transduction, cell-cell recognition, parasite or virus infection, and vesicular trafficking. These structures are operationally defined as membranes resistant to solubilization by nonionic detergents at 4 degree C (detergent-resistant membranes, DRMs). This definition appears to be necessary and sufficient, although additional manoeuvres, not always described with sufficient detail, may be needed to ensure isolation of DRMs, like mechanical homogenization, and changes in the pH and/or ionic strength of the solubilization medium. We show here for the human erythrocyte that the different conditions adopted may lead to the isolation of qualitatively and quantitatively different DRM fractions, thus contributing to the complexity of the notion itself of lipid raft. A significant portion of erythrocyte DRMs enriched in reported lipid raft markers, such as flotillin-1, flotillin-2 and GM1, is anchored to the spectrin membrane-skeleton via electrostatic interactions that can be disrupted by the simultaneous increase in pH and ionic strength of the solubilization medium.

Antigen binding to GM1 ganglioside results in delayed presentation: minimal effects of GM1 on presentation of antigens internalized via other pathways

Plasma membrane rafts are sphingolipid- and cholesterol-rich patches that function as membrane trafficking and surface signalling regions. Ganglioside GM1 is an integral component of these microdomains, and Escherichia coli enterotoxin B subunit (EtxB) is a pentamer that binds with high affinity to GM1 resulting in GM1 cross-linking. We previously demonstrated that antigen coupled directly to EtxB resulted in enhanced presentation relative to antigen taken up by fluid-phase endocytosis. Here we demonstrate a new role for GM1 in antigen presentation by examining the effects of cross-linking GM1 on the kinetics of presentation and processing of antigen by the B-cell receptor (BCR), fluid-phase endocytosis and GM1-targeted antigen. EtxB bound to B cells does not augment the subsequent kinetics or magnitude of presentation of either BCR-internalized antigen or soluble antigen. Moreover, presentation of GM1-bound antigen is significantly slower than antigen presentation following BCR-mediated uptake. In contrast to the rapid internalization of BCR-bound antigen (which has a half life of 60 min), the majority of EtxB-bound antigen forms a plasma membrane depot detectable for many hours after initial incubation (and with a half life of 12 hr). We conclude that cross-linking of GM1 by EtxB minimally affects the processing and presentation of antigens internalized via other pathways. Nevertheless, binding of antigens to GM1 results in delayed presentation that has important implications for in vivo immunization using GM1-targeted adjuvants.

Evidence for a role of ganglioside GM1 in antigen presentation: binding enhances presentation of Escherichia coli enterotoxin B subunit (EtxB) to CD4(+) T cells

Successful antigen presentation by antigen-presenting cells is governed by a number of factors including the efficiency of antigen capture by cell-surface receptors, targeting to compartments of antigen processing, surface expression of MHC II-peptide complexes and presence of co-stimulatory signals. Ganglioside GM1 is an important component of membrane glycosphingolipids, and has been implicated in cell differentiation, apoptosis and signal transduction pathways. Using the B subunit of Escherichia coli enterotoxin (EtxB), a potent immunogen that binds GM1 with high affinity, and a non-binding mutant of EtxB, EtxB(G33D), we demonstrate that GM1 is intimately involved in several aspects of antigen presentation. Thus, GM1-mediated presentation of EtxB by B cells and CD11c(+) dendritic cells (DC) significantly enhanced the proliferation and cytokine expression of EtxB-specific CD4(+) T cells. Investigation regarding potential mechanisms revealed that EtxB binding directly augments the expression of MHC class II on B cells, and fractionation of B cells demonstrated that EtxB binding to GM1 results in rapid internalization and targeting to class II-rich compartments. GM1-mediated uptake of antigens and access to class II compartments in B cells can be exploited to significantly enhance the presentation of ovalbumin-conjugated to EtxB. These results demonstrate that GM1 can play an important role in antigen presentation via the MHC II pathway.

Cross-linking of plasmalemmal cholesterol in lymphocytes induces capping, membrane shedding, and endocytosis through coated pits

By use of a nicked and biotinylated perfringolysin O (BCtheta), which binds to cholesterol specifically, we studied consequences of cross-linking cholesterol in lymphocytes. When bound with BCtheta and then with labeled avidin or streptavidin, capping occurred in most cells within 30 min at 37 degrees C. It was inhibited by cytochalasin D or NaN3, but not by nocodazole. When BCtheta-cholesterol was capped, Thy-1 and transferrin receptor, a GPI-anchored protein and a transmembrane protein, respectively, remained evenly distributed. By fluorescence and electron microscopy, a cluster of small vesicles bound with BCtheta were observed in the cap. They were then shed in the medium or internalized through coated pits. The result indicates that cross-linking of cholesterol in lymphocytes induces capping, but does not affect distribution of membrane proteins, and that the capped cholesterol molecules are either shed as vesicles or endocytosed.

T-cell activation and the dynamic world of rafts

Recent evidence, indicates that T-cell receptor (TCR) triggering and T-cell activation are dynamic processes that involve various aspects of T-cell organization. In addition to the interaction between the TCR molecule and its ligand, T-cell activation depends on a combination of many other events involving coreceptor molecules, actin cytoskeleton and plasma membrane lipids. Altogether, these cell structures organize the formation of a specialized junction between the T cell and the antigen-presenting cell (APC), that plays a critical role in sustaining and amplifying TCR signalling.

Clusters of glycolipid and glycosylphosphatidylinositol-anchored proteins in lymphoid cells: accumulation of actin regulated by local tyrosine phosphorylation

Lateral cross-linking of glycosylphosphatidylinositol (GPI)-anchored proteins and glycosphingolipids can trigger a signaling cascade which leads to activation of lymphoid cells. A possible explanation how the signal is transduced through the plasma membrane has arisen from the concept of raft sphingolipid-cholesterol microdomains in cell membranes. Cross-linking of GPI-anchored proteins, glycolipids and other raft components leads to the formation of stabilized membrane patches in the plasma membrane which enrich members of the Src-tyrosine kinase family. We have studied cellular responses to raft patch formation in the Jurkat T cell line and in particular changes in the actin cytoskeleton. We found that raft patches formed by GPI-anchored CD59 protein and the ganglioside GM1 accumulate filamentous actin. Most interestingly, we observed a strong accumulation of tyrosine-phosphorylated proteins in raft patches, strongly supporting the view that they can function as centers of signal transduction. Using a Lck kinase-deficient variant of Jurkat cells and a specific Lck and Fyn tyrosine kinase inhibitor we found that enrichment of actin in raft patches is dependent on phosphotyrosine accumulation in the patches. These observations show a link between raft-mediated signaling and the interaction of actin cytoskeleton with raft membrane domains.

Cholera toxin stimulates human B-cell DR antigen biosynthesis at the level of translation

Cholera toxin (CT) exerts many diverse regulatory effects on cells of the immune system and is considered a potent adjuvant on gut mucosal immune responses to orally presented antigens. It has been previously described that CT induces surface DR expression in human resting B-cells. As a further step toward understanding this phenomenon, the molecular mechanisms underlying the regulation of DR expression were investigated. By the use of Western analysis, it is shown that CT increases the total levels of DR protein in highly purified human tonsillar cells. [35S]Methionine incorporation studies show that the aforementioned increase is due to de novo biosynthesis of DR protein at as early as 6 hr after CT stimulation and is completed by 24 hr. [3H]Uridine uptake experiments, nuclear transcription runoff assays, and Northern analysis show that CT does not exert its effect at a transcriptional level, indicating translational regulation. Anti-IgM, which mimics B-cell antigen binding, behaves in a manner similar to CT. The B subunit of CT (B-CT) and prostaglandin E2, either alone or in combination, do not induce DR protein biosynthesis nor do they exert any effect on the transcription of DR beta 1 gene. These results show that cAMP elevation as well as binding of B-CT to GM-1 ganglioside, by themselves, do not lead to DR protein biosynthesis suggesting that other activation pathways may be involved.

The effects of cytoskeletal altering agents on the surface topography of GM1 in neuro-2A neuroblastoma cell membranes

Neuro-2a murine neuroblastomal cells exposed to exogenous ganglioside undergo increased neuritogenesis in vitro. To determine if the distribution of exogenous ganglioside (GM1) in neuronal membranes is related to neuritogenesis, the surface topography of exogenous ganglioside in these cells was examined by localization with cholera toxin B-FITC. Following exposure to exogenous ganglioside, levels of fluorescent label appeared similar on perikaryal and neuritic surfaces. Scanning electron microscopic studies using protein G-gold to label antibody against exogenous ganglioside confirmed these observations at higher magnification. Within the general labelling pattern, occasionally labelled material was observed which seemed to form short linear arrays. This suggested that elements of the cytoskeleton might be influencing the surface distribution of exogenous ganglioside. To examine this possibility, Neuro-2a cells were exposed to agents known to alter the stability of specific cytoskeletal components, after which the general distribution of exogenous ganglioside was determined. Treatment with Colcemid, which disrupted microtubules, resulted in restriction of most exogenous ganglioside-positive label to the perikaryal surfaces. In contrast, exposure to taxol which enhanced microtubule stability diminished perikaryal fluorescence and increased neuritic labelling. The disruption of cytochalasin D-sensitive microfilaments did not influence the topographic distribution of exogenous ganglioside. Under the experimental conditions employed, mean neuritic lengths for Colcemid- and taxol-treated cells were nearly equal, indicating that altered neuritic length resulting from treatment with cytoskeletal agents was not a major factor in the redistribution of exogenous ganglioside. These studies suggest that microtubules play a role in determining the distribution of recently incorporated ganglioside in neuronal plasma membranes.

Structure and function of cholera toxin and the related Escherichia coli heat-labile enterotoxin

Cholera and the related Escherichia coli-associated diarrheal disease are important problems confronting Third World nations and any area where water supplies can become contaminated. The disease is extremely debilitating and may be fatal in the absence of treatment. Symptoms are caused by the action of cholera toxin, secreted by the bacterium Vibrio cholerae, or by a closely related heat-labile enterotoxin, produced by Escherichia coli, that causes a milder, more common traveler's diarrhea. Both toxins bind receptors in intestinal epithelial cells and insert an enzymatic subunit that modifies a G protein associated with the adenylate cyclase complex. The consequent stimulated production of cyclic AMP, or other factors such as increased synthesis of prostaglandins by intoxicated cells, initiates a metabolic cascade that results in the excessive secretion of fluid and electrolytes characteristic of the disease. The toxins have a very high degree of structural and functional homology and may be evolutionarily related. Several effective new vaccine formulations have been developed and tested, and a growing family of endogenous cofactors is being discovered in eukaryotic cells. The recent elucidation of the three-dimensional structure of the heat-labile enterotoxin has provided an opportunity to examine and compare the correlations between structure and function of the two toxins. This information may improve our understanding of the disease process itself, as well as illuminate the role of the toxin in studies of signal transduction and G-protein function.

Interaction of gangliosides with the methyltransferase-I of microsomes from normal and neoplastic human mammary gland

The activity of the phospholipid methyltransferase-I, which catalyzes the rapid transmethylation of phosphatidylethanolamine to phosphatidylcholine, was found to be about 6-fold enhanced in microsomal membranes of breast cancer with respect to the level found in normal human mammary gland. Exogenous gangliosides GM1 and GM2 added to neoplastic breast microsomes induced progressive inhibition of the methyltransferase activity. In contrast, in microsomal membranes of non-neoplastic breast tissue treated with these gangliosides, the methyltransferase activity was markedly increased. The addition of cholesterol to these microsomes led to complete inhibition of the GM1-stimulated enzyme activity. The methyltransferase activity was not affected by GM3 alone in either type of tissue. Experiments carried out on non-neoplastic microsomes revealed that the phospholipid methyltransferase-I was affected by that portion of gangliosides which remained stable associated to microsomal membranes.

The T-lymphocyte is the primary cellular target for potentiation of the in vitro T-dependent IgM antibody response by the B subunit of cholera toxin

The B (or binding) subunit of cholera toxin (CTB) was reported previously to potentiate the in vitro T-dependent IgM antibody response by a mechanism independent of the cyclic AMP-generating capacity of the intact toxin. In the present report, experiments were designed to determine the immune cell type mediating potentiation by CTB. Firstly, CTB did not potentiate T-independent antibody responses at concentrations that effectively enhanced T-dependent responses. Secondly, separation/reconstitution studies with splenocytes from CTB- and vehicle-treated mice demonstrated potentiation of T-dependent responses by CTB treatment of either the Sephadex G10 non-adherent population or the T-lymphocyte + macrophage population of cells. Potentiation was not observed by CTB treatment of the plastic adherent population or the B-lymphocyte + macrophage population. The evidence indicates that the T-lymphocyte is the primary cellular target for CTB-induced effects on the T-dependent IgM antibody response. Monosialoganglioside GM1, the putative binding site for CTB, is most likely the site of action for CTB on T-lymphocytes. These studies provide new insight on the mechanism of immunomodulation by cholera toxin, and CTB should provide a useful tool for further understanding the role of gangliosides in cellular immune responses.

Stimulation of transglutaminase activity by GM1-ganglioside and alpha-sialylcholesterol in superior cervical and nodose ganglia excised from adult rat

Changes in transglutaminase (TG) activity in superior cervical ganglia (SCG) and nodose ganglia (NG) excised from adult rats were examined following application of selected membrane transport-altering agents, including GM1-ganglioside (GM1) and alpha-sialylcholesterol (alpha-SC). Although TG activity of freshly dissected SCG and NG was relatively low, it increased gradually during 30 min of incubation, and it stayed at this elevated level for 2 h. Addition of alpha-SC at its maximal effective concentration, 20 microM, stimulated TG activity more than eightfold in SCG and more than twofold in NG by 30 min. Addition of GM1 at its most effective concentration, 5 nM, had similar effects, but of lesser magnitude. Cycloheximide, a potent inhibitor of protein biosynthesis, did not affect the GM1- or alpha-SC-evoked increases in ganglionic TG activity, suggesting that enzyme activation rather than synthesis of new enzyme was occurring. The stimulation of TG activity in both ganglia caused by either GM1 or alpha-SC was associated with a decrease in Km and an increase in Vmax values. Addition of cholera toxin B, which specifically masks the oligosaccharide chain of GM1, reduced the GM1-induced increase in TG activity by approximately 60% in SCG and 88% in NG. The alpha-SC-induced increase in TG activity was only partially mimicked by free cholesterol. Although application of either dibutyryl cyclic AMP or dibutyryl cyclic GMP produced little change in TG activity of either ganglion, phorbol ester clearly inhibited the enzymic activity. Because TG is a calcium-dependent enzyme, we measured 45Ca2+ influx into either ganglion, and found that it was reduced by GM1 and alpha-SC in SCG and by alpha-SC in NG.(ABSTRACT TRUNCATED AT 250 WORDS)

Preliminary investigation of the mechanism of inhibition of bovine lymphocyte proliferation by Pasteurella haemolytica A1 leukotoxin

Pasteurella haemolytica A1 leukotoxic culture supernate has been shown to inhibit bovine lymphocyte blastogenesis induced by concanavalin A (Con A), pokeweed mitogen (PWM) and purified protein derivative (PPD). The various mechanisms by which this inhibition could be overcome were investigated in an effort to determine at which stage of cell activation the leukotoxin exerted its inhibitory effect. For both Con A and PWM stimulated cultures, the addition of partially purified bovine interleukin 1 reduced the leukotoxin-induced inhibition. Recombinant interleukin 2 had a similar effect. Addition of the glycolipid, monosialoganglioside was also able partially to overcome the inhibition.

Cell/substratum adhesions in RSV-transformed rat fibroblasts

Cell/substratum adhesions have been studied in rat fibroblasts transformed by a ts-mutant of Rous sarcoma virus (LA-29) using light and electron microscopy and a variety of preparative methods including immunolabeling. Cells were studied both during the process of transformation, i.e., shifting from 39 degrees to 35 degrees C, and in a fully transformed state (passaged at 35 degrees C continuously). The typical focal contacts observed at 39 degrees C (restrictive temperature) were replaced by "point-contacts" (100-200 per cell) which were classified by immunolabeling as podosome-like adhesions containing actin, beta 1 integrin subunit, vinculin, talin, alpha-actinin, and small membrane patches containing clathrin and integrin. Tyrosine-phosphorylated proteins and pp60src were found in association with groups of small particles on the protoplasmic surface of ventral membranes by gold immunolabeling. Both types of point-contacts were visualized by electron microscopy of ultrathin sections and shadowed replicas and characterized by gold immunolabeling wherever possible. The overall composition of podosome-like adhesions is similar to focal contacts but there are differences in the three-dimensional organization of the microfilaments and in the topography of vinculin which is associated more with actin filaments than with the plasma membrane. The presence of talin and extracellular matrix receptor in podosomes together with the adhesive properties of these actin-containing structures argues against the hypothesis that pp60src affects the interaction of actin with the plasma membrane by phosphorylating the fibronectin receptor and/or other associated proteins.

The B subunit of cholera toxin enhances DNA synthesis in rat hepatocytes induced by insulin and epidermal growth factor

The B subunit of cholera toxin, which binds to ganglioside GM1, enhanced DNA synthesis in rat hepatocytes in primary culture induced by insulin and/or epidermal growth factor. The effect was dose-dependent, and whole cholera toxin, activating adenylate cyclase, showed a higher effect than the B subunit alone. The B subunit acted additively with other agents that also increase cyclic AMP levels. A competitive antagonist of cyclic AMP could not suppress the effect of the B subunit completely. These data suggest that the effect is independent of the cyclic AMP signal pathway, and that GM1 plays a role in hepatocyte proliferation.

Effects of GM1-ganglioside and alpha-sialyl cholesterol on amino acid uptake, protein synthesis, and Na+,K(+)-ATPase activity in superior cervical and nodose ganglia excised from adult rats

We examined the effect of GM1-ganglioside in combination with cholera toxin B, and synthetic alpha-sialyl cholesterol (alpha-SC) on neutral amino acid (tritiated alpha-aminoisobutyric acid, [3H]AIB) uptake, protein synthesis [( 3H]leucine incorporation), and Na+,K(+)-ATPase activity in isolated superior cervical ganglia (SCG) and nodose ganglia (NG) from adult rats after aerobic incubation, usually for 2 h at 37 degrees C in vitro. Cholera toxin B, that specifically masks the oligosaccharide chain of GM1-ganglioside, antagonized the GM1-induced changes in [3H]AIB uptake, [3H]leucine incorporation, and Na+,K(+)-ATPase activity almost completely in SCG, but partially in NG. Although cholesterol itself had little effect on either [3H]AIB uptake and Na+,K(+)-ATPase activity both in SCG and NG, alpha-SC caused considerable reduction of both amino acid uptake and the transport enzyme activity in each ganglia. However, cholesterol was more effective than alpha-SC in decreasing [3H]leucine incorporation in either ganglia. Whereas addition of EGTA markedly reduced either GM1-induced or alpha-SC-induced change in [3H]leucine incorporation into acid-insoluble fraction in both SCG and NG, application of Ca2+ ionophore produced considerable recovery of the protein synthesis from the inhibited level by Ca2(+)-deprivation. ATP and creatine phosphate contents in SCG were elevated by the presence of GM1 or alpha-SC, whereas [3H]AIB uptake and Na+,K(+)-ATPase activity were inhibited, suggesting that utilization for membrane transport was diminished as a result of GM1- or alpha-SC-induced decrease of ATPase activity.

Ganglioside function in the development and repair of the nervous system. From basic science to clinical application

Gangliosides play important roles in the normal physiological operations of the nervous system, in particular that of the brain. Changes in ganglioside composition occur in the mammalian brain not only during development, but also in aging and in several neuropathological situations. Gangliosides may modulate the ability of the brain to modify its response to cues or signals from the microenvironment. For example, cultured neurons are known to respond to exogenous ganglioside with changes characteristic of cell differentiation. Gangliosides can amplify the responses of neurons to extrinsic protein factors (neuronotrophic factors) that are normal constituents of the neuron's environment. The systemic administration of monosialoganglioside also potentiates trophic actions in vivo and improves neural responses following various types of injury to the adult mammalian central nervous system. The possible molecular mechanism(s) underlying the ganglioside effects may reflect an action in modulating ligand-receptor linked transfer of information across the plasma membrane of the cell.

Inhibition of DNA synthesis in C6 glioma cells following cellular incorporation of GM1 ganglioside and choleragenoid exposure

The B subunit of cholera toxin, which is multivalent and binds specifically to GM1 ganglioside on the cell surface, has previously been used as a ganglioside-specific probe to regulate DNA synthesis in thymocytes and fibroblasts. To explore in more detail this growth-regulatory action of gangliosides, C6 glioma cells (which are GM1 ganglioside deficient) were used as a model system. When cultures of C6 cells were first treated with GM1, followed by exposure to the B subunit, proliferation was inhibited, as measured by 3H-labeled thymidine incorporation into DNA. Pretreatment of the cells with 50 microM GM1 for 15 min (followed by washing with fetal calf serum) and incubation with 1 microgram/ml of B subunit for 21 h was sufficient to reduce DNA synthesis to 15% of control values (and confirmed by autoradiographic analysis), although maximal inhibition could be achieved with as little as 30 min exposure to B, followed by washing. Furthermore, the B subunit inhibited the response of the C6 cells to basic fibroblast growth factor only following GM1 pretreatment. The B subunit-induced inhibition of DNA synthesis was specific for the ganglioside GM1, and was unrelated to increases of cyclic AMP. These results demonstrate that cell-incorporated GM1 ganglioside may act as a receptor capable of undergoing a specific ligand interaction, subsequently affecting molecular processes at the nuclear level.

A role for gangliosides in astroglial cell differentiation in vitro

Rat cerebral astroglial cells in culture display specific morphological and biochemical behaviors in response to exogenously added gangliosides. To examine a potential function for endogenous gangliosides in the processes of astroglial cell differentiation, we have used the B subunit of cholera toxin as a ganglioside-specific probe. The B subunit, which is multivalent and binds specifically to GM1 ganglioside on the cell surface, induced a classical star-shaped (stellate) morphology in the astroglial cells and inhibited DNA synthesis in a dose-dependent manner. The morphological response was massive and complete within 2 h, with an ED50 of 0.8 nM, and appeared to depend on the direct interaction of the B subunit with GM1 on the cell surface. A B subunit-evoked inhibition of DNA synthesis and cell division (ED50 = 0.2 nM) was observed when the cells were stimulated with defined mitogens, such as epidermal growth factor and basic fibroblast growth factor. Maximal inhibition approached 80% within 24 h. The effects of the B subunit were unrelated to increases in cAMP. These observations, taken together with previous studies, demonstrate that both endogenously occurring plasma membrane gangliosides and exogenously supplied gangliosides can influence the differentiative state (as judged by morphological and growth behaviors) of astroglial cells in vitro.

Differential binding kinetics of cholera toxin to intestinal microvillus membrane during development

A complete randomized block design was used to compare the binding kinetics of cholera toxin to developing rat enterocyte microvillus membranes prepared from newborn, 2-week-old, 4-week-old, and adult animals. Saturation-binding isotherms were generated on 16 independent samples (four blocks) under steady-state and reversible conditions. Scatchard analyses suggested positive cooperative binding to a single class of receptors, and the isotherms were analyzed by both the Hill-Waud and Michaelis-Menten functions. Receptor density varied significantly with age (P = 0.013). An abrupt rise in receptor density occurred after the neonatal period and normalized in the adult animal. The half-dissociation constant also varied significantly with age (P = 0.019). Microvillus membranes from suckling animals had a slightly higher apparent affinity than those from weaned animals. Neither receptor concentration nor membrane purity confounded these observations. Whereas age-related changes in apparent affinity correlated with cellular responses, changes in receptor density did not. This study suggests that developmental changes in membrane structure which influence binding affinity but not receptor density may, in part, contribute to the increased sensitivity of suckling rats to cholera toxin exposure.

Transmembrane signaling by the B subunit of cholera toxin: increased cytoplasmic free calcium in rat lymphocytes

It has previously been shown that the B subunit of cholera toxin, which binds solely to the plasma membrane ganglioside GM1, stimulates the proliferation of rat thymic lymphocytes (Spiegel, S., P. H. Fishman, and R. J. Weber, 1985, Science [Wash. DC], 230:1285-1287). The purpose of this study was to identify which transmembrane signaling system(s) are activated by the B subunit of cholera toxin. We compared the effects of B subunit and concanavalin A (Con A), a potent mitogenic lectin, on a number of second messenger systems that are putative mediators of T cell activation. Changes in the fluorescence of quin2-loaded cells revealed that mitogenic doses of either B subunit or Con A induced rapid and sustained increases in cytoplasmic free Ca2+ ([Ca2+]i). Within 5 min, [Ca2+]i increased from a basal level of 69 +/- 4 to 136 +/- 17 and 185 +/- 24 nM, respectively. The effects of B subunit and Con A were additive and largely dependent on the presence of extracellular Ca2+, though release of Ca2+ from intracellular stores could be detected for Con A, but not B subunit, using indo-1. The B subunit had no effect on either inositol phosphate levels or on the distribution of protein kinase C, indicating that, unlike Con A, the B subunit does not activate phosphoinositide hydrolysis. Fluorimetric measurements on cells loaded with bis(carboxyethyl)-5,6-carboxyfluorescein revealed that Con A induced a rapid cytoplasmic alkalinization via activation of Na+/H+ exchange, whereas B subunit had no effect on intracellular pH. Finally, by monitoring bis-oxonol fluorescence, we found that Con A induced a small hyperpolarization of the membrane potential, whereas B subunit had no acute effect. These data suggest that the biological effects of B subunit are mediated by an increase in [Ca2+]i resulting from a net influx of extracellular Ca2+.

Cooperativity of ganglioside-dependent with protein-dependent substratum adhesion and neurite extension of human neuroblastoma cells

The potential involvement of gangliosides in the adherence and neurite extension of human neuroblastoma cells (Platt and La-N1) was investigated on tissue culture substrata coated with the ganglioside GM1-binding protein, cholera toxin B (CTB) subunit, for comparison with similar processes on plasma fibronectin (pFN)-coated substrata. Cells attached with reduced efficiency on CTB substrata as compared with pFN substrata and required a much longer time to form neurite processes for a small percentage of cells on CTB. The specificity of these processes for GM1 binding was tested in a variety of ways. Supplementation of the cells with exogenous GM1, but not GD1a, identified a larger population of cells adherent on CTB (comparable to pFN-adherent cells) and dramatically increased the proportion of cells capable of forming neurites without reducing the time requirement. In ultrastructural studies using the scanning electron microscope (SEM) and immunofluorescence (IF) analyses to discriminate microtubule distributions, neurites of GM1-supplemented cells on CTB were virtually identical with pFN-adherent neurites, whereas unsupplemented cells on CTB generated processes with fine-structural differences. Treatment of cells during the GM1 supplementation period with cycloheximide completely abolished the ability of cells to generate neurites on CTB and decreased the adhesive capacity of cells as well; a similar treatment of cells had no adverse effect on adherence or neurite extension on pFN. The importance of one or more proteins in GM1-dependent processes was further confirmed by demonstrating the trypsin sensitivity of a cell surface component(s) required to achieve maximal attachment on CTB; in contrast, adherence and neurite extension on pFN were much more resistant to this treatment process. Therefore, these experiments demonstrate that certain cell surface gangliosides are capable of mediating adherence and neurite outgrowth of human neuroblastoma cells on a suitable ganglioside-binding substratum; this ganglioside dependence is cooperative with one or more cell surface proteins which can now be analysed. These results are discussed in light of the identification in ref. [16] (Exp cell res 169 (1987) 311) of a second 'cell-binding' domain on the pFN molecule competent for adherence and neurite extension of these neuroblastoma cells, as well as the potential role of pFN binding to a complex ganglioside on the surface of these neural tumor cells in these processes.

Affinity-purified tetanus neurotoxin interaction with synaptic membranes: properties of a protease-sensitive receptor component

The pharmacokinetic interaction of an affinity-purified 125I-labeled tetanotoxin fraction with guinea pig brain synaptosomal preparations was investigated. Binding of tetanotoxin was time- and temperature-dependent, was proportional to protein concentration, and was saturable at about 8 X 10(-9) M as estimated by a solid-surface binding assay. Binding was optimal at pH 6.5 under low ionic strength buffer and was almost entirely blocked by gangliosides or antitoxin. In analogy to intact nerve cells, binding of toxin to membranes resulted in a tight association operationally defined as sequestration. Binding and sequestration were abolished after membrane pretreatment with sialidase. The enzyme could not dissociate the membrane-bound toxin formed at 4 or 37 degrees C under low ionic strength conditions, which is in part compatible with internalization as defined in nerve cell cultures. In the latter system the toxin could be removed at 4 degrees C but not at 37 degrees C. Binding was significantly reduced upon pretreatment of guinea pig brain membranes by a variety of hydrolytic enzymes. Trypsin and chymotrypsin inhibited binding between 55% and 68% while bacterial protease abolished it by 91-95%. The effect was species-specific as it was not seen in rat or bovine synaptosomes. Collagenase and hyaluronidase had little or no inhibitory effect when applied to synaptosomes (27% and 9%) but inhibited binding to synaptic vesicles by 56% and 49%, respectively. Phospholipases A2 and C caused 42-43% inhibition of binding in vesicles and less than 22% in synaptosomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular localization and processing of pp60v-src proteins expressed by two distinct temperature-sensitive mutants of Rous sarcoma virus

The transforming protein of Rous sarcoma virus, pp60v-src, is known to be a tyrosine protein kinase, but the mechanism of cell transformation remains unclear. In further investigating pp60v-src structure and function, we have analyzed two temperature-sensitive (ts) Rous sarcoma virus src gene mutants, tsLA29 and tsLA32. The mutations in tsLA29 and tsLA32 map in the carboxy-terminal region and the amino-terminal half of pp60v-src, respectively, and encode mutant proteins with either temperature-labile (tsLA29) or -stable (tsLA32) kinase activities. Here we examined the intracellular processing and localization of these pp60v-src mutants and extended our characterization of transformation parameters expressed by cells infected by the Rous sarcoma virus variants. No obvious defects in functional integrity of the tsLA32 pp60v-src could yet be demonstrated, whereas the tsLA29 pp60v-src was perturbed not only in kinase activity, but also in aspects of protein processing and localization. Analysis of transformation parameters expressed by infected cells demonstrated the complete temperature lability of both mutants.

Neuritogenic and metabolic effects of individual gangliosides and their interaction with nerve growth factor in cultures of neuroblastoma and pheochromocytoma

The 4 major ganglioside species, GM1, GD1a, GD1b and GT1b (200 micrograms/ml), were tested individually for the ability to stimulate neuronal trophic responses. The growth parameters measured were: morphologic changes, quantitated by computer-assisted morphometry of neurite length and number per soma, and metabolic changes, indicated by alterations in ornithine decarboxylase activity (ODC). In addition, the interaction of each ganglioside with nerve growth factor (NGF) was investigated with an NGF-responsive pheochromocytoma PC12 cell line and NGF-insensitive neuroblastoma Neuro-2a cultures. PC12 cells responded to gangliosides only in the presence of NGF (20 micrograms/ml): GM1 produced the greatest morphologic response, but did not alter metabolic levels; GT1b increased both parameters. The presence (5 micrograms/ml) or absence of NGF did not have an effect on the ganglioside-mediated morphologic responses of Neuro-2a cells to each species: GD1b elicited the greatest increase in neurite length, while GD1a and GT1b stimulated both length and number. In contrast, while GT1b alone was able to elevate ODC activity independently of NGF, the simultaneous exposure of Neuro-2a cultures to NGF and GM1 or GD1a resulted in a stimulation of cellular metabolism. These results indicate that each ganglioside species has a specific target action in the stimulation of different trophic responses and that performance in one category is not a predictor of the result in another. In addition, it is possible to confer a sensitivity to NGF by simultaneous treatment with specific gangliosides. This indicates that membrane gangliosides may modulate the actions of neurotrophic factors.

Effect of transformation by Rous sarcoma virus on the character and distribution of actin in Rat-1 fibroblasts: a biochemical and microscopical study

Actin has been measured in subcellular fractions from Rat-1 fibroblasts and in Rous sarcoma virus-transformed Rat-1 cells (VIT), using the DNase 1 inhibition assay. The transformed cells showed a significant shift in the actin monomer (G)in equilibrium with polymer (F) equilibrium within the cell cytosol, and a significant increase in actin in the Triton-insoluble cytoskeletal core in comparison with untransformed cells. This incorporation of actin into the cytoskeletal core fraction is associated with a change in filamentous actin assemblies from 'stress fibre' patterns to punctate filament aggregates. These differences have been correlated with changes in morphology, in actin, vinculin and alpha-actinin distribution, in adhesion plaque formation and with the production of pp60v-src-associated protein kinase activity in the transformed cells. Changes in actin distribution and its polymerization in response to src-gene expression may play an important role in the determination of the transformed cell characteristics.

Variants of BALB/c 3T3 cells lacking complex gangliosides retain a fibronectin matrix and spread normally on fibronectin-coated substrates

Evidence has accumulated that di- and trisialogangliosides are involved in the interaction of cells with fibronectin. We have therefore tested the ability of variants of BALB/c 3T3 deficient in such gangliosides to organize a fibronectin matrix and to spread on fibronectin-coated substrates. Whereas BALB/c 3T3 cells contained gangliosides GM3, GM1, and GD1a, direct chemical analysis showed that five out of six variants isolated contained no detectable GD1a. By the overlaying of thin layer chromatograms of cellular gangliosides with 125I-cholera toxin, these variants were also found to lack ganglioside GM1. In contrast, the sialogalactoprotein profile of these cells, analyzed using an 125I-ricin/SDS polyacrylamide gel overlay technique, was similar to that of the parent cell line. All variants organized an extensive fibronectin matrix comparable to that of BALB/c 3T3, as shown using either immunofluorescence or lactoperoxidase-catalyzed iodination. The variants could also spread on fibronectin-coated substrates and adopt a morphology similar to that of BALB/c 3T3 cells, with little or no difference in the concentration of fibronectin required for 50% cell spreading. Cell spreading of the variants was accompanied by the formation of focal contacts and microfilament bundles, in a manner closely resembling that seen with BALB/c 3T3 cells. Treatment of BALB/c 3T3 cells with neuraminidase, which converts much of the cellular GD1a to GM1, did not affect cell spreading on fibronectin. The results clearly demonstrate that complex gangliosides are not essential for retention of a fibronectin matrix or for spreading on fibronectin-coated substrates.

Direct evidence that endogenous GM1 ganglioside can mediate thymocyte proliferation

The B subunit of cholera toxin, which is multivalent and binds exclusively to a specific ganglioside, GM1, was mitogenic for rat thymocytes. When exposed to the B subunit, the cells proliferated, as measured by 3H-labeled thymidine incorporation. Mitogenesis depended on the direct interaction of the B subunit with GM1 on the surface of the cells. This demonstrates that endogenous plasma membrane gangliosides can mediate proliferation in lymphocytes.

Interaction of tumour promoters with epithelial cells in culture. An immunofluorescence study

12-O-tetradecanoyl phorbol-13-acetate (TPA) has a profound and rapid influence on the cytoskeleton of Madin-Darby Canine Kidney (MDCK) cells. Within 10 min, TPA induces a rapid change in morphology, from a flat, cuboidal state to a rounded or elongated morphology in which the cell membranes become convoluted. Concomitant with this morphological change is a rapid dissolution of stress fibres and a redistribution of F-actin from microfilament bundles to a membrane or sub-membranous location. The rearrangement of actin is paralleled by a rearrangement of alpha-actinin and a reduction in the number of vinculin-containing adhesion plaques. Unusual F-actin configurations are often found emanating from a perinuclear location, usually containing alpha-actinin and terminating in a vinculin-containing adhesion plaque. The cytoskeletal rearrangements occur in the presence of inhibitors of protein synthesis or oxidative phosphorylation, but do not occur if glycolysis is also inhibited. The rearrangements are partly abrogated by the presence of cytochalasin B (CB). Despite these dramatic changes in microfilaments the polymerization state of actin remained unaltered after TPA treatment. Furthermore, although changes in the movement of membrane lipids have been reported, no obvious differences in the ability of glycoproteins to redistribute in the plane of the membrane were found as judged by FITC-concanavalin A (conA) induced patching. The rapidity of the morphological response of MDCK cells to TPA indicates that the cytoskeleton is one of the primary targets of TPA, but that tumour promoters differ from RNA tumour viruses in their effect on the state of actin polymerization.

Evidence for an association between calmodulin and membrane patches containing gonadotropin-releasing hormone--receptor complexes in cultured gonadotropes

participation of calmodulin, clathrin, and actin in receptor mediated endocytosis of gonadotropin-releasing hormone (GnRH) was studied in an in vitro system of dispersed pituitary cells with a triple staining procedure. Cells were incubated in D-Lys6-Pro9-Des10-GnRH-biotin and stained with avidin-peroxidase-diaminobenzidine. Calmodulin, clathrin, and actin as well as luteinizing hormone were identified by indirect immunofluorescence with FITC- and rhodamine-labeled second antibody. The results indicate a close spatial association of calmodulin, but not of clathrin and actin, with GnRH-containing plasma membrane patches.

Glycosphingolipids in detergent-insoluble substrate attachment matrix (DISAM) prepared from substrate attachment material (SAM). Their possible role in regulating cell adhesion

The glycosphingolipids isolated from the detergent-insoluble material (DIM) of whole cells as well as from a similar detergent-insoluble substrate attachment matrix (DISAM) have been investigated in comparison with the glycosphingolipids of whole cells. The proportion of glycolipids in the total lipid extract was enriched in the DISAM as well as DIM fractions as compared to whole cells. The ratio of ganglioside (GM3) to neutral glycolipids was also higher in the DISAM fractions than in whole cells. The radioactivity incorporated into DISAM glycolipids of BHK cells, metabolically labeled with radioactive glucosamine, was greater in confluent cells than in sparsely growing cells; however, label incorporation into glycolipids of the DISAM fraction of BHKpy cells was 2-3-fold higher than that of confluent BHK cells, although the chemical quantity of GM3 in whole cells was much lower in BHKpy cells than in BHK cells. In order to confirm the enhanced label in DISAM glycolipids of BHKpy cells by other procedures, the labeled cells were detached by EGTA, washed, and reattached on plates. The amount of label in DISAM glycolipids of the reattached matrix of BHKpy cells was much higher than that of BHK cells. Cell spreading and cell attachment on plastic plate were inhibited by inclusion of GM3 in the medium. These data suggest that: (i) glycolipids, particularly GM3, at the cell attachment site have different metabolic activity from those of whole cells; the label in glycolipids goes preferentially into cell attachment sites, and may have some functional role in regulating cell attachment of BHK cells; (ii) metabolic activity and turnover of GM3 in cell attachment sites of confluent cells are higher than actively growing cells, yet those of transformed cells are much higher than any state of non-transformed cells.

Direct visualization of redistribution and capping of fluorescent gangliosides on lymphocytes

Fluorescent derivatives of gangliosides were prepared by oxidizing the sialyl residues to aldehydes and reacting them with fluorescent hydrazides. When rhodaminyl gangliosides were incubated with lymphocytes, the cells incorporated them in a time- and temperature-dependent manner. Initially, the gangliosides were evenly distributed on the cell surface but were redistributed into patches and caps by antirhodamine antibodies. When the cells were then stained with a second antibody or protein A labeled with fluorescein, the fluorescein stain revealed the coincident movement of both the gangliosides and the antirhodamine antibodies. When the cells were treated with both rhodamine and Lucifer yellow CH-labeled gangliosides, the antirhodamine antibodies induced patching and capping of both fluorescent gangliosides but had no effect on cells incubated only with Lucifer yellow CH-labeled gangliosides. In addition, capping was observed on cells exposed to cholera toxin, antitoxin antibodies, and rhodamine-labeled protein A, indirectly showing the redistribution of endogenous ganglioside GM1, the cholera toxin receptor. By incorporating Lucifer yellow CH-labeled GM1 into the cells and inducing capping as above, we were able to demonstrate directly the coordinate redistribution of the fluorescent GM1 and the toxin. When the lymphocytes were stained first with Lucifer yellow CH-labeled exogenous ganglioside GM3, which is not a toxin receptor, there was co-capping of endogenous GM1 (rhodamine) and exogenous GM3 (Lucifer yellow CH). These results suggest that gangliosides may self-associate in the plasma membrane which may explain the basis for ganglioside redistribution and capping.

Biological activity of preformed cholera toxin-ganglioside GM1 complex

Synthetic and natural amphiphiles, octyl glucoside, Nonidet P40, sodium dodecyl sulfate (SDS), gangliosides GM1 and GD1a, interact with cholera toxin (CLT) and with its active region (promoter A). The formation of CLT-amphiphile complex leads to inhibition of ADP-ribosyltransferase activity, a characteristic of promoter A elicited after thiol-reagents treatment. In all cases the interaction produces the maximum inhibitory effect above the critical micellar concentration of amphiphiles, although monomers of SDS show inhibition activity as well. The gangliosides appear to be capable of altering bilayer organization of membrane, similar to synthetic detergents. When CLT-ganglioside complexes were incubated with cell culture medium containing 10% fetal calf serum (FCS) and ADP-ribosyltransferase activity was completely restored both in cholera toxin and in promoter A. Some protein of FCS, which is avid of gangliosides, seems to be responsible for reversibility of inhibition. The results indicate that the active site of promoter A may be located in a hydrophobic pocket of the toxin structure. Furthermore, CLT was bound to reconstituted Sendai virus envelopes (RSVEs), containing a small amount of GM1. The RSVEs are made of membranous vesicles, capable of binding and fusing with host cell membrane. The incubation for 1 1hr of RSVE bearing CLT with Friend's erythroleukemic cells produced the stimulation of adenylate cyclase. This stimulation appears to be due to the translocation of the active subunit of CLT in the inner half of plasma membrane.