Biosynthesis and biodegradation of rat brain gangliosides studied in vivo

Comparison of receptors for cholera and Escherichia coli enterotoxins in human intestine

Extraction of lipids from human small intestinal epithelial cells or brush borders removed specific binding sites for cholera toxin completely, but only about 50% of the receptor sites for Escherichia coli heat-labile enterotoxin. Both cholera toxin and E. coli heat-labile enterotoxin bound strongly to ganglioside GM1 in the lipid extract and, to a lesser extent, to another monosialoganglioside and to GD1b. The results suggest that E. coli heat-labile enterotoxin binds to both ganglioside and glycoprotein receptor sites of the human small intestinal epithelium, whereas cholera toxin binding was restricted to the ganglioside receptors.

Gangliosides and sialosylglycoproteins in coated vesicles from bovine brain

The content of gangliosides and sialosylglycoproteins was investigated in a coated-vesicle-enriched fraction prepared from bovine brain by the method of Pearse [(1975) J. Mol. Biol. 97, 93-98] and further purified by g.p.c. (glass-permeation chromatography) [Pfeffer & Kelly (1981) J. Cell Biol. 91, 385-391]. From morphological criteria and from the analysis of the polypeptide pattern on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis the coated-vesicle fraction (CV-fraction) appeared more than 95% pure. The ganglioside-NeuAc (N-acetylneuraminate), glycoprotein-NeuAc, phospholipid and cholesterol contents of CV-fraction were compared with those of bovine brain synaptic plasma membranes (SPM). The cholesterol to phospholipid molar ratio was 0.47 +/- 0.07 in CV-fraction and 1.06 +/- 0.08 in SPM. The ganglioside-NeuAc and glycoprotein-NeuAc to phospholipid molar ratios were 0.047 and 0.020 respectively in CV-fraction and 0.039 and 0.016 respectively in SPM. The (Na+ + K+)-dependent ATPase activity sensitive to ouabain (in mumol of Pi/h per nmol of phospholipid) was 1.04 in CV-fraction and 0.63 in SPM; the ratio between this activity and the activity resistant to ouabain was 2 in CV-fraction and 1.4 in SPM. A t.l.c. analysis of the ganglioside fractions showed that most of the ganglioside species present in SPM were present in CV-fraction. In a rat brain coated-vesicle preparation not subjected to g.p.c., the activities [as sugar-radioactivity (c.p.m.) transferred/h per mumol of phospholipid] of the enzymes CMP-NeuAc:sialosyl-lactosylceramide (GM3) sialosyl-, UDP-Gal:N-acetylgalactosaminyl(sialosyl)lactosylceramide (GM2) galactosyl- and UDP-GalNAc:sialosyl-lactosylceramide (GM3) N-acetylgalactosaminyl-transferases, which were considered Golgi-apparatus markers, were about 19, 16 and 10% respectively of those determined in rat brain neuronal perikaryon-enriched fractions. Taken together, the results indicate that most of the major gangliosides are constituents of coated vesicles.

Biosynthesis of gangliosides in cultured retina from chick embryos

Retina tissue from 7-day chick embryos was maintained in culture for up to 10 days. After 5 days in culture the incorporation of [3H]leucine into proteins and of [3H]glucosamine into gangliosides was similar to that found in retinas from 12-day embryos. The incorporation of [3H]thymidine into DNA decreased steadily with time in culture; after 5 days it was about 20% of the initial value and approximately twice that determined in retinas from 12-day embryos. The radioactivity pattern of gangliosides labeled with [3H]glucosamine showed a predominance of the label in disialosyllactosylceramide (GD3); up to the 3rd day of culture. From then on, there was a progressive increase in the labeling of disialosylgangliotetraosylceramide (GD1a); by day 7 of culture, labeling of GD1a predominated and the labeling pattern was indistinguishable from that found in retinas from 12-day-old embryos. The specific activities of the CMP-NeuAc:GM3 sialosyl- and UDP-GalNAc:GM3 N-acetylgalactosaminyl-transferases decreased to 15% and increased to 400%, respectively, of the values determined in the retinas of 7-day embryos. The cultured retinas progressed in their organization into layers with culture time. The labeling transition from GD3 to GD1a was also detected after inhibition of the histotypic organization by addition of 5-bromo 2-deoxyuridine to the culture medium. Results suggest that high activity of GM3:sialosyl transferase and high labeling of GD3 are associated with the proliferative state of retina cells, while high activity of GM3:N-acetylgalactosaminyltransferase and high labeling of GD1a are associated with the non-proliferative, differentiated state of these cells.

Rabbit intestinal glycoprotein receptor for Escherichia coli heat-labile enterotoxin lacking affinity for cholera toxin

The receptors for cholera toxin and Escherichia coli heat-labile toxin (LT) in rabbit small intestinal epithelium were characterized and compared. (i) In vivo studies in ligated intestinal loops showed that whereas LT B subunits could block the fluid secretogenic action of purified LT as well as cholera toxin, cholera toxin B subunits did not inhibit the LT response even when tested in a concentration 100-fold higher than one which gave complete blocking of cholera toxin action. (ii) In vitro studies indicated that isolated intestinal epithelial cells or brush-border membranes could bind about 10-fold more of E. coli LT than of cholera toxin. (iii) All binding sites for cholera toxin in duodenal, jejunal, or ileal mucosal cells or brush-border membranes were extracted by chloroform-methanol-water (4:8:3), which removed lipids quantitatively but did not extract glycoproteins. The extracted cholera toxin binding sites were to greater than 95% recovered in a monosialoganglioside fraction; quantitatively these sites closely corresponded to the concentration of chromatographically identified mucosal GM1 ganglioside (1 nmol of cholera toxin was bound per 1 to 2 nmol of GM1). In contrast, a substantial fraction of mucosal binding sites for E. coli LT remained in the delipidized tissue residue, and these sites had properties consistent with a glycoprotein nature. Thus, whereas cholera toxin appeared to bind highly selectively to GM1 ganglioside receptor sites of rabbit small intestine, E. coli LT bound both to GM1 ganglioside and to a main glycoprotein receptor for which cholera toxin lacks affinity.

Uniform distribution and similar turnover rates of individual gangliosides along axons of retinal ganglion cells in the chicken

In 5-month-old chickens, an intracranial injection of N-[3H]acetylmannosamine led to a labeling of all optic lobe ganglioside species in a fashion paralleling the relative ganglioside distribution. In contrast, after an intraocular injection of the same precursor, the optic nerve and the optic lobe connected to the injected eye, possessed an exceptionally high labelling of GD1a (in comparison with GD1a-sialic acid), and only negligible incorporation of radioactivity into the myelin-specific GM4 and into a fraction migrating close to GM1. Subtracting both these very low labelling fractions from the total gave a percentage distribution of ganglioside sialic acid which now corresponded well to the distribution of radioactivity along the whole optic nerve, including the region of nerve terminals in the optic lobe. This pattern of ganglioside labelling, which indicates that GD1a carries about 60% of total ganglioside sialic acid of retinal ganglion cell axons, did not change remarkably during post-hatching development up to 5 months. Long-time incorporation studies revealed similar turnover rates of the main retinal ganglion cell gangliosides. The average half-lives were 34 (GD1a), 35 (GQ1b), 36.3 (GT1b) and 38.5 days (GD3). The findings suggest that the retinal ganglion cell axons and their presynaptic terminals possess a similar ganglioside pattern, characterized by a high content of GD1a.

Disposition of gangliosides and sialosylglycoproteins in neuronal membranes

Labeled gangliosides and glycoproteins were obtained by incubation of homogenized neuronal perikarya from rat brain with CMP-[3H]N-acetyl neuraminic acid. The highest degree of labelling was observed in a subcellular fraction that also showed the highest specific activities for several ganglioside glycosyltransferases. The [3H]sialosylglycoconjugates of this fraction remained associated with the membranes after treatment with 1 M-KCl, 125 mM-EDTA, repeated freezing and thawing, or controlled sonication, but were solubilized by sodium deoxycholate (DOC) at a concentration high enough to solubilize the choline phospholipids. About 75% of th neuraminidase-labile sialosyl residues of these labeled endogenous gangliosides and glycoproteins were protected from the action of added neuraminidase or pronase or both enzymes added together. The protection was not abolished by pretreatment of the membranes with high ionic strength or with EDTA but was abolished by sonication or low concentration of DOC. Between 50 and 80% of the neuraminidase-labile sialosyl residues of the gangliosides of the neuronal perikaryon membrane fraction labelled in vivo by an intracerebral injection of N-[3H]acetylmannosamine were, at 3 h after the injection, also protected from the action of added neuraminidase. The protection was abolished by the addition of DOC. In contrast with behaviour of the labeled glycoconjugates of this neuronal perikaryon fraction, the gangliosides and sialosylglycoproteins from intact synaptosomes were accessible to neuraminidase. It is suggested that most gangliosides and sialosylglycoproteins are sialosylated as intrinsic components of the neuronal perikaryon membrane fraction and that at some stage of the process of transport through the axon and incorporation into the synaptic plasma membrane they change their accessibility to added enzymes.

Chemical structures of three fucogangliosides isolated from nervous tissue of mini-pig

In the nervous tissue of miniature pig, type Göttingen, three fucosyl-containing gangliosides are major gangliosides. All three were found in dorsal-root ganglia and spinal cord, but only two of them in the forebrain. The concentrations of these gangliosides were increased in chloroquine intoxication. Their chemical structures were determined by component analysis, partial acid and enzymatic degradation and assay of the partially methylated sugars after hydrolysis, reduction and acetylation. Mass spectrometric analysis of the intact gangliosides as permethylated, permethylated-reduced and permethylated-reduced-silylated derivatives confirmed the following structures of the three gangliosides. (See Formula in text).

Turnover of free sialic acid, CMP-sialic acid, and bound sialic acid in rat brain

Adult male rats were injected intraventricularly with N-[3H]acetylmannosamine. After different time intervals the rats were killed and free sialic acid, CMP-sialic acid, lipid- and protein-bound sialic acid were isolated from brain and the specific radioactivities determined. Maximal specific radioactivity was reached after approximately 4 h for CMP-sialic acid, after 10-12 h for free sialic acid and after approximately 42 h for lipid- and protein-bound sialic acid. After some days the specific radioactivities of all four pools were the same and decreased equally, with a calculated turnover rate of approximately 3.5 weeks. The conclusion was that this phenomenon was the result of reutilisation of sialic acid and/or precursors. Therefore, the calculated turnover is not the turnover of bound sialic acid, but merely the rate of leakage of sialic acid and/or precursors out of the brain, so that no real turnover can be measured by this method. The first few hours after injection the specific radioactivity of CMP-sialic acid rose above that of free sialic acid. It is supposed that a compartmentalization exists of free sialic acid. The newly synthesised sialic acid molecules are not secreted into the cytoplasmic pool but are preferentially used for the synthesis of CMP-sialic acid. The results and conclusions are discussed in view of the general problems concerning turnover measurements of glycoconjugates.

Turnover in vivo of lactosyl-ceramide and other glycosphingolipids in the adult rat brain

The quantitative contribution of glucose to the biosynthesis of lactosyl-ceramide and other glycosphingolipids was studied in the adult rat brain in vivo using a semicompartmental model. Half-lives of glucose carbon in both the total carbon pool and the carbohydrate residue of the lipid were calculated. In all glycolipids the half-life of carbohydrate units was six to eight times shorter than the half-life of carbon in the total carbon pool of the same lipid. This carbohydrate half-life appears to be closely related to the turnover rate of the glycolipid. The shortest carbohydrate half-life (2.2 days) was obtained for lactosyl-ceramide followed by gangliosides, galactosyl-ceramides, and sulphatides. The results indicate that lactosyl-ceramide may serve as a branch point for the biosynthesis of cerebral gangliosides in vivo, rather than occur as a breakdown product of more complex molecules.

Studies on glycosphingolipid biosynthesis by lectin-stimulated human lymphocytes

1. Human peripheral lymphocytes, normal and stimulated by phytohemagglutinin or concanavalin A. were investigated with respect to their ability to biosynthesize neutral and acidic glycosphingolipids from D-[U-14C]glucose, D-[U-14C]galactose and D-[U-14C]glucosamine as precursors. 2. Galactose and glucosamie are taken up selectively, in the presence of excessive glucose concentrations. Labeling of total neutral glycosphingolipids from D-[U-14]galactose in normal cells decreases after the first 6 h while in stimulated cells there is a fourfold and a sevenfold increase after 6 and 12 h of incubation respectively. Under similar conditions stimulation with concanavalin A gives a fourfold increase after 12 h of incubation. 3. Analysis of individual glycosphingolipids biosynthesized from [U-14C]galactose shows that lactosylceramide is the major radioactive neutral glycosphingolipid and that stimulation by phytohemagglutinin yields an almost sevenfold and 13-fold increase in the radioactivity incorporated within 6 and 12 h of incubation respectively. Glucosylceramide shows about an eightfold increase, globotetraosylceramide a threefold increase and globotriaosylceramide a fourfold increase. The rate of incorporation into glucosylceramide of stimulated lymphocytes declines after 6 h of incubation, accompanied by a concomitant increase of incorporation into lactosylceramide. 4. At 12 h of incubation the ratios of radioactivities incorporated into neutral glycosphingolipids of phytohemagglutinin-stimulated cells compared to normal cells were 1.0 for D-[U-14C]glucose 7.0 for D-[U-14C]galactose and 2.5 for D-[U-14C]glucosamine. Respective ratios for lactosylceramide are 1.0 for [U-14C]glucose and 13.0 for [U-14C]galactose. These differences did not arise from changes of the uptake of the glycosyl precursor by the cell due to stimulation. 5. Incorporation of D-[U-14C]galactose into II3-N-acetylneuraminosyllactosylceramide by cells stimulated by phytohemagglutinin and concanavalin A is enhanced 14-times and 15-times respectively. With [U-14C]glucosamine as precursor, this increase in the labeling is much more impressive, 80-fold after 12 h of incubation by the phytohemagglutinin-stimulated lymphocytes. Neuraminidase treatment and gas radiochromatographic analysis of the labeled compound derived from [U-14C]-galactose as precursor indicate that 89% of the radioactivity was incorporated into the glucosyl and galactosyl moieties, in a ratio 1:1. With [U-14C]glucosamine as precursor, a selective labeling of the sialyl moiety of the II3-N-acetylneuraminosyllactosylceramide was indicated. 6. The pattern of complex gangliosides (more complex than II3-N-acetylneuraminosyllactosylceramide) which are biosynthesized after phytohemagglutinin stimulation of the cells, show no significant differences when compared to the patterns obtained from normal cells.

Characteristics of the brain tissue disialoganglioside G2a

Disialoganglioside G2a was separated by one dimensional two-step TLC between the two ganglioside fractions using long plate precoated with silica H+ florisil in the particular solvent system. Behaviour of this fraction to the enzyme effect and molecule percentages of its components showed the characteristics of the ganglioside containing two N-acetylneuraminic acids. Synthesis dynamics of this fraction in rat brain was very intensive in the myelination period and it practically ended in 40th day. In mutant "Jimpy" mice brain ganglioside G2a served as an indicator of the normal ganglioside synthesis in relation to the changes in myelin sphingolipid molecular structure. Comparative brain studies undertaken in various species, intracellular rat brain fractions, various structures of Torpedo ocellata electric fish, emphasized the importance of this ganglioside in cell membrane formation where it is obligatory component. Quantitative determination of this fraction in Astrocytoma with various malignancy degree offers great help in differential diagnosis.

The biosynthesis of brain gangliosides. Separation of membranes with different ratios of ganglioside sialylating activity to gangliosides

Brain subcellular fractions were analysed for ganglioside-sialylating activity by measuring the incorporation of N-[3H]acetylneuraminic acid from CMP-N-[3H]acetylneuraminic acid into endogenous ganglioside acceptors (endogenous incorporation) and into exogenous lactosyceramide (haematoside synthetase activity). The ratios of endogenous incorporation to gangliosides and of haematoside synthetase to gangliosides for the synaptosomal and mitochondrial fractions from a washed crude mitochondrial fraction were lower than those obtained for other membrane fractions. The differences appear to reflect intrinsic characteristics of each membrane fraction. The results of labelling in vitro and the time course of labelling of gangliosides of the different subcellular fractions in vivo after injection of N-[3H]acetylmannosamine are consistent with the possibility of a subcellular site for synthesis of gangliosides different from that of ganglioside deposition.

Changes in rat brain gangliosides following active avoidance conditioning

Brain gangliosides of rats trained in a conditioned avoidance Sidman task and undisturbed rats in their cages were studied. The (14C) acetate was injected intracerebrally seven days before the starting of 30 days training. Thirty-seven days after injection all rats were killed and ganglioside fractions were isolated from neocortex, hippocampus, brain stem, cerebellum and residual cerebral tissue of each one brain. Trained rats had higher levels of (14C)-labeled polysialogangliosides (G1, G2, G3) in hippocampus and neocortex than the controls. Regarding the rest of the brain areas, a significant increase of G2 in the residual cerebral tissue of the trained as compared with the controls was found. The results suggest that the sialic acid rich gangliosides of only certain parts of the brain are affected by the Sidman avoidance conditioning of the animals.

Biochemical evidence on the role of gangliosides in nerve-endings

From the biochemical and developmental data in the literature, there appears to be some specific relationship of gangliosides to synapses. Whether the concentration of polysialo-gangliosides is located in the immediately pre-synaptic part of the neuronal plasma membrane is not known. There are many unanswered questions about the gangliosides, which we have attempted to pose in this article. There are as many hypotheses about roles for gangliosides, and virtually no data to back them up. In fact there are not even real indications of possible functions. We believe that it will be very difficult to define the functions of gangliosides until more of the basic questions are answered-even if their synaptic localization is a constant temptation to speculate.

The metabolism of Tay-Sachs ganglioside: catabolic studies with lysosomal enzymes from normal and Tay-Sachs brain tissue

The catabolism of Tay-Sachs ganglioside, N-acetylgalactosaminyl- (N-acetylneuraminosyl) -galactosylglucosylceramide, has been studied in lysosomal preparations from normal human brain and brain obtained at biopsy from Tay-Sachs patients. Utilizing Tay-Sachs ganglioside labeled with (14)C in the N-acetylgalactosaminyl portion or (3)H in the N-acetylneuraminosyl portion, the catabolism of Tay-Sachs ganglioside may be initiated by either the removal of the molecule of N-acetylgalactosamine or N-acetylneuraminic acid. The activity of the N-acetylgalactosamine-cleaving enzyme (hexosaminidase) is drastically diminished in such preparations from Tay-Sachs brain whereas the activity of the N-acetylneuraminic acid-cleaving enzyme (neuraminidase) is at a normal level. Total hexosaminidase activity as measured with an artificial fluorogenic substrate is increased in tissues obtained from patients with the B variant form of Tay-Sachs disease and it is virtually absent in the O-variant patients. The addition of purified neuraminidase and various purified hexosaminidases exerted only a minimal synergistic effect on the hydrolysis of Tay-Sachs ganglioside in the lysosomal preparations from the control or patient with the O variant of Tay-Sachs disease.