Specific monosaccharide inhibition of active sodium channels in neuroblastoma cells

Oligosaccharide composition of the neurotoxin-responsive sodium channel of mouse neuroblastoma and requirement of sialic acid for biological activity

A glycoprotein, M(r) 200,000, which has the biological activity of the neurotoxin-responsive Na+ channel, was isolated from a clonal line of mouse neuroblastoma cells, N-18. The glycoprotein was purified to homogeneity in 18% yield by methods used to purify glycoproteins, which included metabolic labeling of the cells with L-[3H]fucose and binding of the radioactive glycoproteins to WGA- and lentil-Sepharose, and DEAE-cellulose. The glycoprotein has biological activity of neurotoxin-responsive ion flux when reconstituted into artificial phospholipid vesicles. This activity was shown to depend on the presence of sialic acid since treatment of the purified, reconstituted glycoprotein with Vibrio cholerae neuraminidase abolished the response to neurotoxins of 86Rb flux. The [3H]fucose-containing glycopeptides derived by Pronase digestion of the glycoprotein were characterized by affinity to immobilized lectins and contained di-, tri-, and tetra-antennary oligosaccharides in a ratio of 2:4:3. Most of the glycopeptides were sialylated as shown by binding characteristics to immobilized serotonin-Sepharose with and without neuraminidase. The structure of the diantennary oligosaccharides was elucidated by 500-MHz 1H NMR spectroscopy. The Con A-bound fraction contains alpha-NeuNAc-(2-->6)-bound group on the GlcNAc5' antenna and an alpha-NeuNAc-(2-->3)-bound groups on the GlcNAc5 antenna. An alpha-L-fucosyl group is (1-->6)-bound to the Asn core GlcNAc1 residue.

Perturbation of glycoprotein processing affects the neurotoxin-responsive Na+ channel in neuroblastoma cells

The activity of neurotoxin-responsive Na+ channels in mouse neuroblastoma cells, N-18, was examined after treating the cells with compounds that are reported to perturb intracellular traffic. The compounds used have been shown to either alter glycoprotein synthesis and processing, (swainsonine, castanospermine, monensin, and retinoic acid) or receptor mediated endocytosis (mevinolin, 7-ketocholesterol, and chloroquine), or both. All of these compounds inhibited the activity of the neurotoxin-responsive Na+ channel with the exception of retinoic acid which increased the activity. Na+ channel activity was measured by two methods: (a) In vivo, the efflux of 86Rb was measured by use of the cells in monolayer culture, and (b) in vitro, the flux of 86Rb was measured from artificial phospholipid vesicles containing the partially purified Na+ channel. In both cases, 86Rb flux responded to stimulating neurotoxins, veratridine and scorpion venom, and was inhibited by tetrodotoxin as characteristic of excitable membranes. One of the perturbing compounds, swainsonine, was examined in detail. Treatment of N-18 cells with 10 microM swainsonine for 24 h markedly reduced the activity of the neurotoxin-responsive Na+ channel, as shown by the neurotoxin-stimulated efflux of 86Rb in vivo. In addition, after reconstitution into phospholipid vesicles of the partially purified Na+ channel from swainsonine-treated cells, reduced 86Rb flux was observed when compared with that of nontreated cells. Furthermore, the activity was not recovered in other less purified fractions. A comparison of the glycopeptides from the treated and nontreated cells by size, charge, and lectin-binding affinities was consistent with the formation of hybrid oligosaccharides after swainsonine treatment. It is concluded that the oligosaccharide residues of the Na+ channel glycoprotein must be processed to the mature complex-type for full activity. The stimulation of channel activity by treatment with retinoic acid supported this conclusion.

Reconstitution in vitro of neurotoxin-responsive ion efflux by using membrane glycoproteins of neuroblastoma cells

Glycoproteins were purified from a clonal cell line of mouse neuroblastoma, N-18, labeled metabolically with L-[3H]fucose. The purified radioactive glycoproteins were reconstituted into artificial phosphatidylcholine vesicles. When the vesicles were preloaded with cesium acetate and treated with neurotoxins to activate the Na+ channel, a shift in intravesicular density was observed to a less dense position after centrifugation on sucrose gradients. This shift was partially inhibited by tetrodotoxin, which prevents the activation of the Na+ channel. A similarly derived fraction of [14C]fucose-containing glycoproteins from a neuroblastoma cell line that does not possess excitable membranes, N1A-103, was reconstituted into phospholipid vesicles, and, after preloading with cesium ions, the fraction was combined with those of the 3H-labeled glycoproteins of the differentiated cells, N-18, which have excitable membranes. Only the 3H-labeled glycoprotein-containing vesicles were responsive to the neurotoxins, as shown by a shift in intravesicular density on sucrose gradients. These results are interpreted as a demonstration of the reconstitution of glycoproteins to form the activated Na+ channel. Comparison of the radioactive glycoprotein profiles after polyacrylamide gel electrophoresis showed that glycoproteins of Mr 200,000, Mr 165,000, and Mr 65,000 were common to the reconstituted fractions that were biologically active.