Exposure of the major human red-cell glycolipid, globoside, to galactose oxidase

Sphingolipid transport in eukaryotic cells

Sphingolipids constitute a sizeable fraction of the membrane lipids in all eukaryotes and are indispensable for eukaryotic life. First of all, the involvement of sphingolipids in organizing the lateral domain structure of membranes appears essential for processes like protein sorting and membrane signaling. In addition, recognition events between complex glycosphingolipids and glycoproteins are thought to be required for tissue differentiation in higher eukaryotes and for other specific cell interactions. Finally, upon certain stimuli like stress or receptor activation, sphingolipids give rise to a variety of second messengers with effects on cellular homeostasis. All sphingolipid actions are governed by their local concentration. The intricate control of their intracellular topology by the proteins responsible for their synthesis, hydrolysis and intracellular transport is the topic of this review.

Change of ganglioside accessibility at the plasma membrane surface of cultured neurons, following protein kinase C activation

While the mechanism of signal transduction across the plasma membrane from the exo- to the endoplasmic side has been extensively investigated, the possible return of messages back to the outer layer is less known. We studied the effect of protein kinase C activation on the ganglioside accessibility at the exoplasmic face of intact rat cerebellar granule cells in culture, using the enzyme sialidase as the probing molecule. Under the experimental conditions (1 milliunit/mL enzyme, 2 min incubation at 37 degreesC), only GT1b and GD1a gangliosides were partially affected by the enzyme (28.6 and 25.7% hydrolysis, respectively). After cell treatment with phorbol 12-myristate 13-acetate, inducing protein kinase C activation, GT1b and GD1a ganglioside susceptibility to sialidase was strongly decreased (8.6 and 15.9% hydrolysis, respectively). A reduction of ganglioside hydrolysis was also observed when protein kinase C activation was induced by cell treatment for 15 min with 100 microM glutamate. On the contrary, accessibility did not vary when protein kinase C translocation was not effective (either in the absence of Ca2+ in the medium or using 1 microM glutamate) or when the kinase activity was inhibited by staurosporine. These data suggest that following PKC activation, a key step of inbound transmembrane signaling, cell may dispatch outbound messages to the plasma membrane outer layer, changing the selective recognition and crypticity of glycolipids at the cell surface, possibly through a modulation of their segregation state.

Exposure to galactose oxidase of GM1 ganglioside molecular species embedded into phospholipid vesicles

The exposure of GM1 molecular species present in the native ganglioside, carrying C18:1 or C20:1 long-chain bases (LCB), to Dactylium dendroides galactose oxidase was studied. When native GM1 (49.3% C18:1 and 50.7% C20:1 LCB, respectively), was inserted in dipalmitoylphosphatidylcholine vesicles and partially oxidized (10%), the proportion of C18:1 and C20:1 species in the oxidized GM1 was 59.6% and 40.4%, respectively, suggesting a preferential action of the enzyme on the shorter species. The Vmax of the enzyme was higher on C18:1 GM1 than on C20:1 GM1. The molecular species were affected without any preference after partial (10%) oxidation of GM1 incorporated in egg phosphatidylcholine vesicles or in micellar form. These data indicate that the exposure of the terminal galactose moiety of GM1 ganglioside to galactose oxidase is affected by the ganglioside ceramide composition as well as the phospholipid environment, that presumably determine the distribution (molecular dispersion, segregation) of the ganglioside within the membrane.

Specific hydrolysis of intact erythrocyte cell-surface glycosphingolipids by endoglycoceramidase. Lack of modulation of erythrocyte glucose transporter by endogenous glycosphingolipids

This study represents the specific hydrolysis of cell-surface glycosphingolipids (GSLs) of intact cells by endoglycoceramidase (EGCase; EC.3.2.1.123) which cleaves the linkage between oligosaccharides and ceramides of various GSLs. After a 2-h incubation of horse intact erythrocytes with 20 mU EGCase II in the presence of activator at 37 degrees C, 68% of the N-glycolylneuraminic-acid-containing ganglioside GM3(NeuGc) and 70% of 4-O-acetyl GM3(NeuGc) were found to be hydrolyzed without hemolysis, accompanied by a corresponding increase in ceramide but not sphingosine or N,N-dimethylsphingosine. No hydrolysis was observed for sphingomyelin, phosphatidylcholine, phosphatidylethanolamine, cholesterol or membrane proteins. The decrease in immunoreactivity with GMR8 antibody, specific to NeuGc alpha 2,3Gal- of GM3(NeuGc), corresponded to that of cell-surface GM3(NeuGc) by the enzyme, and almost no immunoreactivity was found when 70% of the GM3(NeuGc) was hydrolyzed. Besides the cell-surface GM3(NeuGc) of horse erythrocytes, Gg3Cer of guinea pig, GM3(NeuAc) and LcCer of human, and bovine and rabbit erythrocyte IV3Gal alpha-nLc4Cer were found to be efficiently hydrolyzed by EGCase II even when present in intact cells, while human erythrocyte Gb4Cer is quite resistant to hydrolysis by the enzyme on the cell surface as well as in detergent micelles. Glucose incorporation via the glucose transporter in erythrocytes was not affected at all by the specific and exhaustive hydrolysis of cell-surface GSLs by EGCase II. This result strongly suggested that glucose transporter function was not directly modulated by endogenous GSLs. In summary, this paper demonstrates that, together with the assistance of activator protein, EGCase II will become a powerful tool for selectively removing sugar chains from cell-surface GSLs without damaging other cell membrane components, and will be useful for describing the biological functions of endogenous GSLs.

Rotaviruses specifically bind to the neutral glycosphingolipid asialo-GM1

Rotaviruses are the major etiologic agents of severe diarrhea in children. Many rotaviruses encode a hemagglutinin which binds to sialic acids. We report that rotaviruses specifically recognize the neutral glycosphingolipid gangliotetraosylceramide (asialo-GM1 or GA1). GA1 resolved by thin-layer chromatography is bound by rotavirus, and binding is blocked by neutralizing rotavirus antiserum. Similar glycosphingolipid structures, such as globoside, gangliotriaosylceramide, and GA1 oxidized with galactose oxidase are ineffective in binding rotavirus. Other enteric viruses also specifically bind GA1. GA1 adsorbed to polystyrene beads inhibits rotavirus replication in vitro (as do anti-GA1 antibodies). The use of orally administered immobilized GA1 or anti-GA1 antibodies may prove useful in preventing or attenuating rotaviral and other enteric viral infections.

Liberation of oligosaccharides from glycosphingolipids on PC12 cell surface with endoglycoceramidase

Endoglycoceramidase (EGCase) cleaves the linkage between the oligosaccharide and ceramide of glycosphingolipids (Ito, M., and Yamagata, T., J. Biol. Chem. 261, 14278-14282, ibid, 264, in press). Intact cells of rat pheochromocytoma line PC12 were treated with the highly purified EGCase I and the oligosaccharides released were analyzed by HPLC. Cleavage of the oligosaccharides with the enzyme reached a plateau as the amount of the enzyme was increased. At maximum, 42% of the oligosaccharides from globoside, 40% from GalGb3Cer, and 60% from Gb3Cer were liberated during 1h of incubation without impairing the viability of cells. The only partial liberation indicates that not all oligosaccharides of cell surface glycosphingolipids are accessible to the enzyme. The use of EGCase offers an important new method to access the functions of glycosphingolipids on cell surface in situ.

Oxidation of glycolipids in liposomes by galactose oxidase

Small unilamellar phosphatidylcholine vesicles containing globo-series glycolipids were labeled by the galactose oxidase/NaB[3H]4 procedure. The major glycolipid of human red cells, globoside, was the best substrate for galactose oxidase both in vesicles and in tetrahydrofuran-containing buffer. The oxidation rates of membrane-bound ceramide trihexoside and Forssman glycolipid were one-fourth and one-tenth, respectively, of the oxidation rate of globoside. Membrane-bound ceramide dihexoside was not a substrate for galactose oxidase, although it was readily oxidized in tetrahydrofuran-containing buffer. Soluble sialoglycoproteins and membrane-incorporated glycophorin A stimulated the oxidation of globoside-containing vesicles, whereas membrane-bound GD1a ganglioside had no effect on globoside oxidation.

Degradation of the human erythrocyte membrane band 3 studied with monoclonal antibody directed against an epitope on the cytoplasmic fragment of band 3

The mouse hybridoma monoclonal antibody BIII.136 of the IgG2a class is specific for human erythrocyte band-3 protein. It was shown by means of immunoblotting and immunoprecipitation assays that the antibody recognized an epitope located in the cytoplasmic pole of the band-3 molecule within approximately 20 kDa from the N-terminal end. The N-terminal fragments of band-3 protein, migrating in SDS/polyacrylamide gel electrophoresis in the 60-kDa, 40-kDa and 20-kDa regions, were detected with the antibody in untreated red-cell membranes as products of autolysis of band-3 protein. A correlation was found between the amount of these fragments and erythrocyte age, which suggests that partial degradation of band 3 proceeds in vivo during senescence of erythrocytes. The further degradation of band-3 protein in vitro was not observed in intact erythrocytes stored at 4 degrees C, but progressed distinctly after hemolysis of red cells, during washing and storing the membranes.

Interaction of viruses, bacteria and bacterial toxins with host cell surface glycolipids. Aspects on receptor identification and dissection of binding epitopes

An overview and perspective is presented on animal cell surface carbohydrate (primarily lipid-linked oligosaccharides) as specific receptors for viruses, bacteria and bacterial toxins. Although carbohydrate has been known for many years to be specific attachment sites for these ligands, it is only in very recent time that carbohydrate technology and receptor assays in combination afford a rational approach. One generalization from present experience is the property of microbiological ligands to recognize sequences placed internally in an oligosaccharide chain which differs from antibody recognition of short sequences which most often involves terminally placed determinants. This is of both biological and technical importance. Biologically it may assure attachment by avoiding differences between host individuals often residing in terminal parts (e.g. blood group determinants), and may also make a shift of target cells by mutations more efficient. Technically this property is an important help when dissecting narrow binding epitopes, and for disclosing receptor-binding variants with only slight differences in binding epitopes (e.g. different epitopes on the same disaccharide). Such variants representing a kind of "epitope drift" are probably a consequence of point mutations in the binding site of the lectin-like proteins to select a proper host environment. Current technology allows an efficient screening for carbohydrate receptors with interesting consequences for applications within medicine (diagnosis and therapy) and biotechnology.

Exposure of major neutral glycolipids in red cells to galactose oxidase. Effect of neuraminidase

The exposure of several major red-cell glycolipids to galactose oxidase was studied by oxidizing the cells with the enzyme and reducing them with NaB2H4. After isolation, the deuterium label was detected by mass fragmentography. 60-70% globoside in human and porcine erythrocytes was exposed as measured by this method. In contrast, asialo-GM2 in guinea-pig erythrocytes and Forssman glycolipid in sheep erythrocytes were mainly in a cryptic state. Neuraminidase treatment increased the incorporation of deuterium label to asialo-GM2 4-8-fold. A similar effect was seen in Forssman glycolipid when sheep red cells were labeled with the neuraminidase/galactose oxidase/NaB3H4 method. In contrast, the increase in labeling was only about 10-40% in porcine and human globosides, which were efficiently exposed to galactose oxidase already in native red cells.