Inhibition of human neuroblastoma cell proliferation and EGF receptor phosphorylation by gangliosides GM1, GM3, GD1A and GT1B

The inhibitory action of gangliosides GT1B, GD1A, GM3 and GM1 on cell proliferation and epidermal growth factor receptor (EGFR) phosphorylation was determined in the N-myc amplified human neuroblastoma cell line NBL-W. The IC50 of each ganglioside was estimated from concentration-response regressions generated by incubating NBL-W cells with incremental concentrations (5-1000 microm) of GT1B, GD1A, GM3 or GM1 for 4 days. Cell proliferation was quantitatively determined by a colourimetric assay using tetrazolium dye and spectrophotometric analysis, and EGFR phosphorylation by densitometry of Western blots. All gangliosides assayed, with the exception of GM1, inhibited NBL-W cell proliferation in a concentration-dependent manner. The IC50s for gangliosides GT1B [molecular weight (MW) 2129], GM3 (MW 1236), and GD1A (MW 1838) were (mean +/- SEM) 117 +/- 26, 255 +/- 29, and 425 +/- 44 m, respectively. In contrast, the IC50 for GM1 (MW 1547) could not be determined. Incubation of NBL-W cells with epidermal growth factor (EGF) concentrations ranging from 0.1 to 1000 ng/ml progressively increased cell proliferation rate, but it plateaued at concentrations above 10 ng/ml. EGFR tyrosine phosphorylation, however, was incrementally stimulated by EGF concentrations from 1 to 100 ng/ml. The suppression of EGF-induced EGFR phosphorylation differed for each ganglioside, and their respective inhibitory potencies were as follows: EGFR phosphorylation [area under curve (+ EGF)/area under curve (- EGF)]: control (no ganglioside added) = 8.2; GM1 = 8.3; GD1A = 6.7; GM3 = 4.87, and GT1B = 4.09. The lower the ratio, the greater the inhibitory activity of the ganglioside. Gangliosides GD1A and GT1B, which have terminal N-acetyl neuraminic acid moieties, as well as one and two N-acetyl neuraminic acid residues linked to the internal galactose, respectively, both inhibited cell proliferation and EGFR phosphorylation. However, GD1A was a more potent suppressor of cell proliferation and GT1B most effective against EGFR phosphorylation. GM3, which only has a terminal N-acetyl neuraminic acid, inhibited cell proliferation and EGFR phosphorylation almost equivalently. These data suggest that gangliosides differ in their potency as inhibitors of NBL-W neuroblastoma cell proliferation and EGFR tyrosine phosphorylation, and that perturbations in the differential expression of membrane glycosphingolipids may play a role in modulating neuroblastoma growth.     

Effects of gangliosides on the differentiation of human mesenchymal stem cells into osteoblasts by modulating epidermal growth factor receptors

Gangliosides are sialic acid-conjugated glycosphingolipids that are believed to regulate cell differentiation as well as the signals of several signal molecules, including epidermal growth factor receptors (EGFR). These compounds are localized in a glycosphingolipid-enriched microdomain on the cell surface and regulated by the glycosphingolipid composition. However, the role that gangliosides play in osteoblastogenesis is not yet clearly understood, therefore, in this study, the relationship between gangliosides and EGFR activation was investigated during osteoblast differentiation in human mesenchymal stem cells (hMSCs). The results of high-performance thin-layer chromatography (HPTLC) showed that ganglioside GM3 expression was decreased, whereas ganglioside GD1a expression was increased during the differentiation of hMSCs into osteoblasts. In addition, an increase in the activation of alkaline phosphatase (ALP) was observed in response to treatment with EGF (5 ng/ml) and GD1a (1 μM) (p < 0.05). The activation of ALP was significantly elevated in response to treatment of ganglioside GD1a with EGF when compared to control cells (p < 0.01). However, treatment with GM3 (1 μM) resulted in decreased ALP activation (p < 0.01), and treatment of hMSCs with a chemical inhibitor of EGFR, AG1478, removed the differential effect of the two gangliosides. Moreover, incubation of the differentiating cells with GD1a enhanced the phosphorylation of EGFR, whereas treatment with GM3 reduced the EGFR phosphorylation. However, AG1478 treatment inhibited the effect of ganglioside GD1a elicitation on EGFR phosphorylation. Taken together, these results indicate that GD1a promotes osteoblast differentiation through the enhancement of EGFR phosphorylation, but that GM3 inhibits osteoblast differentiation through reduced EGFR phosphorylation, suggesting that GM3 and GD1a are essential molecules for regulating osteoblast differentiation in hMSCs.     

Identity of GA1, GM1a and GD1b synthase in Golgi vesicles from rat liver

Synthesis of ganglioside GD1b from ganglioside GD2 was demonstrated using Golgi membranes isolated from rat liver. Competition experiments using gangliosides GA2, GM2 and GD2 as substrates, and as mutual inhibitors for ganglioside galactosyltransferase activity in preparations of Golgi vesicles derived from rat liver, suggested that galactosyl transfer to these three compounds, leading to gangliosides GA1, GM1a and GD1b respectively, is catalyzed by one enzyme. These results strengthen the hypothesis that the main site for the regulation of ganglioside biosynthesis occurs within the reaction sequence LacCer----GA3----GD3----GT3.     

Studies on the binding substances on human erythrocytes for the heat-labile enterotoxin isolated from chicken enterotoxigenic Escherichia coli

To study the predominant binding substance for the heat-labile enterotoxin (LTc) isolated from chicken enterotoxigenic Escherichia coli, competitive binding assays were performed with neuraminidase-treated human type B erythrocytes and 125I-labeled B subunit of LTc (LTc-B). Of all inhibitors used, the ganglioside GM1 was the most effective in inhibiting the binding of 125I-labeled LTc-B to the erythrocytes. The other gangliosides used as inhibitors, gangliosides GD1b, GD1a, GM2, GT1b and GM3, were about 24, 166, 250, 440 and at least 440 times less reactive than ganglioside GM1, respectively. With glycoproteins as inhibitors, on the other hand, hog A + H, porcine thyroglobulin and bovine salivary mucin were over 10(4) times less potent. No inhibition was obtained by other mono-, di- and polysaccharides at the highest concentrations used. These findings suggest that the predominant binding substance on neuraminidase-treated human type B erythrocytes for the LTc-B is ganglioside GM1 and that the combining site of LTc-B may be specific for the terminal disaccharide (galactose-N-acetyl-D-galactosamine)-linked portion of ganglioside GM1.     

Ganglioside binding pattern of CD33-related siglecs

Our study deals with the interaction of CD33 related-siglecs-5,-7,-8,-9,-10 with gangliosides GT1b, GQ1b, GD3, GM2, GM3 and GD1a. Siglec-5 bound preferentially to GQ1b, but weakly to GT1b, whereas siglec-10 interacted only with GT1b ganglioside. Siglec-7 and siglec-9 displayed binding to gangliosides GD3, GQ1b and GT1b bearing a disialoside motif, though siglec-7 was more potent; besides, siglec-9 interacted also with GM3. Siglec-8 demonstrated low affinity to the gangliosides tested compared with other siglecs. Despite high structural similarity of CD33 related siglecs, they demonstrated different ganglioside selectivity, in particular to the Neu5Acalpha2-8Neu5Ac motif.     

Growth- and development-dependent expression of gangliosides in rat hepatocytes and liver tissues.

Expression of gangliosides in the liver was examined in primary cultures of hepatocytes from adult rats and liver tissues from rats of different ages. Hepatocytes were isolated from 7-week-old rat liver and cultured in L-15 medium containing insulin, dexamethasone and 10% fetal bovine serum. Hepatocytes proliferated only on the first day, and then ceased proliferation. The content of GD3 and GD1a increased during the period of active proliferation and reached a nearly constant level, whereas GM1, GD1b, GT1b, and GQ1b gradually increased throughout culture. Addition of EGF to the culture medium caused significant increases in the content of GD3, and to a lesser degree of GM3, but exhibited little effect on the expression of other ganglioside species. The specific induction of GD3 and GM3 expression by EGF was reproduced under serum-free conditions, despite the lack of hepatocyte proliferation. Expression of gangliosides in cultured hepatocytes was also modulated by cell density; higher cell density brought about increased content of GM1, GD1a, GD1b, GT1b, and GQ1b with concomitant reduction of GM3 in cells. The composition of gangliosides in liver tissues demonstrated a unique developmental pattern. GD3 and GD1a were strongly expressed in E-16 embryonic tissue and rapidly decreased with increasing age. GD1b, GT1b, and GQ1b were found only in postnatal liver tissues. These findings suggest that the expression of gangliosides in rat hepatocytes and liver tissues are regulated by growth- and development-dependent factors.     

Binding specificities of heat-labile enterotoxins isolated from porcine and human enterotoxigenic Escherichia coli for different gangliosides

The binding specificities of heat-labile enterotoxins (LTp and LTh) isolated from porcine and human enterotoxigenic Escherichia coli on human erythrocytes were studied by competitive binding assays using different gangliosides as inhibitors. The binding of 125I-labeled LTp to neuraminidase-treated human type A erythrocytes was most effectively inhibited by ganglioside GM1. Ganglioside GM1 was 11 and 105 times more potent than gangliosides GD1b and GM2, respectively. Gangliosides GD1a, GT1b, and GM3 were much less potent. Similar results were also obtained in competitive binding assays with the 125I-labeled B subunit of LTh and neuraminidase-treated human type B erythrocytes, and in those with 3H-labeled ganglioside GM1 and LTp-coupled Sepharose 4B. The binding of 3H-labeled ganglioside GM1 to LTp was not effectively inhibited by galactose-beta(1----3)N-acetyl-D-galactosamine at the highest concentration used. These findings suggest that the combining sites of LTp and LTh may be specific for at least the galactose-N-acetyl-D-galactosamine-galactose (N-acetyl-neuraminic acid) portion of ganglioside GM1.     

Interleukin-2 binds to ganglioside GD(1b)

We have developed a solid matrix immunoassay to determine the binding of interleukin-2 (IL-2) to specific gangliosides. The assay establishes that recombinant human IL-2 binds to ganglioside GD(1b) but not to any other gangliosides (GM(1), GM(2), GM(3), GD(1a), GD(2), GD(3), and GT(1b)). The binding varies with the ratio of GD1b and IL-2. This assay enables distinguishing the nature of the sugar moiety of the ganglioside recognized by IL-2 and establishes the dosimetry of the ganglioside-IL-2 interaction. Since rIL-2 is administered systematically into stage IV melanoma patients, we have examined 45 tumor biopsies for GD(1b) content. The incidence of GD(1b) in tumor biopsies is 51%. We postulate that GD(1b) associated on the tumor or in the circulation of cancer patients may bind to rIL-2 and prevent the availability of rIL-2 to augment antitumor-immune response.     

Gangliosides and N-glycoproteins function as Newcastle disease virus receptors

The interaction of enveloped viruses with cell surface receptors is the first step in the viral cycle and an important determinant of viral host range. Although it is established that the paramyxovirus Newcastle Disease Virus binds to sialic acid-containing glycoconjugates the exact nature of the receptors has not yet been determined. Accordingly, here we attempted to characterize the cellular receptors for Newcastle disease virus. Treatment of cells with tunicamycin, an inhibitor of protein N-glycosylation, blocked fusion and infectivity, while the inhibitor of O-glycosylation benzyl-N-acetyl-alpha-D-galactosamide had no effect. Additionally, the inhibitor of glycolipid biosynthesis 1-phenyl-2-hexadecanoylamino-3-morpholino-1-propanol blocked viral fusion and infectivity. These results suggest that N-linked glycoproteins and glycolipids would be involved in viral entry but not O-linked glycoproteins. The ganglioside content of COS-7 cells was analyzed showing that GD1a was the major ganglioside component; the presence of GM1, GM2 and GM3 was also established. In a thin-layer chromatographic binding assay, we analyzed the binding of the virus to different gangliosides, detecting the interaction with monosialogangliosides such as GM3, GM2 and GM1; disialogangliosides such as GD1a and GD1b, and trisialogangliosides such as GT1b. Unlike with other viruses, our results seem to point to the absence of a specific pattern of gangliosides that interact with Newcastle disease virus. In conclusion, our results suggest that Newcastle disease virus requires different sialic acid-containing compounds, gangliosides and glycoproteins for entry into the target cell. We propose that gangliosides would act as primary receptors while N-linked glycoproteins would function as the second receptor critical for viral entry.     

Exogenous gangliosides enhance the interaction of fibronectin with ganglioside-deficient cells

The major cell-surface glycoprotein fibronectin mediates a variety of cellular adhesive interactions that have been reported to be competitively inhibited by gangliosides. These effects suggest a possible function of gangliosides as receptors for fibronectin. To test this hypothesis more directly, we examined the interaction of endogenous fibronectin with a ganglioside-deficient cell line, NCTC 2071. These cells, which grow in serum-free medium, synthesized fibronectin. The fibronectin did not bind to these cells, but instead bound diffusely to the culture substratum. When the cells were cultured in medium containing ganglioside, the fibronectin became bound to the cell surface in fibrillar strands. The order of effectiveness of purified gangliosides was GT1b greater than GD1a greater than GM1 greater than GM2 greater than GM3. The effect with mixed gangliosides was accompanied by a restoration of cellular capacity to bind and to respond to cholera toxin. Treatment of the cells with several phospholipids did not alter fibronectin binding. Our results support the hypothesis that gangliosides can help mediate the binding of fibronectin to fibroblasts.     

Selective ganglioside desialylation in the plasma membrane of human neuroblastoma cells*

Gangliosides of the plasma membrane are important modulatorsof cellular functions. Previous work from our laboratory hadsuggested that a plasma membrane sialidase was involved in growthcontrol and differentiation in cultured human neuroblastomacells (SK-N-MC), but its substrates had remained obscure. Wenow performed sialidase specificity studies in subcellular fractionsand found ganglioside GM3 desialylating activity in presenceof Triton X-100 to be associated with the plasma membrane, butabsent in lysosomes. This Triton-activated plasma membrane enzymedesialylated also gangliosides GDla, GD1b, and GT1b, therebyforming GM1; cleavage of GM1 and GM2, however, was not observed.Sialidase activity towards the glycoprotein fetuin with modifiedC-7 sialic acids and towards 4-methylumbelliferyl neuraminatewas solely found in lysosomal, but not in plasma membrane fractions. The role of the plasma membrane sialidase in ganglioside desialylationof living cells was examined by following the fate of [³H]galactose-labelledindividual gangliosides in pulse-chase experiments in absenceand presence of the extracellular sialidase inhibitor 2-deoxy-2,3-dehydro-N-acetylneuraminicacid. When the plasma membrane sialidase was inhibited, radioactivityof all gangliosides chased at the same rate. In the absenceof inhibitor, GM3, GD1a, GD1b, GD2, GD3 and GT1b were degradedat a considerably faster rate in confluent cultures, whereasthe GM1-pool seemed to be filled by the desialylation of highergangliosides. The results thus suggest that the plasma membranesialidase causes selective ganglioside desialylation, and thatsuch surface glycolipid modification triggers growth controland differentiation in human neuroblastoma cells. ganglioside neuroblastoma cells plasma membrane sialidase     

Developmental Changes in Ganglioside Composition and Synthesis in Embryonic Rat Brain

Developmental changes in ganglioside composition and biosynthesis was studied in rat brain between embryonic day (E) 14 and birth. In E14 brains, GM3 and GD3 were predominant. At E16, "b" series gangliosides, such as GD1b, GT1b, and GQ1b, increased in content. After E18, "a" series gangliosides such as GM1, GD1a, and GT1a increased in content, and the content of GM3 and GD3 markedly decreased. Because of these changes in composition, we determined the activities, in homogenates of embryonic brains, of two key enzymes of ganglioside synthesis: sialyltransferase for the synthesis of GD3 from GM3 and N-acetylgalactosaminyltransferase for GM2 synthesis from GM3. The sialyltransferase activity (GM3----GD3) was constant between E14 and E18 but decreased rapidly from E18 to birth. In contrast, the N-acetylgalactosaminyltransferase activity (GM3----GM2) increased between E14 and E18 but was constant from E18 to birth. These changes in ganglioside composition and enzymatic activities indicate that during development there is a shift from synthesis of the simplest gangliosides of the "a" and "b" pathways to synthesis of the more complex gangliosides.     

Stimulatory effect of gangliosides on phagocytosis, phagosome–lysosome fusion, and intracellular signal transduction system by human polymorphonuclear leukocytes

Gangliosides are known to be differentiation-inducing molecules in mammalian stem cells. We studied the interaction between the molecular structure of glycosphingolipids (GSLs) and their promoting mechanisms of the phagocytic processes in human polymorphonuclear leukocytes (PMN). The effect of various gangliosides from mammalian tissues on adhesion, phagocytosis, phagosome–lysosome (P–L) fusion and superoxide anion production was examined by human PMN using heat-killed cells of Staphylococcus aureus coated with GSLs. Gangliosides GM3, GD1a, GD3 and GT1b showed a marked stimulatory effect on the phagocytosis and P–L fusion in a dose-dependent manner, while ganglioside GM1, asialo GM1 and neutral GSLs did not. The relative phagocytic rate of ganglioside GM3-coated S. aureus was the highest among the tested GSLs. Both P–L fusion rate and phagocytosis of S. aureus were elevated significantly when coated with ganglioside GD1a, GD3 or GT1b, and GT1b gave a five times higher rate than that of the non-coated control. These results suggest that the terminal sialic acid moiety is essential for the enhancement of phagocytosis and that the number of sialic acid molecules in the ganglioside is related to the enhancement of the P–L fusion process. On the other hand, the superoxide anion release from PMN was not affected by ganglioside GM2, GM3, GD1a or GT1b. Furthermore, to clarify the trigger or the signal transduction mechanism of phagocytic processes, we examined the effect of protein kinase inhibitors such as H-7, staurosporine (protein kinase C inhibitor), H-89 (protein kinase A inhibitor), genistein (tyrosine kinase inhibitor), ML-7 (myosin light chain kinase inhibitor), and KN-62 (Ca2+/calmodulin-dependent protein kinase II inhibitor) on ganglioside-induced phagocytosis. H-7, staurosporine and KN-62 inhibited ganglioside-induced phagocytosis in the range of concentration without cell damage, while H-89, genistein and ML-7 did not. Moreover, H-7 and KN-62 inhibited ganglioside-induced P–L fusion. These results suggest that protein kinase C and Ca2+/calmodulin-dependent protein kinase II may be involved in the induction of phagocytosis and P–L fusion stimulated by gangliosides. This revised version was published online in November 2006 with corrections to the Cover Date.     

Genetic polymorphism of rat liver gangliosides

Liver ganglioside patterns of eight rat strains were classified according to two phenotypes: SHR type, characterized by predominance of b-series gangliosides (GD1b, GT1b, GQ1b), and DA type, characterized by predominance of a-series gangliosides (GM1, GD1a). Comparison of ganglioside pattern expressed in the liver of F1 hybrids and backcross F2 hybrids indicated that SHR type is controlled by a single autosomal-dominant gene which probably determines the expression of sialytransferase 2 activity for synthesis of GD3 from GM3.     

Ganglioside patterns and cholera toxin--peroxidase labeling of aggregating cells from the chick optic tectum.

2The ganglioside compositions of the chick optic tectum and aggregating tectal cell cultures were examined. Both showed similar trends in changes in ganglioside patterns during development. GD3 and GD1b were the predominant gangliosides early in development, while GD1a and several other multisialogangliosides increased in relative amounts with increasing age in vivo and in vitro. Four gangliosides were present early in development which have not previously been reported. These gangliosides are not present at later developmental times suggesting a possible role for them during the critical early stages of nervous tissue differentiation. Some differences were noted when comparing in vivo versus in vitro ganglioside patterns; these differences may possibly be due to the lack of normal retinotectal connections in the cultures. Cytochemical studies on the localization of the presumed cholera toxin--peroxidase binding site GM1 showed conjugate binding correlates with increasing levels of GM1 in the cultures. In older cultures, the conjugate was uniformly localized on all cells and processes in the aggregates. The conjugate also bound to synaptic membranes and intensely stained the synaptic cleft. This latter observation suggests an enrichment of GM1 in the synaptic cleft region.     

Theoretical studies on the conformation of monosialogangliosides and disialogangliosides

The preferred conformation of gangliosides GM3, GM2, GM1, GD1a and GD1b have been studied by computing their potential energies. The conformation of NeuNAc in GM3 differs from that expected for the same residue in GM2 and GM1. The NeuNAc residues in GM2 and GM1 exhibit identical conformations. Theory predicts that the terminal NeuNAc of GD1a is conformationally similar to that of GM3 and that the internal one is similar in conformation to those present in GM2 and GM1 in agreement with NMR studies. The differences in chemical shifts of the C2 and C3 carbons of the internal and terminal NeuNAc of GD1a have been attributed to differences in orientation. The present studies suggest that the binding site of cholera toxin is much smaller than that of tetanus toxin. The preferred shape of these gangliosides correlate well with their biological properties.     

Ganglioside GM3 inhibition of EGF receptor mediated signal transduction

Ganglioside GM3 is a membrane component that has been describedto modulate cell growth through inhibition of EGF receptor associatedtyrosine kinase. In order to determine if the inhibition ofcell growth by this ganglioside is specifically mediated throughEGF receptor signaling, the effects of GM3 on key enzymes implicatedin EGF signaling were determined and compared to another inhibitorof the EGF receptor kinase. Treatment of A1S cells in cultureby GM3 or a tyrosine kinase inhibitor, leflunomide, led to theinhibition of MAP kinase and PI3 kinase activities. There wasno detectable effect on phosphotyrosine phosphatases. In a cellfree system, however, GM3 had no effect on the activity of thesesignaling intermediates. Leflunomide was able to directly inhibitMAP kinase activity. GM3 and leflunomide were also found toact differently on the expression of the early immediate genes.The expression of c-fos and c-jun was inhibited by both GM3and leflunomide. The expression of c-myc, however, was onlyinhibited by leflunomide. These findings suggest that the actionof GM3 on cell growth and signaling is specifically mediatedby EGF receptor and that this ganglioside does not act directlyon the intracellular intermediates of EGF receptor signaling.In addition, soluble small molecule tyrosine kinase inhibitorssuch as leflunomide can directly affect the activity of MAPkinases and possibly other signaling intermediates. The directeffects of leflunomide on signaling intermediates may explainthe differential effects of leflunomide and GM3 on gene expressionand cell growth. cell growth epidermal growth factor gangliosides GM3 signal transduction     

Specific ganglioside changes in extraneural tissues of adult rats with hypothyroidism

Adults rats with hypothyroidism were prepared by administration of 6-propyl-2-thiouracil (PTU) or methimazole, and the tissues were examined for their gangliosides through methods including glycolipid-overlay techniques. Normal thyroid tissue contained GM3, GD3, and GD1a as the major gangliosides, with GM1, GD1b, GT1b, and GQ1b in lesser amounts. The goitrous tissue of PTU-induced hypothyroid rats had higher concentrations of GM1 and GD1a with a concomitant decrease of GM3. The amount of GT3 in thyroid tissue was increased in hypothyroid animals. While normal liver tissue had a complex ganglioside pattern with a- and b-series gangliosides, the PTU-induced hypothyroid tissue showed a simpler ganglioside profile that consisted mainly of a-series gangliosides with almost undetectable amounts of b-series gangliosides. The expression of c-series gangliosides was suppressed in the hypothyroid liver tissue. Heart tissue had higher contents of GM3 and GT3 than control. No apparent change was observed in the compositions of major and c-series gangliosides in other extraneural tissues (i.e., kidney, lung, spleen, thymus, pancreas, testis, skeletal muscle, and eye lenses), and neural tissues (i.e., cerebrum and cerebellum) from PTU-induced hypothyroid rats. The ganglioside changes of thyroid, liver, and heart tissues were reproduced in corresponding tissues of methimazole-induced hypothyroid rats. These results suggest that hypothyroid conditions affect the biosynthesis and expression of gangliosides in specific tissue and cell types.     

Modulation of EGF receptor activity by changes in the GM3 content in a human epidermoid carcinoma cell line, A431

Gangliosides have been described as modulators of growth factor receptors. For example, GM3 addition in cell culture medium inhibits epidermal growth factor (EGF)-stimulated receptor autophosphorylation. Furthermore, depletion of ganglioside by sialidase gene transfection appeared to increase EGF receptor (EGFR) autophosphorylation. These data suggested that changes in GM3 content may result in different responses to EGF. In this study, the ceramide analog d-threo-1-phenyl-2-decannoylamino-3-morpholino-1-propanol ([D]-PDMP), which inhibits UDP-glucose-ceramide glucosyltransferase, and addition of GM3 to the culture medium were used to study the effects of GM3 on the EGFR. Addition of 10 microM [D]-PDMP to A431 cells resulted in significant GM3 depletion. Additionally, EGFR autophosphorylation was increased after EGF stimulation. When exogenous GM3 was added in combination with [D]-PDMP, the enhanced EGFR autophosphorylation was returned to control levels. [D]-PDMP also increased EGF-induced cell proliferation, consistent with its effect on autophosphorylation. Once again, the addition of GM3 in combination with [D]-PDMP reversed these effects. These results indicate that growth factor receptor functions can be modulated by the level of ganglioside expression in cell lines. Addition of GM3 inhibits EGFR activity and decrease of GM3 levels using [D]-PDMP treatment enhances EGFR activity. Modulation of growth factor receptor function may provide an explanation for how transformation-dependent ganglioside changes contribute to the transformed phenotype.     

Ganglioside Composition of Normal and Mutant Mouse Embryos

The enrichment of gangliosides in neuronal membranes suggests that they play an important role in CNS development. We recently found a marked tetrasialoganglioside deficiency in twl/twl mutant mouse embryos at embryonic day (E)-11. The recessive twl/twl mutants die at embryonic ages E-9 to E-18 from failed neural differentiation in the ventral portion of the neural tube. In the present study, we examined the composition and distribution of gangliosides in twl/twl mutant mouse embryos at E-12. The total ganglioside sialic acid concentration was significantly lower in the mutants than in normal (+/-) embryos. The mutants also expressed significant deficiencies of gangliosides in the "b" metabolic pathway (GD3, GD1b, GT1b, and GQ1b) and elevations in levels of gangliosides in the "a" metabolic pathway (GM3, GM2, GM1, and GD1a). These findings suggest that the mutants have a partial deficiency in the activity of a specific sialyltransferase in the b pathway. Regional ganglioside distribution was also studied in E-12 normal mouse embryos. The ganglioside composition in heads and bodies was similar to each other and to whole embryos. Total ganglioside concentration and the distribution of b pathway gangliosides were significantly higher in neural tube regions than in nonneural tube regions. These findings suggest that b pathway gangliosides accumulate in differentiating neural cells and that the deficiency of these gangliosides in the twl/twl mutants is closely associated with failed neural differentiation.