Translocation of newly synthesized gangliosides to the cell surface

Chemical Characterization of N-Linked Oligosaccharide As the Antigen Epitope Recognized by an Anti-Sperm Auto-Monoclonal Antibody, Ts4

Ts4, an anti-sperm auto-monoclonal antibody, possesses immunoreactivity to the acrosomal region of mouse epididymal spermatozoa. In addition, the mAb shows specific immunoreactivity to reproduction-related regions such as testicular germ cells and early embryo. Our qualitative study previously showed that the antigen epitope for Ts4 contained a N-linked common oligosaccharide (OS) chain on testicular glycoproteins as determined by Western blotting for testicular glycoproteins after treatment with several glycohydrolases. Since the distribution of the Ts4-epitope is unique, the OS chain in Ts4-epitope may have role(s) in the reproductive process. The aim of this study was to clarify the molecular structure of the Ts4-epitope, particularly its OS moiety. Using Ts4 immunoprecipitation combined with liquid chromatography and multiple-stage mass spectrometry, the candidate carbohydrate structure in the Ts4-epitope is proposed to be N-linked fucosylated agalacto-biantennary with bisecting N-acetylglucosamine (GlcNAc) or with N-acetylgalactosamine-GlcNAc motif. Further binding analyses using various lectins against the mouse testicular Ts4-immunoprecipitants revealed that Phaseolus vulgaris erythroagglutinin and Pisum sativum agglutinin showed positive staining of the bands corresponding to Ts4 reactive proteins. Moreover, the immunoreactivity of Ts4 against the testicular extract was completely abrogated after digestion with β-N-acetylglucosaminidase. These results show that the Ts4-epitope contains agalacto-biantennary N-glycan with bisecting GlcNAc carrying fucose residues.

Regulation of ganglioside biosynthesis in the nervous system

Ganglioside biosynthesis is strictly regulated by the activities of glycosyltransferases and is necessarily controlled at the levels of gene transcription and posttranslational modification. Cells can switch between expressing simple and complex gangliosides or between different series within these two groups during brain development. The sequential biosynthesis of gangliosides in parallel enzymatic pathways, however, requires fine-tuned subcellular sequestration and orchestration of glycosyltransferases. A popular model predicts that this regulation is achieved by the vectorial organization of ganglioside biosynthesis: sequential biosynthetic steps occur with the traffic of ganglioside intermediates through subsequent subcellular compartments. Here, we review current models for the subcellular distribution of glycosyltransferases and discuss results that suggest a critical role of N-glycosylation for the processing, transport, and complex formation of these enzymes. In this context, we attempt to illustrate the regulation of ganglioside biosynthesis as well as the biological significance of N-glycosylation as a posttranslational regulatory mechanism. We also review the results of analyses of the 5' regulatory sequences of several glycosyltransferases in ganglioside biosynthesis and provide insights into how their synthesis can be regulated at the level of transcription.

Ganglioside/glycosphingolipid turnover: new concepts

In this review focus is given to the metabolic turnover of gangliosides/glycosphingolipids. The metabolism and accompanying intracellular trafficking of gangliosides/glycosphingolipids is illustrated with particular attention to the following events: (a) the de novo biosynthesis in the endoplasmic reticulum and Golgi apparatus, followed by vesicular sorting to the plasma membrane; (b) the enzyme-assisted chemical modifications occurring at the plasma membrane level; (c) the internalization via endocytosis and recycling to the plasma membrane; (d) the direct glycosylations taking place after sorting from endosomes to the Golgi apparatus; (e) the degradation at the late endosomal/lysosomal level with formation of fragments of sugar (glucose, galactose, hexosamine, sialic acid) and lipid (ceramide, sphingosine, fatty acid) nature; (f) the metabolic recycling of these fragments for biosynthetic purposes (salvage pathways); and (g) further degradation of fragments to waste products. Noteworthy, the correct course of ganglioside/glycosphingolipid metabolism requires the presence of the vimentin intracellular filament net work, likely to assist intracellular transport of sphingoid molecules. ut of the above events those that can be quantitatively evaluated with acceptable reliability are the processes of de novo biosynthesis, metabolic salvage and direct glycosylation. Depending on the cultured cells employed, the percentage of distribution of de novo biosynthesis, salvage pathways, and direct glycosylation, over total metabolism were reported to be: 35% (range: 10-90%) for de novo biosynthesis, 7% (range: 5-10%) for direct glycosylation, and 58% (range: 10-90%) for salvage pathways. The attempts made to calculate the half-life of overall ganglioside turnover provided data of unsure reliability, especially because in many studies salvage pathways were not taken into consideration. The values of half-life range from 2 to 6.5 h to 3 days depending on the cells used. Available evidence for changes of ganglioside/glycosphingolipid turnover, due to extracellular stimuli, is also considered and discussed.

Enzymatic N-glycan analysis of 31 kDa molecule in plerocercoid of Spirometra mansoni (sparganum) and its antigenicity after chemical oxidation

A highly specific antigenic protein of 31 kDa from plerocercoid of Spirometra mansoni (sparganum) was obtained by gelatin affinity and Mono Q anion-exchange column chromatography. The purified 31 kDa protein was subjected to N-glycan enzymatic digestion for structural analysis. The relative electrophoretic mobility was analyzed by SDS-PAGE, before and after digestion. On SDS-PAGE after enzymatic digestion, the 31 kDa protein showed a molecular shift of approximately 2 kDa, which indicated the possession of complex N-linked oligosaccharides (N-glycosidase F sensitive) but not of high-mannose oligosaccharides (endo-beta-N-acetylglucosaminidase H, non-sensitive). Chemically periodated 31 kDa protein showed statistically non-significant changes with human sparganosis sera by enzyme linked immunosorbent assay (ELISA). Therefore, the dominant epitopes of the 31 kDa molecule in human sparganosis were found to be mainly polypeptide, while N-glycans of the antigenic molecule in sparganum was minimal in anti-carbohydrate antibody production.

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.

Cell surface display and intracellular trafficking of free glycosylphosphatidylinositols in mammalian cells

In addition to serving as membrane anchors for cell surface proteins, glycosylphosphatidylinositols (GPIs) can be found abundantly as free glycolipids in mammalian cells. In this study we analyze the subcellular distribution and intracellular transport of metabolically radiolabeled GPIs in three different cell lines. We use a variety of membrane isolation techniques (subcellular fractionation, plasma membrane vesiculation to isolate pure plasma membrane fractions, and enveloped viruses to sample cellular membranes) to provide direct evidence that free GPIs are not confined to their site of synthesis, the endoplasmic reticulum, but can redistribute to populate other subcellular organelles. Over short labeling periods (2.5 h), radiolabeled GPIs were found at similar concentration in all subcellular fractions with the exception of a mitochondria-enriched fraction where GPI concentration was low. Pulse-chase experiments over extended chase periods showed that although the total amount of cellular radiolabeled GPIs decreased, the plasma membrane complement of labeled GPIs increased. GPIs at the plasma membrane were found to populate primarily the exoplasmic leaflet as detected using periodate oxidation of the cell surface. Transport of GPIs to the cell surface was inhibited by Brefeldin A and blocked at 15 degrees C, suggesting that GPIs are transported to the plasma membrane via a vesicular mechanism. The rate of transport of radiolabeled GPIs to the cell surface was found to be comparable with the rate of secretion of newly synthesized soluble proteins destined for the extracellular space.

Multi-enzyme kinetic analysis of glycolipid biosynthesis

Gangliosides are acidic glycosphingolipids synthesized sequentially by a series of glycosyltransferases acting in parallel biosynthetic pathways. While most glycosyltransferases are highly specific, some, however, may catalyze equivalent steps in each pathway using different gangliosides as substrates (e.g. N-acetylgalactosaminyltransferase, sialyltransferase-IV). A multi-enzyme kinetic analysis was developed on the condition that serial enzymatic reactions operate below substrate saturation. A multi-enzyme kinetic analysis enabled a simultaneous calculation of the Vmax/Km value of each enzyme derived from the equilibrium concentration of the respective substrate. Substrate concentrations [S] were determined by radioactive labelling of gangliosides in intact cells with the precursor sugars [14C]galactose and [14C]glucosamine, followed by high-performance thin-layer chromatography and autoradiography of the radiolabelled glycolipids. On the basis of Michaelis-Menten kinetics, Vmax/Km values were derived from [S] by a system of linear equations. The procedure was used to analyze the development of the glycolipid composition during differentiation of rat gliomaxmurine neuroblastoma (NG108-15) cells. The Vmax/Km values calculated by multi-enzyme kinetic analysis were consistent with the kinetic data obtained with solubilized enzymes. Application of multi-enzyme kinetic analysis to published data on the correlation of enzyme activities with ganglioside levels in various cell lines and tissues indicated the validity of this method for analysis of the glycolipid biosynthesis, in particular, of its initial steps. On the basis of the kinetic analysis, it is suggested that the cell lines can be divided into two groups with respect to the substrate pools of GM3 used by sialyltransferase-II and N-acetylgalactosaminyltransferase-I. The first group encompasses the majority of the neuroblastoma cell lines and the embryonic rat brain where the two enzymes share a common pool of GM3. In the second group, the two enzymes do not compete for the same pool of GM3, indicating a different subcellular localization of CMP-NeuAc:GM3 alpha2-8-sialyltransferase and UDP-N-acetylgalactosaminyl:GM3 N-acetylgalactosaminyltransferase. In this study, the theory of a multi-enzyme kinetic analysis is discussed and its application to analysis of the glycolipid biosynthesis in neuroblastoma cells is demonstrated. A multi-enzyme kinetic analysis can be applied to other biosynthetic pathways and provides the advantage of analyzing kinetic data with intact cells or tissue samples.

Salvage of catabolic products in ganglioside metabolism: a study on rat cerebellar granule cells in culture

Cerebellar granule cells in culture were subjected to a pulse (0.5-4 h)-chase (0-4 h) of 10(-6) M [3H]ganglioside GM1 carrying the radioactive label at the level of NeuAc ([3H-NeuAc]GM1), Sph ([3H-Sph]GM1) or Gal ([3H-Gal]GM1) and the formed [3H]metabolites were determined. With all forms of [3H]GM1, there was formation of [3H]catabolites, including [3H]H2O and [3H]biosynthetic products obtained by recycling of [3H]NeuAc, [3H]Sph and [3H]Gal released during intralysosomal ganglioside degradation (salvage processes). Much higher amounts of [3H]H2O were produced from [3H-Gal]GM1 than [3H-Sph]GM1 and [3H-NeuAc]GM1; conversely, more products from salvage processes (polysialogangliosides GD1a, GD1b, GT1b, O-acetylated GT1b, protein-bound radioactivity) were obtained with [3H-NeuAc]GM1 than the two other forms of [3H]GM1. Liberated [3H]NeuAc produced 10-fold less tritiated water and 10-fold higher salvage products than [3H]Gal. Using [3H-NeuAc]GM1, granule cells appeared to metabolize 7.7% of membrane-incorporated exogenous GM1 per hour with a high degree of NeuAc recycling and the calculated metabolic half-life was 6.5 h.

Sorting of newly synthesized galactosphingolipids to the two surface domains of epithelial cells

The high concentration of glycosphingolipids on the apical surface of epithelial cells may be generated by selective transport from their site of synthesis to the cell surface. Previously, we showed that canine kidney MDCK and human intestinal Caco-2 cells converted a ceramide carrying the short fluorescent fatty acid C6-NBD to glucosylceramide (GlcCer) and sphingomyelin (SM), and that GlcCer was preferentially transported to the apical surface as compared to SM. Here, we address the point that not all glycosphingolipid classes are apically enriched in epithelia. We show that a ceramide containing the 2-hydroxy fatty acid C6OH was preferentially converted by MDCK and Caco-2 cells to galactosylceramide (GalCer) and its derivatives galabiosylceramide (Ga2Cer) and sulfatide (SGalCer) as compared to SM and GlcCer--all endogenous lipid classes of these cells. Transport to the apical and basolateral cell surface was monitored by a BSA-depletion assay. In MDCK cells, GalCer reached the cell surface with two- to sixfold lower apical/basolateral polarity than GlcCer. Remarkably, in Caco-2 cells GalCer and GlcCer displayed the same apical/basolateral polarity, but it was sixfold lower for lipids with a C6OH chain than for C6-NBD lipids. Therefore, the sorting of a sphingolipid appears to depend on lipid structure and cell type. We propose that the different ratios of gluco- and galactosphingolipid synthesis in the various epithelial tissues govern lipid sorting in the membrane of the trans Golgi network by dictating the composition of the domains from where vesicles bud to the apical and basolateral cell surface.

Carbohydrate epitopes on Haemonchus contortus antigens

Extracts of infective larvae and adults of the trichostrongylid Haemonchus contortus were studied for the presence of carbohydrate moieties. Several different lectin-binding sites were demonstrated in both stages using a panel of nine lectins. The carbohydrate specificity of the lectins used strongly suggests that alpha-D-mannose, alpha-D-glucose, and D-N-acetylglucosamine are the most important carbohydrate epitopes present on H. contortus proteins. Thus, N-linked oligosaccharides form the major part of the carbohydrate moieties on these glycoproteins. Treatment with sodium periodate was performed to investigate the immunoreactivity towards the carbohydrate moieties. This treatment resulted in a reduction in the immunoreactivity of these antigens as demonstrated by enzyme-linked immunosorbent assay (ELISA) and immunoblotting, suggesting that a substantial part of the host immune response against H. contortus is directed against the carbohydrate epitopes on the parasite antigens.

GM2 ganglioside and pyramidal neuron dendritogenesis

GM2 ganglioside, although scarce in normal adult brain, is the predominant ganglioside accumulating in several types of lysosomal disorders, most notably Tay-Sachs disease. Pyramidal neurons of cerebral cortex in Tay-Sachs, as well as many other types of neuronal storage disorders, are known to exhibit a phenomenon believed unique to storage disorders: growth of ectopic dendrites. Recent studies have shown that a common metabolic abnormality shared by storage diseases with ectopic dendrite growth is the abnormal accumulation of GM2 ganglioside. The correlation between increased levels of GM2 and the presence of ectopic dendrites has been found in both ganglioside and nonganglioside storage disorders, the latter including sphingomyelin-cholesterol lipidosis, mucopolysaccharidosis, and alpha-mannosidosis. Quantitative HPTLC analysis has shown that increases in GM2 occur in proportion to the incidence of ectopic dendrite growth, whereas other gangliosides, including GM1, lack similar increases. Immunocytochemical studies of all nonganglioside storage diseases which exhibit ectopic dendritogenesis have revealed heightened GM2 ganglioside-immunoreactivity in the cortical pyramidal cell population, whereas nerurons in normal adult brain exhibit little or no staining for this ganglioside. Further, studies examining disease development have consistently shown that accumulation of GM2 ganglioside precedes growth of ectopic dendrites, indicating that it is not simply occurring secondary to new membrane production. These findings have prompted an examination for a similar relationship between GM2 ganglioside and dendritogenesis in cortical neurons of normal developing brain. Results show that GM2 ganglioside-immunoreactivity is consistently elevated in immature neurons during the period when they are undergoing active dendritic initiation, but this staining diminishes dramatically as the dendritic trees of these cells mature. Collectively, these studies on diseased and normal brain offer compelling evidence that GM2 ganglioside plays a pivotal role in the regulation of dendritogenesis in cortical pyramidal neurons.

Functional coupling of glycosyl transfer steps for synthesis of gangliosides in Golgi membranes from neural retina cells

The synthesis of the oligosaccharide of gangliosides is carried out in the Golgi complex by successive sugar transfers to proper glycolipid acceptors. To examine how the product of one glycosylation step couples with the next transfer step, the endogenous gangliosides of Golgi membranes from 14-day-old chick embryo retina were labeled from CMP-[3H]NeuAc or UDP-[3H]GalNAc or UDP-[3H]Gal in conditions which do not allow vesicular intercompartmental transport. After saturation of the endogenous acceptor capacity, labeling was mostly in the immediate acceptors of the corresponding labeled sugars. However, some labeled intermediates progressed to more glycosylated gangliosides if the membranes were incubated in a second step in the presence of the necessary unlabeled sugar nucleotides. This was particularly evident in the case of membranes incubated with UDP-[3H]Gal, in which most of the [3H]Gal-labeled lactosylceramide synthesized in the first step was converted to GM3 and GD3, or to GM2 or to GD1a in a second incubation step in the presence of unlabeled CMP-NeuAc alone, or together with UDP-GalNAc, or together with UDP-Gal plus UDP-GalNAc, respectively. Conversion was time dependent and dilution-independent. Since prior reports using brefeldin A indicate that transfer steps catalyzed by GalNAc-T, Gal-T2, and Sial-T4 localize in the trans-Golgi network (TGN), our results lead to the following major conclusions: (a) transfer steps catalyzed by GalNAc-T, Gal-T2, and Sial-T4 colocalize and are functionally coupled in the TGN; (b) proximal Golgi Gal-T1, Sial-T1, and Sial-T2, and their corresponding glycolipid acceptors, extend their presence to the TGN, and (c), GalNAc-T and Sial-T2 compete for a common pool of acceptor GM3 in the synthesis of GM2 and GD3.

Intracellular lipid heterogeneity caused by topology of synthesis and specificity in transport. Example: sphingolipids

The differences in lipid composition between intracellular membranes cannot be adequately explained by local synthesis and degradation. Especially in the case of sphingolipids, which are synthesized in the Golgi complex but enriched on the cell surface and in endocytotic organelles, there is evidence for a cellular machinery that preferentially shuttles these lipids in vesicles to the cell surface. The machinery appears to involve the formation of domains of sphingolipid and cholesterol in the lumenal leaflet of Golgi membranes. Several pieces of evidence suggest that the selective anterograde transport of plasma membrane proteins may be mechanistically related to the sphingolipid domains.

Rapid internalization and intracellular metabolic processing of exogenous ganglioside by cerebellar granule cells differentiated in culture

Ganglioside GM1, tritiated at the level of the long chain base (sphingosine) [( Sph-3H]GM1), sialic acid (N-acetylneuraminic acid) [( NeuAc-3H]GM1), or terminal galactose [( Gal-3H]GM1) was supplied to cerebellar granule cells differentiated in vitro, and its metabolic processing was followed with pulse time. Using [Sph-3H]GM1 and [NeuAc-3H]GM1 the formation of radioactive compounds of catabolic origin (GM2, GM3, lactosylceramide, glucosylceramide, and ceramide) started being detectable at 10-15 min of pulse, whereas compounds of biosynthetic origin (GD1a, GD1b, GT1b, O-acetylated GT1b, spingomyelin, and sialoglycoprotein) appeared after 15-30 min of pulse. Using [Gal-3H]GM1 two radioactive substances were formed, GD1a and GT1b, with the former (produced by direct sialosylation of GM1) appearing after 30 min of pulse and the latter (formed by biosynthetic recycling of released galactose) appearing after 2 h. The radioactivity linked to all metabolites increased with increasing pulse time until 4 h. The percentage of GM1 taken up and subjected to metabolic processing was found to increase from 1.8% after 10 min of pulse to 12.5% after 4 h. Cerebellar granule cells were able to release enzymes of lysosomal origin, beta-D-N-acetylhexosaminidase and beta-D-galactosidase, into the culture medium, with the release being markedly decreased by the absence in the medium of fetal calf serum, a condition that was used for studying exogenous GM1 uptake and metabolization. However, these enzymes exerted no activity at the pH of the culture medium, and no radioactive gangliosides, besides GM1, were detected in the culture medium during pulse.(ABSTRACT TRUNCATED AT 250 WORDS)

Organelle biogenesis and intracellular lipid transport in eukaryotes

The inter- and intramembrane transport of phospholipids, sphingolipids, and sterols involves the most fundamental processes of membrane biogenesis. Identification of the mechanisms involved in these lipid transport reactions has lagged significantly behind that for intermembrane protein traffic until recently. Application of methods that include fluorescently labeled and spin-labeled lipid analogs, new cellular fractionation techniques, topographically specific chemical modification techniques, the identification of organelle-specific metabolism, permeabilized cell methodology, and yeast molecular genetics has contributed to revealing a diverse biochemical array of transport processes for lipids. Compelling evidence now exists for ATP-dependent, ATP-independent, vesicle-dependent, and vesicle-independent transport processes that are lipid and membrane specific. ATP-dependent transport processes include the transbilayer movement of phosphatidylserine and phosphatidylethanolamine at the plasma membrane and the transport of phosphatidylserine from its site of synthesis to the mitochondria. ATP-independent processes include the transbilayer movement of virtually all lipids at the endoplasmic reticulum, the movement of phosphatidylserine between the inner and outer mitochondrial membranes, and the transfer of nascent phosphatidylcholine and phosphatidylethanolamine to the plasma membrane. The ATP-independent movement of lipids between organelles is believed to be due to the action of lipid transfer proteins, but this still remains to be proved. Vesicle-based transport mechanisms (which are also inherently ATP dependent) include the transport of nascent cholesterol, sphingomyelin, and glycosphingolipids from the Golgi apparatus to the plasma membrane and the recycling of sphingolipids and selected pools of phosphatidylcholine from the plasma membrane to the cell interior. The vesicles involved in cholesterol transport to the plasma membrane are different from those involved in bulk protein transport to the cell surface. The vesicles involved in recycling sphingomyelin to and from the cell surface are different from those involved in the assembly of newly synthesized sphingolipids into the plasma membrane. The preliminary characterization of these lipid translocation processes suggests divergent rather than unifying mechanisms for lipid transport in organelle assembly.

Ganglioside GD3 biosynthesis in normal and mutant mouse embryos

CMP-sialic acid:GM3 sialyltransferase (GD3 synthase; EC 2.4.99.8) was characterized in a membrane-enriched preparation (P2 pellet) from mouse embryos at embryonic day 12 (E-12). Gangliosides GD3 and GM3 were the major radiolabeled products of the reaction. Optimum GD3 synthase activity was obtained at pH 6.0 using 0.1% detergent Triton CF-54. The Km values for GM3 and CMP-sialic acid were 55 and 80 microM, respectively. The Vmax value was calculated as 622 pmol/mg protein/hr. Ganglioside GD3, as end product, induced a two-step reduction of enzyme activity in the range of concentrations from 0 to 34 microM (40%) and from 150 to 300 microM (65%). The rate of GD3 formation was similar in whole embryos and in embryo head and body regions. GD3 synthase activity in tw1/tw1 mutant mouse embryos, which express defects in neuronal differentiation, was only 40% of that in normal wild-type (+/+) embryos. Enzyme activity in heterozygous (+/twl) embryos was similar to that in +/+ embryos. These findings suggest that the reduced GD3 synthase activity in the mutants might arise as a consequence of failed nervous system development and might reflect a secondary rather than a primary effect of the mutation.

Brief treatment of sensory ganglion neurons with GM1 ganglioside enhances the efficacy of opioid excitatory effects on the action potential

In previous studies, we showed that low (nM) concentrations of opioid prolong the action potential duration (APD) of many mouse dorsal root ganglion (DRG) neurons via Gs-linked excitatory opioid receptors, whereas micromolar opioid levels shorten the APD via Gi/Go-linked inhibitory receptors. In addition, cholera toxin-B subunit (CTX-B) selectively blocks opioid- but not forskolin-induced prolongation of the APD in DRG neurons. Since CTX-B binds with selective high affinity to GM1 ganglioside located on the cell surface, the results suggest that GM1 plays an essential role in regulating excitatory opioid receptor functions. This hypothesis was tested by treating DRG neurons in mouse DRG-cord explants with exogenous gangliosides and determining whether the efficacy of opioid agonists in prolonging the APD is enhanced. The threshold concentration of the opioids, dynorphin(1-13) and morphine required to prolong the APD in many DRG neurons was markedly decreased from nM to fM levels after bath exposure to 10 nM to 1 microM GM1 ganglioside for less than 5 min. In contrast, GM2 and GM3 gangliosides and asialo-GM1 ganglioside were ineffective, even when DRG neurons were exposed to high concentrations (1-10 microM) for periods greater than 1 h. Although GD1a, GD1b and GQ1b gangliosides appeared to be as effective as GM1 when tested at microM concentrations for 15 min, tests at lower concentrations, shorter periods, and/or at lower temperature (24 degrees vs 34 degrees C), showed that they were significantly less effective than GM1.(ABSTRACT TRUNCATED AT 250 WORDS)

Glycolipid and glycoprotein transport through the Golgi complex are similar biochemically and kinetically. Reconstitution of glycolipid transport in a cell free system

Glycolipid transport between compartments of the Golgi apparatus has been reconstituted in a cell free system. Transport of lactosylceramide (galactose beta 1-4-glucose-ceramide) was followed from a donor to an acceptor Golgi population. The major glycolipid in CHO cells is GM3 (sialic acid alpha 2-3 galactose beta 1-4-glucose-ceramide). Donor membranes were derived from a Chinese hamster ovary (CHO) cell mutant (Lec2) deficient in the Golgi CMP-sialic acid transporter, and therefore contained lactosylceramide as the predominant glycolipid. Acceptor Golgi apparatus was prepared from another mutant, Lec8, which is defective in UDP-Gal transport. Thus, glucosylceramide is the major glycolipid in Lec8 cells. Transport was measured by the incorporation of labeled sialic acid into lactosylceramide (present originally in the donor) by transport to acceptor membranes, forming GM3. This incorporation was dependent on ATP, cytosolic components, intact membranes, and elevated temperature. Donor membranes were prepared from Lec2 cells infected with vesicular stomatitus virus (VSV). These membranes therefore contain the VSV membrane glycoprotein, G protein. Donor membranes derived from VSV-infected cells could then be used to monitor both glycolipid and glycoprotein transport. Transport of these two types of molecules between Golgi compartments was compared biochemically and kinetically. Glycolipid transport required the N-ethylmaleimide sensitive factor previously shown to act in glycoprotein transport (Glick, B. S., and J. E. Rothman. 1987. Nature [Lond.]. 326:309-312; Rothman, J. E. 1987. J. Biol. Chem. 262:12502-12510). GTP gamma S inhibited glycolipid and glycoprotein transport similarly. The kinetics of transport of glycolipid and glycoprotein were also compared. The kinetics of transport to the end of the pathway were similar, as were the kinetics of movement into a defined transport intermediate. It is concluded that glycolipid and glycoprotein transport through the Golgi occur by similar if not identical mechanisms.

Glycolipid transfer protein and intracellular traffic of glucosylceramide

Glycolipid transfer protein (GL-TP), a nonglycosylated protein with a molecular weight of 22,000 K, has been purified from pig brain. The protein transfers, by a carrier mechanism, glycolipids with a beta-glucosyl or beta-galactosyl residue directly linked to either ceramide or diacylglycerol. GL-TP appears to be present in most animal cells, and evidence has been obtained which indicates that it is a cytoplasmic protein. Little is known about the function of GL-TP. Current evidence indicates that glycosphingolipid glycosylation occurs at the luminal side of the Golgi apparatus, except for the glucosylation of ceramide, which has been shown to occur at the cytoplasmic side of the Golgi or endoplasmic membrane. It appears most likely that GL-TP participates in the intracellular traffic of glucosylceramide.

Subcellular biosynthesis and transport of gangliosides formed from exogenous lactosylceramide in rat liver

In order to clarify the mechanisms of ganglioside biosynthesis and transport we intravenously administered a liposomal dispersion of radiolabelled lactosylceramide (LacCer) to rats and then followed the time course of the individual gangliosides which became radioactive in the Golgi-apparatus and plasma-membrane fractions prepared from the liver. After administration of radiolabelled LacCer the liver retained a substantial amount of radioactivity, which was distributed among an organic phase (mainly residual LacCer), a fraction containing low-Mr substances (mainly 3H2O) and a ganglioside fraction. The hepatocytes were found to provide the bulk of gangliosides biosynthesized from exogenous LacCer. After subcellular fractionation, the total radioactive gangliosides increased in the Golgi apparatus up to 8 h, to then decrease and practically disappear at 24 h; in the plasma membranes they were progressively concentrated, accounting for high absolute values. Ganglioside patterns were greatly modified with time in both the Golgi apparatus and plasma membrane, but without significant differences between them. Biosynthesis in the Golgi apparatus and accumulation in the plasma membrane of each individual ganglioside followed a precursor-product relationship. The obtained results indicated that once a ganglioside is biosynthesized in the Golgi apparatus, it is in part made available for translocation to the plasma membrane, which rapidly occurs, and is in part retained in the Golgi apparatus, where it acts as a precursor for the biosynthesis of more glycosylated gangliosides.

Localization of Forssman glycolipid and GM1 ganglioside intracellularly and on the surface of germ cells during fetal testicular and ovarian development of mice

Changes in the expression pattern and intracellular localization of Forssman glycolipid (FA) and GM1 ganglioside (GM1) in fetal mouse gonads were examined during germ cell differentiation by immunofluorescence microscopy and immunoelectron microscopy. In male germ cells from the 12th to 14th day p.c., anti-FA binding was localized in granular structures aggregated on one side of the cytoplasm and/or in the plasma membrane. On day 16 p.c., some germ cells still showed patch-like positive reactions in the plasma membrane, but by day 18 p.c., positive reactions for FA had completely disappeared. The female germ cells showed granular bindings of anti-FA scattered throughout their cytoplasm during the 13th to 16th day p.c., although the positive reactions in female germ cells on day 12 p.c. tended to be found in one side of cytoplasm and/or plasma membrane similar to those in male germ cells from 12th to 14th day p.c. On day 18 p.c., positive reactions remained in the plasma membrane of some germ cells, but these positive reactions disappeared before birth. Immunoelectron microscopic observation showed that the sites of anti-FA bindings were equivalent to the "small dense bodies" (SDB) and the Golgi lamellae both in male and female germ cells. On the other hand, GM1 was not detected in male germ cells at any time during fetal testicular development, whereas an anti-GM1 reaction was detected in the plasma membrane of female germ cells from the 16th to 18th day p.c. (oocytes in the first meiotic prophase).(ABSTRACT TRUNCATED AT 250 WORDS)

Subcellular distribution and biosynthesis of rat liver gangliosides

Gangliosides have generally been assumed to be localized primarily in the plasma membrane. Analysis of gangliosides from isolated subcellular membrane fractions of rat liver indicated that 76% of the total ganglioside sialic acid was present in the plasma membrane. Mitochondria and endoplasmic reticulum fractions, while containing only low levels of gangliosides on a protein basis, each contained approx. 10% of total ganglioside sialic acid. Gangliosides also were present in the Golgi apparatus and nuclear membrane fractions, and soluble gangliosides were in the supernatant. Individual gangliosides were non-homogeneously distributed and each membrane fraction was characterized by a unique ganglioside composition. Plasma membrane contained only 14 and 28% of the total GD1a and GD3, respectively, but 80-90% of the GM1, GD1b, GT1b and GQ1b. Endoplasmic reticulum, when corrected for plasma membrane contamination, contained only trace amounts of GM1, GD1b, GT1b and GQ1b, but 11 and 5% of the total GD1a and GD3, respectively. The ganglioside composition of highly purified endoplasmic reticulum was similar. Ganglioside biosynthetic enzymes were concentrated in the Golgi apparatus. However, low levels of these enzymes were present in the highly purified endoplasmic reticulum fractions. Pulse-chase experiments with [3H]galactose revealed that total gangliosides were labeled first in the Golgi apparatus, mitochondria and supernatant within 10 min. Labeled gangliosides were next observed at 30 min in the endoplasmic reticulum, plasma membrane and nuclear membrane fractions. Analysis of the individual gangliosides also revealed that GM3, GM1, GD1a and GD1b were labeled first in the Golgi apparatus at 10 min. These studies indicate that gangliosides synthesized in the Golgi apparatus may be transported not only to the plasma membrane, but to the endoplasmic reticulum and to other internal endomembranes as well.

Spontaneous transfer of ganglioside GM1 between phospholipid vesicles

The transfer kinetics of the negatively charged glycosphingolipid II3-N-acetylneuraminosyl-gangliotetraosylceramide (GM1) were investigated by monitoring tritiated GM1 movement between donor and acceptor vesicles. After appropriate incubation times at 45 degrees C, donor and acceptor vesicles were separated by molecular sieve chromatography. Donors were small unilamellar vesicles produced by sonication, whereas acceptors were large unilamellar vesicles produced by either fusion or ethanol injection. Initial GM1 transfer to acceptors followed first-order kinetics with a half-time of about 40 h assuming that GM1 is present in equal mole fractions in the exterior and interior surfaces of the donor vesicle bilayer and that no glycolipid flip-flop occurs. GM1 net transfer was calculated relative to that of [14C]cholesteryl oleate, which served as a nontransferable marker in the donor vesicles. Factors affecting the GM1 interbilayer transfer rate included phospholipid matrix composition, initial GM1 concentration in donor vesicles, and the GM1 distribution in donor vesicles with respect to total lipid symmetry. The findings provide evidence that GM1 is molecularly dispersed at low concentrations within liquid-crystalline phospholipid bilayers.

Incorporation of 14C-galactose into gangliosides of rabbit lens

Rabbit lenses were incubated in organ culture with 14C-galactose for 6, 12 and 20 hours. Gangliosides were extracted using the Folch-Suzuki method, purified by dialysis and analyzed by thin-layer chromatography. Six radioactive bands, including the origin, were observed. Tentative identification of these bands as N-acetylneuraminylgalactosylglucosylceramide (GM3), N-acetylgalactosaminyl-(N-acetylneuraminyl)-galactosylglucosylceramide (GM2), galactosyl-N-acetylgalactosaminyl-(N-acetylneuraminyl)- galactosylglucosylceramide (GM1), N-acetylneuraminylgalactosyl-N- acetylgalactosaminyl-(N-acetylneuraminyl)-galactosylglucosylceramide (GD1a), N-acetylneuraminylgalactosyl-N-acetylgalactosaminyl-(N- acetylneuraminyl-N-acetylneuraminyl)-galactosylglucosylceramide (GT) was made by comparison with authentic standard gangliosides. Galactose incorporation into GM3 and GM2 increased during the first 12 hours but decreased during the period from 12 to 20 hours. GD1a and GT incorporated the greatest amount of label during the period from 12 to 20 hours. Incorporation of labeled galactose into GM1 was nearly constant during this time period. Specific activities for GM1, GM3 and GT were nearly the same at 6 hours and were about half those of GM2 and GD1a for the same time period.

Fibrillar organization of fibronectin is expressed coordinately with cell surface gangliosides in a variant murine fibroblast

NCTC 2071A cells, a line of transformed murine fibroblasts, grow in serum-free medium, are deficient in gangliosides, synthesize fibronectin, but do not retain and organize it on the cell surface. When the cells are exposed to exogenous gangliosides, fibrillar strands of fibronectin become attached to the cell surface. A morphologically distinct variant of NCTC 2071A cells was observed to both retain cell surface fibronectin and organize it into a fibrillar network when the cells were stained with anti-fibronectin antibodies and a fluorescent second antibody. A revertant cell type appeared to resemble the parental NCTC 2071A cells in terms of morphology and fibronectin organization. All three cell types were subjected to mild NaIO4 oxidation and reduction with KB3H4 of very high specific radioactivity in order to label the sialic acid residues of surface gangliosides. The variant had much more surface gangliosides than the parental, particularly more complex gangliosides corresponding to GM1 and GD1a. The surface gangliosides of the revertant were intermediate between the parental and the variant. By using sialidase, which hydrolyzes GD1a to GM1, and 125I-labeled cholera toxin, which binds specifically to GM1, the identity and levels of these gangliosides were confirmed in the three cell types. When variant cells were exposed to sialidase for 2 d, there appeared to be little change in fibronectin organization. Concomitant treatment of the cells with the B subunit of cholera toxin, which bound to all the surface GM1 including that generated by the sialidase, however, eliminated the fibrillar network of fibronectin. In addition, exposure of the variant cells to a 70,000-mol-wt fragment of fibronectin, which lacks the cell attachment domain but contains a matrix assembly domain, inhibited the formation of fibers. Finally, all three cell types were assayed for their ability to attach to and spread on fibronectin-coated surfaces; no significant differences were found. Our results further establish that the ability of a cell to organize fibronectin into an extracellular matrix is dependent on certain gangliosides, but they also indicate that cell adhesion to fibronectin is independent of these gangliosides. We suggest that matrix organization and cell attachment and spreading are based on separate mechanisms and that these functions are associated with different cell surface "receptors."

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.

Defining lipid transport pathways in animal cells

A new technique for studying the metabolism and intracellular transport of lipid molecules in living cells based on the use of fluorescent lipid analogs is described. The cellular processing of various intermediates (phosphatidic acid and ceramide) and end products (phosphatidylcholine and phosphatidylethanolamine) in lipid biosynthesis is reviewed and a working model for compartmentalization during lipid biosynthesis is presented.

Studies on the turnover and subcellular localization of membrane gangliosides in cultured neuroblastoma cells

To compare the subcellular distribution of endogenously synthesized and exogenous gangliosides, cultured murine neuroblastoma cells (N1E-115) were incubated in suspension for 22 h in the presence of D-[1-3H]galactose or [3H]GM1 ganglioside, transferred to culture medium containing no radioisotope for periods of up to 72 hr, and then subjected to subcellular fractionation and analysis of lipid-sialic acid and radiolabeled ganglioside levels. The results indicated that GM2 and GM3 were the principal gangliosides in the cells with only traces of GM1 and small amounts of disialogangliosides present. About 50% of the endogenously synthesized radiolabelled ganglioside in the four major subcellular membrane fractions studied was recovered from plasma membrane and only 10-15% from the crude mitochondrial membrane fraction. In contrast, 45% of the exogenous [3H]GM1 taken up into the same subcellular membrane fractions was recovered from the crude mitochondrial fraction; less than 15% was localized in the plasma membrane fraction. The results are similar to those obtained from previously reported studies on membrane phospholipid turnover. They suggest that exogenous GM1 ganglioside, like exogenous phosphatidylcholine, does not intermix freely with any quantitatively major pool of endogenous membrane lipid.

Effect of drugs and temperature on biosynthesis and transport of glycosphingolipids in cultured neurotumor cells

Neuroblastoma and glioma cells were grown in the presence of [3H]galactose, and the incorporation of 3H into gangliosides and the transport of newly synthesized gangliosides to the cell surface were examined under different experimental conditions. A variety of drugs, including inhibitors of protein synthesis and energy metabolism, modulators of the cytoskeleton and the ionophore monensin, had no effect on the transport of newly synthesized GD1a in neuroblastoma cells. Only low temperature effectively blocked translocation to the plasma membrane. Monensin, however, had marked effects on the biosynthesis of gangliosides and neutral glycosphingolipids. Whereas incorporation of 3H into complex glycosphingolipids was reduced, labeling of glucosylceramide was increased in cells exposed to monensin. In addition, biosynthesis of the latter glycolipid was less susceptible to low temperatures than that of more complex ones. Previous studies have implicated the Golgi apparatus as the predominant site of glycosylation of gangliosides. As monensin has been reported to interfere with the Golgi apparatus, our results indicate that glucosylceramide may be synthesized at a site that is separate from the site where further glycosylation occurs. Once synthesis of a ganglioside is completed, transport of the molecule to the cell surface proceeds under conditions of cytoskeletal disruption, energy depletion and ionic inbalance , but not low temperature.

Approaches in the study of ganglioside metabolism

Ganglioside GM1, 3H-labeled in the sphingosine or terminal galactose moiety was injected into mice and its metabolic fate in the liver was followed. After administration of sphingosine-labeled GM1 all major liver gangliosides (GM3, GM2, GM1, GD1a-NeuAc, NeuG1) became radioactive, the radioactivity residing in all cases on the sphingosine moiety. The specific radioactivity was highest on GM1, followed by GM2, GM3 and GD1a-NeuAc, NeuG1. Several neutral glycosphingolipids and sphingomyelin were also formed. After administration of galactose-labelled GM1 the only radioactive gangliosides present in the liver were GM1 and GD1a-NeuAc, NeuG1, both carrying the radioactivity on the terminal galactose residue, with no formation of labelled neutral glycosphingolipids. Subcellular studies gave clear evidence that GM1, after being taken up by the liver, was mainly degraded to GM2, GM3 and neutral glycosphingolipids at the level of lysosomes. A part of it was sialylated to more complex gangliosides and some of its metabolic by-products were used for the biosynthesis of other sphingolipid species, likely at the level of the Golgi apparatus. All this suggests that exogenous GM1 is introduced in the metabolic routes of endogenous gangliosides and of other sphingolipids, which are operating in the liver.

Fluorescence labeling of cell surface glycoconjugates with Lucifer yellow CH

Lucifer yellow CH, a fluorescent hydrazide, reacted with cell surface glycoconjugates on murine thymocytes oxidized with NaIO4 or galactose oxidase. Surface labeling was monitored by fluorescence microscopy, spectrophotometry and by flow microfluorometry. No labeling was detected in unoxidized cells and fluorescence was reduced by neuraminidase and by trypsin. Lucifer yellow CH derivatives of gangliosides also were prepared and incorporated into the surface membranes of thymocytes.

Sphingolipid metabolism in cultured fibroblasts: microscopic and biochemical studies employing a fluorescent ceramide analogue

A fluorescent analogue of ceramide, N-[7-(4-nitrobenzo-2-oxa-1,3-diazole)]-epsilon-aminocaproyl sphingosine (C6-NBD-ceramide), was used to investigate sphingolipid metabolism in Chinese hamster fibroblasts. C6-NBD-ceramide was incorporated into small unilamellar dioleoyl phosphatidylcholine vesicles and incubated with cells in monolayer culture at 2 degrees C, resulting in rapid and preferential transfer of the labeled ceramide from vesicles to cells. The cells were then washed and subsequently incubated at 37 degrees C for various intervals. The metabolism of C6-NBD-ceramide was monitored by lipid extraction and analysis, and the intracellular distribution of the labeled molecule was followed by fluorescence microscopy. Initially, fluorescence was detected almost exclusively in mitochondria, with over 90% of the extractable lipid fluorescence due to C6-NBD-ceramide. After 30 min at 37 degrees C, intense fluorescence appeared in the Golgi apparatus. This organelle was identified by colocalization of NBD fluorescence with a Golgi-apparatus-specific stain. At later times the plasma membrane became visibly labeled as well, at which point 90% of the cell-associated fluorescence was recovered as NBD-labeled sphingomyelin and NBD-labeled cerebroside. These metabolites were identified by enzymatic and biochemical analysis and by thin-layer chromatography of the fluorescent lipid extracts. The finding that C6-NBD-ceramide is used by these cells in standard pathways of sphingolipid biosynthesis suggests that this fluorescent precursor will be a valuable tool for correlating the metabolism of sphingolipids with their intracellular distribution and translocation. In addition, during its metabolism by Chinese hamster fibroblasts, this compound acts as a vital stain for the Golgi apparatus.

The confined function model of the Golgi complex: center for ordered processing of biosynthetic products of the rough endoplasmic reticulum

The organized and characteristic elements of the Golgi complex (GC) are the stacked smooth-surfaced cisternae, which are found in the centrosphere of all eukaryotic cells. These cisternae, in conjunction with other associated smooth-surfaced membranes, are responsible for executing net unidirectional intracellular transport (ICT) from the rough endoplasmic reticulum (RER) toward more distally located structures. This chapter focuses on the broad range of accessory activities that occur during transport, the family of “posttranslational modifications.” These events are, in all likelihood, not essential for the “primary” function of the GC yet they are crucial in allowing the cell to tailor its biosynthetic products for its own needs and the needs of the organism as a whole. In addition to modifying products of the rough endoplasmic reticulum, the GC may be involved in processing events because of its participation in other routes of vesicular traffic—for example, centripetal traffic from the cell surface. Various nonequivalent criteria have been used to ascribe processing events to the GC-autoradiography, preparative or analytic subcellular fractionation, interruption by ICT inhibitors, and delay in the impact of cycloheximide.